The term tachyarrhythmias typically refers to nonsustained and sustained forms of tachycardia originating from myocardial foci or reentrant circuits. The standard definition of tachycardia is a rhythm that produces a ventricular rate >100 beats per minute. This definition has some limitations in that atrial rates can exceed 100 beats per minute despite a slow ventricular rate. Furthermore, ventricular rates may exceed the baseline sinus rate and be <100 beats per minute but still represent an important “tachycardia” response, as is observed with accelerated ventricular rhythms. Premature complexes (depolarizations) are considered under the category of tachyarrhythmias because they may cause arrhythmia-related symptoms and/or serve as triggering events for more sustained forms of tachycardia.
SYMPTOMS DUE TO TACHYARRHYTHMIAS
Tachyarrhythmias classically produce symptoms of palpitations or racing of the pulse. With premature beats, skipping of the pulse or a pause may be experienced, and patients may even sense slowing of the heart rate or dizziness. A more dramatic irregularity of the pulse is experienced with chaotic rapid rhythms or tachyarrhythmias that originate in the atrium and conduct variably to the ventricles. With rapid tachyarrhythmias, hemodynamic compromise can occur, as can dizziness or syncope due to a decrease in cardiac output or breathlessness due to a marked increase in cardiac filling pressures. Occasionally, chest discomfort may be experienced that mimics symptoms of myocardial ischemia. The underlying cardiac condition typically dictates the severity of symptoms at any specific heart rate. Even patients with normal systolic left ventricular (LV) function may experience severe symptoms if diastolic compliance due to hypertrophy or valvular obstruction is present and a tachycardia develops. Hemodynamic collapse with the development of ventricular fibrillation (VF) can lead to sudden cardiac death (SCD) (Chap. 29).
DIAGNOSTIC TESTS IN EVALUATING TACHYARRHYTHMIAS
In patients who present with nonlife-threatening symptoms such as palpitations or dizziness, electrocardiographic (ECG) confirmation of an arrhythmia with the development of recurrent symptoms is essential. A 24-h Holter monitor should be considered only for patients with daily symptoms. For intermittent symptoms that are of prolonged duration, a patient-activated event monitor can be used to obtain the ECG information without the need for continuous ECG lead attachment and recordings. A patient-activated monitor with a continuously recorded memory loop (“loop recorder”) can be used to document short-lived episodes and the onset of the arrhythmia. This is the preferred monitoring technique for symptomatic patients with less frequent arrhythmia events, but it requires continuous ECG recording. A monitor that automatically triggers to record a fast rhythm can be used to detect asymptomatic arrhythmias. Patients with infrequent, severe symptoms that cannot be identified by intermittent ECG monitoring may receive an implanted loop ECG monitor that provides more extended periods of monitoring and automatic arrhythmia detection (Fig. 16-1).
Spontaneous termination of atrial fibrillation at the time of a syncopal episode identified from implantable loop ECG recording.
In patients who present with more severe symptoms, such as syncope, outpatient monitoring may be insufficient. In patients with structural heart disease and syncope in whom there is suspicion of ventricular tachycardia (VT), hospitalization and diagnostic electrophysiologic testing are warranted, with strong consideration of an implantable cardioverter/defibrillator (ICD) device. The 12-lead ECG recorded in sinus rhythm should be assessed carefully in patients without structural heart disease for evidence of ST-segment elevation in leads V1 and V2 consistent with Brugada syndrome, QT interval changes consistent with long or short QT syndromes, or a short PR interval and delta wave consistent with Wolff-Parkinson-White (WPW) syndrome. These ECG patterns identify a possible arrhythmogenic substrate that may cause intermittent life-threatening symptoms and warrant further evaluation and therapy. The individual syndromes are discussed in detail later in this chapter.
Monitoring for asymptomatic tachyarrhythmias is indicated in several specific situations. In patients with a suspected tachycardia-induced cardiomyopathy marked by chamber dilation and depression in systolic function, the demonstration of arrhythmia control is essential. Monitoring for asymptomatic ventricular premature complexes (VPCs) and nonsustained VT can be helpful in stratifying the risk of SCD in patients with depressed LV function after myocardial infarction (MI). Finally, in patients with asymptomatic atrial fibrillation (AF), anticoagulation treatment strategies depend on an accurate assessment of the presence of this arrhythmia. The duration of monitoring for asymptomatic arrhythmias may have to be extended to optimize detection capabilities.
A 12-lead ECG recording during the tachycardia can be an important diagnostic tool in identifying the mechanism and origin of a tachycardia to a degree not afforded by one- or two-lead ECG recordings. A 12-lead ECG of the tachyarrhythmia should be recorded and incorporated as a permanent part of the medical record whenever possible. For patients whose arrhythmias are provoked by exercise, an exercise test may provide an opportunity to obtain 12-lead ECG recordings of the arrhythmia and may obviate the need for more extended periods of monitoring.
Many paroxysmal supraventricular tachyarrhythmias are not associated with a significant risk of structural heart disease, and an evaluation for the presence of ischemic heart disease and cardiac function is required infrequently unless dictated by the severity or characteristics of the symptoms. However, in patients with focal or macroreentrant atrial tachycardias (ATs), atrial flutter (AFL), or AF, an evaluation of cardiac chamber size and function and of valve function is warranted. In patients with VT, an echocardiographic assessment of LV and right ventricular (RV) size and function should be the norm. Ventricular tachycardia that occurs in the setting of depressed LV function should raise the suspicion of advanced coronary artery disease (CAD). Ventricular tachycardia in the setting of isolated RV dilation should raise concern about the diagnosis of arrhythmogenic RV cardiomyopathy. Polymorphic VT in the absence of QT prolongation should always raise concern for a potentially unstable ischemic process that may need to be corrected to effect VT control.
MECHANISMS OF TACHYARRHYTHMIAS
Tachycardias are due to abnormalities of impulse formation and/or abnormalities of impulse propagation (Fig. 16-2).
Schematic representation of the different mechanisms for arrhythmias. A. Abnormal automaticity due to an increased slope of phase 4 of the action potential or a decrease in the threshold for phase 0. B.Triggered activity due to early afterdepolarizations (EADs) during phase 3 of the action potential due to alteration of plateau currents or delayed afterdepolarizations (DADs) during phase 4 of the action potential due to intracellular calcium accumulation. C.Reentry with basic requirements of two pathways that have heterogeneous electrophysiologic properties which allows conduction to block in one pathway and propagate slowly in the other, allowing for sufficient delay so that the blocked site has time for recovery to allow for reentry or circus movement tachycardia. Shown is a typical schema for reentry in the AV node. AV, atrioventricular; APC, atrial premature complex.
Abnormalities in impulse formation
An increase in automaticity normally causes an increase in sinus rate and sinus tachycardia (Fig. 16-2A). Abnormal automaticity is due to an increase in the slope of phase 4 depolarization or a reduced threshold for action potential depolarization in myocardium other than the sinus node. Abnormal automaticity is thought to be responsible for most atrial premature complexes (APCs) and VPCs and some ATs. Pacing does not provoke automatic rhythms. Less commonly, abnormal impulse formation is due to triggered activity. Triggered activity is related to cellular afterdepolarizations that occur at the end of the action potential, during phase 3, and are referred to as early afterdepolarizations; when they occur after the action potential, during phase 4, they are referred to as late afterdepolarizations. Afterdepolarizations are attributable to an increase in intracellular calcium accumulation. If sufficient afterdepolarization amplitude is achieved, repeated myocardial depolarization and a tachycardic response can occur. Early afterdepolarizations may be responsible for the VPCs that trigger the polymorphic ventricular arrhythmia known as torsades des pointes (TDP). Late afterdepolarizations are thought to be responsible for atrial, junctional, and fascicular tachyarrhythmias caused by digoxin toxicity and also appear to be the basis for catecholamine-sensitive VT originating in the outflow tract. In contrast to automatic tachycardias, those due to triggered activity (Fig. 16-2B) frequently can be provoked with pacing maneuvers.
Abnormalities in impulse propagation
Reentry is due to inhomogeneities in myocardial conduction and/or recovery properties. The presence of a unidirectional block with slow conduction to allow for retrograde recovery of the blocked myocardium allows the formation of a circuit that, if perpetuated, can sustain a tachycardia (Fig. 16-2C). These inhomogeneities are somewhat inherent but are minimized in normal myocardial activation/recovery. The inhomogeneities can be exaggerated by the presence of extra pathways, as occurs with the WPW syndrome; generalized genetically determined myocardial ion channel abnormalities, as occur with long QT syndrome (LQTS); or the interruption of normal myocardial patterns of activation due to the development of fibrosis.
Reentry appears to be the basis for most abnormal sustained supraventricular tachycardias (SVTs) and VTs. In general, reentry can be anatomically driven (fixed) based on the presence of “extra” pathways, natural anatomic barriers of conduction such as the crista terminalis, the vertical crest on the interior wall of the right atrium that separates the nontrabeculated posterior right atrium from the rest of the trabeculated right atrium located lateral to the structure, and/or extensive fibrosis created by underlying myocardial disease. This form of reentry seems to be more stable and results in a tachycardia that has a uniform (often monomorphic), repetitive appearance. Other forms of reentry appear to be more functional and are more dependent on dynamic changes in electrophysiologic properties of the myocardium. These tachycardias tend to be more unstable and may result in tachycardias that have a polymorphic appearance. Two classic examples of reentry that are primarily functional are VF due to acute myocardial ischemia and polymorphic VT in patients with a genetically determined ion channel abnormality such as Brugada syndrome, LQTS, or catecholaminergic polymorphic VT.
ATRIAL PREMATURE COMPLEXES
Atrial premature complexes are the most common arrhythmia identified during extended ECG monitoring. The incidence of APCs frequently increases with age and with the presence of structural heart diseases. Atrial premature complexes typically are asymptomatic, although some patients experience palpitations or an irregularity of the pulse.
ECG diagnosis of APCs
The ECG diagnosis of APCs is based on the identification of a P wave that occurs before the anticipated sinus beat (Fig. 16-3A and B). The source of the APC appears to parallel the typical sites of origin for ATs. The P-wave contour typically differs from that noted during sinus rhythm, although APCs from the right atrial appendage, superior vena cava (SVC), and superior aspect of the crista terminalis in the region of the sinus node may mimic the sinus P-wave morphology. In response to an APC, the PR interval lengthens, although APCs that originate near the atrioventricular (AV) nodal region may actually have a shorter PR interval because the atrial conduction time to the junction is shortened. A very early APC may not conduct to the ventricle and can create a pulse irregularity that may be perceived as a pause or “dropped beat.” If the APC conducts rapidly through the AV node, a partially recovered His-Purkinje system will be encountered and a QRS pattern consistent with a right or left bundle branch block may occur. This wide QRS pattern and the failure to recognize the preceding P wave may result in misdiagnosis of VPCs. APCs characteristically reset the sinus node. The resulting sum of the pre- and post-APC RR interval is less than two sinus PP intervals.
Atrial and ventricular premature complexes (APCs, VPCs). The APC resets the sinus node, and no compensatory pause is present A. even when conducted aberrantly in the ventricles with a bundle branch block-type QRS pattern B. VPCs tend not to reset the sinus activity (arrows) and will demonstrate a full compensatory pause C.
TREATMENT Atrial Premature Complexes
Atrial premature complexes generally do not require intervention. For extremely symptomatic patients who do not respond to explanation and reassurance, an attempt can be made to suppress the APCs with pharmacologic agents. The repetitive focus can even be targeted for catheter ablation. Beta blockers may be tried. Of note, these agents may uncommonly exacerbate symptoms if AV block occurs with the APC and irregularity of the pulse consequently becomes more profound. The use of class IC antiarrhythmic agents may eliminate the APCs but should be avoided if structural heart disease is present.
JUNCTIONAL PREMATURE COMPLEXES
Junctional premature complexes are extremely uncommon. The complexes originate from the AV node and His bundle region and may produce retrograde atrial activation with the P wave distorting the initial or terminal portions of the QRS complex, producing pseudo Q or S waves in leads II, III, and aVF. Extrasystoles originating in the bundle of His that do not conduct to the ventricle and also block the atria can produce unexplained surface ECG PR prolongation that does not follow a typical Wenckebach periodicity (i.e., gradual PR prolongation culminating in atrial activity that fails to conduct to the ventricles). Intracardiac recordings frequently can identify a His depolarization, thus identifying the origin of the complex to the AV junction. Symptomatic patients typically may be treated with beta blockers or, if there is no structural heart disease, class IC antiarrhythmic agents.
Physiologic sinus tachycardia represents a normal or appropriate response to physiologic stress, such as that which occurs with exercise, anxiety, or fever. Pathologic conditions such as thyrotoxicosis, anemia, and hypotension also may produce sinus tachycardia. It is important to distinguish sinus tachycardia from other SVTs. Sinus tachycardia will produce a P-wave contour consistent with its origin from the sinus node located in the superior-lateral and posterior aspect of the right atrium. The P wave is upright in leads II, III, and aVF and negative in lead aVR. The P-wave morphology in lead V1 characteristically has a biphasic, positive/negative contour. Onset of sinus tachycardia is gradual, and in response to carotid sinus pressure there may be some modest and transient slowing but no abrupt termination. Importantly, the diagnosis should not be based on the PR interval or the presence of a P wave before every QRS complex. The PR interval and the presence of 1:1 AV conduction properties are determined by AV nodal and His-Purkinje conduction; therefore, the PR interval can be dramatically prolonged while sinus tachycardia remains the atrial mechanism.
TREATMENT Physiologic Sinus Tachycardia
Treatment of physiologic sinus tachycardia is directed at the underlying condition causing the tachycardia response. Uncommonly, beta blockers are used to minimize the tachycardia response if it is determined to be potentially harmful, as may occur in a patient with ischemic heart disease and rate-related anginal symptoms.
Inappropriate sinus tachycardia represents an uncommon but important medical condition in which the heart rate increases either spontaneously or out of proportion to the degree of physiologic stress/exercise. Dizziness and even frank syncope often accompany the sinus tachycardia and symptoms of palpitations. The syndrome can be quite disabling. Associated symptoms of chest pain, headaches, and gastrointestinal upset are common. In many patients, the syndrome occurs after a viral illness and may resolve spontaneously over the course of 3–12 months, suggesting a postviral dysautonomia.
Excluding the diagnosis of an automatic AT that originates in the region of the sinus node can be difficult and may require invasive electrophysiologic evaluation. Frequently, patients are misdiagnosed as having an anxiety disorder with physiologic sinus tachycardia.
TREATMENT Inappropriate Sinus Tachycardia
For symptomatic patients, maintaining an increased state of hydration, salt loading, and careful titration of beta blockers to the maximum tolerated dose, administered in divided doses, frequently minimize symptoms. For severely symptomatic patients who are intolerant of or unresponsive to beta blockers, catheter ablation directed at modifying the sinus node may be effective. Because of the high recurrence rate after ablation and the frequent need for atrial pacing therapy, this intervention remains second-line treatment.
(Fig. 16-4) Atrial fibrillation is the most common sustained arrhythmia. It is marked by disorganized, rapid, and irregular atrial activation. The ventricular response to the rapid atrial activation is also irregular. In an untreated patient, the ventricular rate also tends to be rapid and is entirely dependent on the conduction properties of the AV junction. Although typically the rate will vary between 120 and 160 beats per minute, in some patients it can be >200 beats per minute. In other patients, because of heightened vagal tone or intrinsic AV nodal conduction properties, the ventricular response is <100 beats per minute and occasionally even profoundly slow. The mechanism for AF initiation and maintenance, although still debated, appears to be a complex interaction between drivers responsible for the initiation and the complex anatomic atrial substrate that promotes the maintenance of multiple wavelets of (micro)reentry. The drivers appear to originate predominantly from the atrialized musculature that enters the pulmonary veins and represent either focal abnormal automaticity or triggered firing that is somewhat modulated by autonomic influences. Sustained forms of microreentry as drivers also have been documented around the orifice of pulmonary veins; nonpulmonary vein drivers also have been demonstrated. The role these drivers play in maintaining the tachycardias may be significant and may explain the success of pulmonary vein isolation procedures in eliminating more chronic or persistent forms of AF.
Supraventricular tachycardias with irregular ventricular rates. Atrial fibrillation (A), atrial flutter (B), atrial tachycardia (C), and multifocal atrial tachycardia (MAT; D) are shown. The characteristics of the atrial activity with respect to the morphology and rate provide the clues to the diagnosis. The variable ventricular response to the atrial flutter and the atrial tachycardia suggest a Wenckebach-type periodicity.
Although AF is common in the adult population, it is extremely unusual in children unless structural heart disease is present or there is another arrhythmia that precipitates the AF, such as paroxysmal SVT in patients with WPW syndrome. The incidence of AF increases with age such that >5% of the adult population over 70 will experience the arrhythmia. As many patients are asymptomatic with AF, it is anticipated that the overall incidence, particularly that noted in the elderly, may be more than double the previously reported rates. Occasionally, AF appears to have a well-defined etiology, such as acute hyperthyroidism, an acute vagotonic episode, or acute alcohol intoxication. Acute AF is particularly common during the acute or early recovery phase of major vascular, abdominal, and thoracic surgery, in which case autonomic fluxes and/or direct mechanical irritation potentiate the arrhythmia. AF also may be triggered by other supraventricular tachycardias, such as AV nodal reentrant tachycardia (AVNRT), and elimination of these arrhythmias may prevent AF recurrence.
AF has clinical importance related to (1) the loss of atrial contractility, (2) the inappropriate fast ventricular response, and (3) the loss of atrial appendage contractility and emptying leading to the risk of clot formation and subsequent thromboembolic events.
Symptoms from AF vary dramatically. Many patients are asymptomatic and have no apparent hemodynamic consequences from the development of AF. Other patients experience only minor palpitations or sense irregularity of the pulse. Many patients, however, experience severe palpitations. The hemodynamic effect in patients can be quite dramatic, depending on the need for normal atrial contractility and the ventricular response. Hypotension, pulmonary congestion, and anginal symptoms may be severe in some patients. In patients with LV diastolic dysfunction that occurs with hypertension, hypertrophic cardiomyopathy, or obstructive aortic valvular disease, symptoms may be even more dramatic, especially if the ventricular rate does not permit adequate ventricular filling. Exercise intolerance and easy fatigability are the hallmarks of poor rate control with exertion. Occasionally, the only manifestation of AF is severe dizziness or syncope associated with the pause that occurs upon termination of AF before sinus rhythm resumes (Fig. 16-1).
The ECG in AF is characterized by the lack of organized atrial activity and the irregularly irregular ventricular response. Occasionally, one needs to record from multiple ECG leads simultaneously to identify the chaotic continuous atrial activation. Lead V1 frequently shows the appearance of organized atrial activity that mimics AFL. This occurs because the crista terminalis serves as an effective anatomic barrier to electrical conduction, and the activation of the lateral right atrium may be represented by a more uniform activation wave front that originates over the roof of the right atrium. ECG assessment of the PP interval (<200 ms) and the chaotic P-wave morphology in the remaining ECG leads will confirm the presence of AF.
Evaluation of a patient with AF should include a search for a reversible cause of the arrhythmia, such as hyperthyroidism or anemia. An echocardiogram should be performed to determine whether there is structural heart disease. Persistent or labile hypertension should be identified and treated.
TREATMENT Atrial Fibrillation
Treatment for AF must take into account the clinical situation in which the arrhythmia is encountered, the chronicity of the AF, the status of the patient’s level of anticoagulation, risk factors for stroke, the patient’s symptoms, the hemodynamic impact of the AF, and the ventricular rate.
ACUTE RATE CONTROL In the absence of hemodynamic compromise that might warrant emergent cardioversion to terminate the AF, the initial goals of therapy are to (1) establish control of the ventricular rate and (2) address anticoagulation status and begin IV heparin treatment if the duration of AF is >12 h and risk factors for stroke with AF are present (Table 16-1). Ventricular rate control for acute AF is best established with beta blockers and/or the calcium channel blocking agents verapamil and diltiazem. The route of administration and dose will be dictated by the ventricular rate and clinical status. Digoxin may add to the rate-controlling benefit of the other agents but is uncommonly used as a stand-alone agent, especially in acute AF.
RISK FACTORS FOR STROKE IN ATRIAL FIBRILLATION
Anticoagulation is of particular importance in patients who have known risk factors for stroke associated with AF. Factors associated with the highest risk of stroke include a history of stroke, transient ischemic attack (TIA) or systemic embolism, and the presence of rheumatic mitral stenosis. Other identified risk factors include age >65 years, history of congestive heart failure (CHF), diabetes mellitus, hypertension, LV dysfunction, and evidence of marked left atrial enlargement (>5.0 cm). Chronic anticoagulation with warfarin targeted to achieve an international normalized ratio (INR) between 2.0 and 3.0 is recommended in patients with persistent or frequent and long-lived paroxysmal AF and risk factors. If patients have not been adequately anticoagulated and the AF is more than 24–48 h in duration, a transesophageal echocardiogram (TEE) can be performed to exclude the presence of a left atrial thrombus that might dislodge with the attempted restoration of sinus rhythm with either nonpharmacologic or pharmacologic therapy. Anticoagulation must be instituted coincident with the TEE and maintained for at least 1 month after restoration of sinus rhythm if the duration of AF has been prolonged or is unknown. Heparin is maintained routinely until the INR is 1.8 with the administration of warfarin after the TEE. For patients who do not warrant early cardioversion of AF, anticoagulation should be maintained for at least 3 weeks with the INR confirmed to be >1.8 on at least two separate occasions before attempts at cardioversion.
Termination of AF acutely may be warranted on the basis of clinical parameters and/or hemodynamic status. Confirmation of appropriate anticoagulation status as described earlier in the chapter must be documented unless symptoms and clinical status warrant emergent intervention. Direct current transthoracic cardioversion during short-acting anesthesia is a reliable way to terminate AF. Conversion rates using a 200-J biphasic shock delivered synchronously with the QRS complex typically are >90%. Pharmacologic therapy to terminate AF is less reliable. Oral and/or IV administration of amiodarone or procainamide has only modest success. The acute IV administration of ibutilide appears to be somewhat more effective and may be used in selected patients to facilitate termination with direct current (DC) cardioversion (Tables 16-2 and 16-3).
COMMONLY USED ANTIARRHYTHMIC AGENTS—INTRAVENOUS DOSE RANGE/PRIMARY INDICATION
COMMONLY USED ANTIARRHYTHMIC AGENTS: CHRONIC ORAL DOSING/PRIMARY INDICATIONS
Pharmacologic therapy to maintain sinus rhythm can be instituted once sinus rhythm has been established or in anticipation of cardioversion to attempt to maintain sinus rhythm (Table 16-3). A single episode of AF may not warrant any intervention or only a short course of beta blocker therapy. To prevent recurrent AF unresponsive to beta blockade, a trial of antiarrhythmic therapy may be warranted, particularly if the AF is associated with rapid rates and/or significant symptoms. The selection of antiarrhythmic agents should be dictated primarily by the presence or absence of CAD, depressed LV function not attributable to a reversible tachycardia-induced cardiomyopathy, and/or severe hypertension with evidence of marked LV hypertrophy. The presence of any significant structural heart disease typically narrows treatment to the use of sotalol, amiodarone, dofetilide, or dronedarone. Severely depressed LV function with heart failure symptoms precludes the use of dronedarone and may limit sotalol therapy. Owing to the risk of QT prolongation and polymorphic VT, sotalol and dofetilide have to be initiated in the hospital in most cases.
In patients without evidence of structural heart disease or hypertensive heart disease without evidence of severe hypertrophy, the use of the class IC antiarrhythmic agents flecainide or propafenone appears to be well tolerated and does not have significant proarrhythmia risk. It is important to recognize that no drug is uniformly effective, and arrhythmia recurrence should be anticipated in over one-half of the patients during long-term follow-up regardless of the type and number of agents tried. It is also important to recognize that although the maintenance of sinus rhythm has been associated with improved long-term survival, the survival outcome of patients randomized to the pharmacologic maintenance of sinus rhythm was not superior to that of patients treated with rate control and anticoagulation in the AFFIRM and RACE trials. The AFFIRM and RACE trials compared outcome with respect to survival and thromboembolic events in patients with AF and risk factors for stroke using the two treatment strategies. It is believed that the poor outcome related to pharmacologic therapy used to maintain sinus rhythm was primarily due to the common inefficacy of such drug therapy and an increased incidence of asymptomatic AF. Many of the drugs used for rhythm control, including sotalol, amiodarone, propafenone, dronedarone, and flecainide, enhance slowing of AV nodal conduction. The absence of symptoms frequently leads to stopping anticoagulant therapy, and asymptomatic AF without anticoagulation increases stroke risk. Any consideration for stopping anticoagulation therefore must be accompanied by a prolonged period of ECG monitoring to document asymptomatic AF. It is also recommended that patients participate in monitoring by learning to take their pulse on a twice-daily basis and reliably identify its regularity if discontinuing anticoagulant therapy is contemplated seriously.
It is clear that to reduce the risk of drug-induced complications in treating AF, a thorough understanding of the drug planned to be used is critical—its dosing, metabolism, and common side effects and important drug-drug interactions. This information has been summarized in Tables 16-2, 16-3, 16-4, and 16-5 and serves as a starting point for a more complete review. In using antiarrhythmic agents that slow atrial conduction, strong consideration should be given to adding a beta blocker or a calcium channel blocker (verapamil or diltiazem) to the treatment regimen. This should help avoid a rapid ventricular response if AF is converted to “slow” AFL with the drug therapy (Fig. 16-5).
Atrial fibrillation. A. Transitions to “slow” atrial flutter during antiarrhythmic drug therapy. B. A rapid ventricular response with 1:1 atrioventricular conduction occurred with exercise, leading to C.symptoms of dizziness.
COMMON NONARRHYTHMIC TOXICITY OF MOST FREQUENTLY USED ANTIARRHYTHMIC AGENTS
PROARRHYTHMIC MANIFESTATIONS OF MOST FREQUENTLY USED ANTIARRHYTHMIC AGENTS
CHRONIC RATE CONTROL This is an option in patients who are asymptomatic or symptomatic due to the resulting tachycardia. Rate control is frequently difficult to achieve in patients who have paroxysmal AF. In patients with more persistent forms of AF, rate control with beta blockers, the calcium channel blockers diltiazem and verapamil, and/or digoxin frequently can be achieved. Using the drugs in combination may avoid some of the common side effects seen with high-dose monotherapy. An effort should be made to document the adequacy of rate control to reduce the risk of a tachycardia-induced cardiomyopathy. Heart rates >80 beats/min at rest or 100 beats/min with very modest physical activity are indications that rate control may be inadequate in persistent AF. Extended periods of ECG monitoring and assessment of heart rate with exercise should be considered.
In patients with symptoms resulting from inadequate rate control with pharmacologic therapy or worsening LV function due to the persistent tachycardia, ablative therapy to attempt to eliminate atrial fibrillation, or an AV junction ablation can be performed. The AV junction ablation must be coupled with the implantation of an activity sensor pacemaker to maintain a physiologic range of heart rates. Recent evidence that RV pacing can occasionally modestly depress LV function should be taken into consideration in identifying which patients are appropriate candidates for the “ablate and pace” treatment strategy. Occasionally, biventricular pacing may be used to minimize the degree of dyssynchronization that can occur with RV apical pacing alone. Rate control treatment options must be coupled with chronic anticoagulation therapy in all cases. Trials evaluating the elimination of embolic risk by elimination or isolation of the left atrial appendage or by endovascular insertion of a left atrial appendage-occluding device may provide other treatment options that can eliminate the need for chronic anticoagulation.
CATHETER AND SURGICAL ABLATIVE THERAPY TO PREVENT RECURRENT AF Although the optimum ablation strategy has not been defined, most ablation strategies incorporate techniques that isolate the atrial muscle sleeves entering the pulmonary veins; these muscle sleeves have been identified as the source of the majority of triggers responsible for the initiation of AF. Ablation therapy is currently considered an alternative to additional pharmacologic therapy trials in patients with recurrent symptomatic AF or AF associated with poor rate control who have failed an initial attempt at rhythm control with pharmacologic management. Ablative therapy appears superior to additional pharmacologic treatment aimed at rhythm control in this setting. Elimination of AF in 50–80% of patients with a catheter-based ablation procedure should be anticipated, depending on the chronicity of the AF, with additional patients becoming responsive to previously ineffective medications.
Catheter ablative therapy also holds promise in patients with more persistent forms of AF and even those with severe atrial dilation. Its confirmed efficacy suggests an important alternative to His bundle ablation and pacemaker insertion in many patients. Serious risks related to the left atrial ablation procedure, albeit low (overall 2–4%), include pulmonary vein stenosis, atrioesophageal fistula, systemic embolic events, perforation/tamponade, and phrenic nerve injury.
Surgical ablation of AF is typically performed at the time of other cardiac valve or coronary artery surgery and, less commonly, as a stand-alone procedure. The surgical Cox-Maze procedure is designed to interrupt all macroreentrant circuits that might potentially develop in the atria, thereby precluding the ability of the atria to fibrillate. In an attempt to simplify the operation, the multiple incisions of the traditional Cox-Maze procedure have been replaced with linear lines of ablation and pulmonary vein isolation using a variety of energy sources.
Severity of AF symptoms and difficulties in rate and/or rhythm control with pharmacologic therapy frequently dictate the optimum AF treatment strategy. Similar to the approach with pharmacologic rhythm control, a cautious approach to eliminating anticoagulant therapy is recommended after catheter or surgical ablation. Careful ECG monitoring for asymptomatic AF, particularly in patients with multiple risk factors for stroke, should be considered until guidelines are firmly established. If the left atrial appendage has been removed surgically, the threshold for stopping anticoagulation may be lowered. Antiarrhythmic therapy typically can be discontinued after catheter or surgical ablation of AF. However, in selected patients, satisfactory AF control may require maintenance of previously ineffective drug therapy after the ablation intervention.
ATRIAL FLUTTER AND MACROREENTRANT ATRIAL TACHYCARDIAS
Macroreentrant arrhythmias involving the atrial myocardium are referred to collectively as AFL. The terms AFL and macroreentrant AT frequently are used interchangeably, with both denoting a nonfocal source of an atrial arrhythmia. The typical or most common AFL circuit rotates in a clockwise or counterclockwise direction in the right atrium around the tricuspid valve annulus. The posterior boundary of the right AFL circuit is defined by the crista terminalis, the eustachian ridge, and the inferior and superior vena cavae. Counterclockwise right AFL represents ~80% of all AFL with superiorly directed activation of the interatrial septum, which produces the saw-toothed appearance of the P waves in ECG leads II, III, and aVF. Clockwise rotation of the same right atrial circuit produces predominantly positive P waves in leads II, III, and aVF (Fig. 16-4). Macroreentrant left AFL also may develop, albeit much less commonly. This type of arrhythmia may be the sequela of surgical or catheter-based ablation procedures that create large anatomic barriers or promote slowing of conduction in the left atrium, especially around the mitral valve annulus or partially disconnected pulmonary veins. Atypical AFL or macro-reentrant AT can also develop around incisions created during surgery for valvular or congenital heart disease or in and/or around large areas of atrial fibrosis.
Classic or typical right AFL has an atrial rate of 260–300 beats per minute with a ventricular response that tends to be 2:1, or typically 130–150 beats per minute. In the setting of severe atrial conduction disease and or antiarrhythmic drug therapy, the atrial rate can slow to <200 beats per minute. In this setting, a 1:1 rapid ventricular response may occur, particularly with exertion, and produce adverse hemodynamic effects (Fig. 16-5). Atypical AFL or macroreentrant AT related to prior surgical incisions and atrial fibrosis demonstrates less predictability in terms of the atrial rate and is more likely to demonstrate slower rates that overlap with those identified with focal atrial tachycardias.
Because lead V1 is frequently monitored in a hospitalized patient, coarse AF may be misdiagnosed as AFL. This occurs because in both typical right AFL and coarse AF the crista terminalis in the right atrium may serve as an effective anatomic barrier. The free wall of the right atrium, whose electrical depolarization is best reflected on the body surface by lead V1, may demonstrate a uniform wave front of atrial activation in both conditions. The timing of atrial activation is much more rapid in AF and always demonstrates variable atrial intervals with some intervals between defined P waves <200 ms (Fig. 16-6). A review of the other ECG leads demonstrates the disorganized atrial depolarization that is characteristic of AF. Frequently, an individual patient may alternate between AF and AFL or, less commonly, may manifest AF in one atrium and AFL in the other, making the distinction more difficult.
Atrial flutter/atrial fibrillation. Coarse atrial fibrillation (A) contrasted with organized atrial flutter (B).
TREATMENT Atrial Flutter
Because of the anticipated rapid regular ventricular rate associated with AFL and the failure to respond to pharmacologic therapy directed at slowing the ventricular rate, patients frequently are treated with DC cardioversion. The organized atrial flutter activity frequently can be terminated with low-energy external cardioversion of 50–100 J. The risk of thromboembolic events associated with typical AFL is high, and anticoagulation must be managed similarly to what was described for patients with AF.
Asymptomatic patients with AFL may develop heart failure symptoms with tachycardia-induced severe LV dysfunction. In all patients, an effort should be made to control the ventricular rate pharmacologically or restore sinus rhythm. Rate control with calcium antagonists (diltiazem or verapamil), beta blockers, and/or digoxin may be difficult. Even higher-grade AV slowing, such as a 4:1 AV response, may be only transient and is easily overcome with activity or emotional stress. Owing to the typically faster ventricular rate, AFL tends to be poorly tolerated in comparison to AF.
In selected patients with high anesthestic risk, an attempt at pharmacologic cardioversion with procainamide, amiodarone, or ibutilide is appropriate. antiarrhythmic drug therapy may also enhance the efficacy of DC cardioversion and the maintenance of sinus rhythm after cardioversion. Recurrence rates of AFL with pharmacologic attempts at rhythm control exceed 80% by 1 year.
Patients who manifest recurrent AFL appear to be effectively treated with catheter ablative therapy. For typical right AFL, an isthmus ablation line from the tricuspid annulus to the opening of the inferior vena cava can permanently eliminate flutter, with an anticipated success rate of >90% in most experienced centers. In patients with macroreentrant atrial tachycardia or AFL involving prior surgical incisions or catheter ablation or in areas of atrial fibrosis, detailed mapping of the arrhythmia circuit is required to design the best ablation strategy to interrupt the circuit. In selected patients with AF and typical right AFL, pharmacologic therapy may help prevent the AF but not the AFL. In this type of patient, hybrid therapy with antiarrhythmic agents coupled with a right atrial isthmus ablation may produce AF and AFL control.
MULTIFOCAL ATRIAL TACHYCARDIA
Multifocal AT (MAT) is the signature tachycardia of patients with significant pulmonary disease. The atrial rhythm is characterized by at least three distinct P-wave morphologies and often at least three different PR intervals, and the associated atrial and ventricular rates are typically between 100 and 150 beats per minute. The presence of an isoelectric baseline distinguishes this arrhythmia from AF (Fig. 16-4). The absence of any intervening sinus rhythm distinguishes MAT from normal sinus rhythm with frequent multifocal APCs, although this distinction may be moot as these processes define an electrophysiologic continuum.
TREATMENT Multifocal Atrial Tachycardia
Therapy for MAT should be directed at improving the underlying medical condition, which is typically, although not invariably, chronic obstructive or restrictive lung disease. Treatment with the calcium channel blocker verapamil also may provide some benefit. The judicious use of flecainide or propafenone may also decrease atrial arrhythmias. Patients should be screened for the presence of significant ventricular dysfunction or CAD before these agents are started. Low-dose amiodarone therapy may also control the arrhythmia and minimize the risk of pulmonary toxicity noted with the drug.
FOCAL ATRIAL TACHYCARDIAS The two general mechanisms for focal ATs can be distinguished by observations made at AT initiation and in response to adenosine. Automatic ATs start with a “warm-up” period over the first 3–10 complexes and, similarly, slow in rate before termination. They may respond to adenosine not only with evidence of AV block but also with gradual slowing of the atrial rhythm and termination. The initiation of automatic ATs frequently can be provoked by isoproterenol infusion. The first P wave of the tachycardia has the same morphology as the remaining waves. Some of the ATs may be triggered or provoked by burst atrial pacing but are not reliably initiated by programmed atrial stimulation.
In contrast, evidence supporting a focal reentrant AT includes the initiation of the tachycardia with programmed atrial stimulation or spontaneous premature beats. The P wave initiating the tachycardia will characteristically have a different morphology than the P wave during the sustained AT. In response to adenosine, reentrant ATs will demonstrate AV block but typically do not slow and/or terminate. Most focal ATs in the absence of structural heart disease originate from specific anatomic locations. These anatomic locations appear to be associated with anatomic ridges, such as the crista terminalis, the valve annuli, and the limbus of the fossa ovalis. ATs also appear to originate from the muscular sleeves associated with the cardiac thoracic veins, i.e., the SVC, the coronary sinus, and the pulmonary veins. As was indicated, repetitive firing of these foci also appears to serve as the triggering mechanism for AF in most patients.
It is important to distinguish focal ATs from reentrant tachycardias that incorporate the AV node in the circuit (Fig. 16-7). The primary distinction is related to the persistence of the AT in the presence of AV block that occurs spontaneously or is created by carotid sinus massage or the administration of adenosine (Fig. 16-4). Atrial activity drives the ventricles in AT and all changes in the PP interval accompanied by correlative changes in the RR intervals; in addition, the V–A relationship changes when the atrial rate changes. The P wave in AT is characteristically distinct from the sinus P-wave morphology, and unless there is significant AV nodal conduction delay, the PR interval is shorter than the measured RP interval when there is a 1:1 relationship between atria and ventricles (Fig. 16-7).
The P wave for ATs depends on the anatomic site of origin. In addition to attempting to create an AV block to establish the diagnosis of AT, analysis of the P-wave morphology on the 12-lead ECG may help exclude AV nodal reentry, AV bypass tract–mediated reentrant tachycardias, and physiologic or inappropriate sinus tachycardia (Fig. 16-7).
Pattern of atrial and ventricular activation and characteristic relationship of P-wave and QRS complex as recorded in leads II and V1 during regular supraventricular tachycardias. A. Sinus tachycardia. B. Atrial tachycardia from top of the atria. C. Atrioventricular nodal reentry. D. Accessory pathway–mediated orthodromic supraventricular tachycardia.
The ECG distinction between focal automatic or microreentrant and macroreentrant AT or atypical AFL is not always possible. Although sustained focal ATs tend to be slower, the atrial rates frequently overlap. Focal ATs, which are more common in the absence of structural heart disease, tend to demonstrate an isolectric baseline between P waves, whereas macroreentrant ATs represent atrial activation that is continuous and an isoelectric baseline between P waves frequently is absent. In patients with a history of prior atrial surgery, one must suspect a macroreentrant mechanism. These distinctions are less important with respect to acute management but have importance related to ablation strategies and anticipated outcome.
TREATMENT Atrial Tachycardia
Pharmacologic treatment of AT generally is approached in a similar fashion to that of AF and AFL. AV nodal blocking agents are administered in the setting of rapid ventricular rates. Acute IV administration of procainamide or amiodarone may terminate the tachycardia. Tachycardias that do not respond to pharmacologic therapy may be terminated with electrical cardioversion. Typically, anticoagulation before treatment is not needed unless there is evidence of severe atrial dilatation, >5 cm left atrial diameter with a high risk of AF, and/or a history of coincident paroxysmal AF. Most focal ATs are readily amenable to catheter ablative therapy. In patients who fail to respond to medical therapy or who are reluctant to take chronic drug therapy, this option should be considered, with an anticipated 90% cure rate. A parahisian location for the AT and/or a focus that is located in the left atrium may modestly increase the risk related to the procedure, and for this reason, every effort should be made to determine the likely origin of the AT based on an analysis of the P-wave morphology on 12-lead ECG before the procedure.
AV NODAL TACHYCARDIAS
AV nodal reentrant tachycardia
Atrioventricular nodal reentrant tachycardia is the most common paroxysmal regular SVT. It is more commonly observed in women than in men and is typically manifest in the second to fourth decades of life. In general, because AVNRT tends to occur in the absence of structural heart disease, it is usually well tolerated. Neck pulsations are usually felt because of the simultaneous atrial and ventricular contraction, and a “frog sign” can be identified on physical examination during the arrhythmia. In the presence of hypertension or other forms of structural heart disease that limit ventricular filling, hypotension or syncope may occur.
Atrioventricular nodal reentrant tachycardia develops because of the presence of two electrophysiologically distinct pathways for conduction in the complex syncytium of muscle fibers that make up the AV node. The fast pathway in the more superior part of the node has a longer refractory period, whereas the pathway lower in the AV node region conducts more slowly but has a shorter refractory period. As a result of the inhomogeneities of conduction and refractoriness, a reentrant circuit can develop in response to premature stimulation. Although conduction occurs over both pathways during sinus rhythm, only the conduction over the fast pathway is manifest, and as a result, the PR interval is normal. APCs occurring at a critical coupling interval are blocked in the fast pathway because of the longer refractory period and are conducted slowly over the slow pathway. When sufficient conduction slowing occurs, the blocked fast pathway can recover excitability and atrial activation can occur over the fast pathway to complete the circuit. Repetitive activation down the slow and up the fast pathway results in typical AV nodal reentrant tachycardia (Fig. 16-7).
ECG Findings in AVNRT
The APC initiating AVNRT is characteristically followed by a long PR interval consistent with conduction via the slow pathway. AVNRT is manifest typically as a narrow QRS complex tachycardia at rates that range from 120 to 250 beats/min. The QRS-P wave pattern associated with typical AVNRT is quite characteristic, with simultaneous activation of the atria and ventricles from the reentrant AV nodal circuit. The P wave frequently is buried inside the QRS complex and either will not be visible or will distort the initial or terminal portion of the QRS complex (Fig. 16-7). Because atrial activation originates in the region of the AV node, a negative deflection will be generated by retrograde atrial depolarization when recording ECG leads II, III, or aVF.
Occasionally, AVNRT occurs with activation in the reverse direction, conducting down the fast pathway and returning up the slow pathway. This form of AVNRT occurs much less commonly and produces a prolonged RP interval during the tachycardia with a negative P wave in leads II, III, and aVF. This atypical form of AVNRT is more easily precipitated by ventricular stimulation.
TREATMENT Atrioventricular Nodal Reentrant Tachycardia
ACUTE TREATMENT Treatment is directed at altering conduction within the AV node. Vagal stimulation, such as that which occurs with the Valsalva maneuver or carotid sinus massage, can slow conduction in the AV node sufficiently to terminate AVNRT. In patients in whom physical maneuvers do not terminate the tachyarrhythmia, the administration of adenosine, 6–12 mg IV, frequently does so. Intravenous beta blockade or calcium channel therapy should be considered as second-line treatment. If hemodynamic compromise is present, R-wave synchronous DC cardioversion using 100–200 J can terminate the tachyarrhythmia.
PREVENTION Prevention may be achieved with drugs that slow conduction in the antegrade slow pathway, such as digitalis, beta blockers, and calcium channel blockers. In patients who have a history of exercise-precipitated AVNRT, the use of beta blockers frequently eliminates symptoms. In patients who do not respond to drug therapy directed at the antegrade slow pathway, treatment with class IA or IC agents directed at altering conduction of the fast pathway may be considered.
Catheter ablation, directed at elimination or modification of slow pathway conduction, is very effective in permanently eliminating AVNRT. Patients with recurrent AVNRT that produces significant symptoms or heart rates >200 beats/min and patients reluctant to take chronic drug therapy should be considered for ablative therapy. Catheter ablation can cure AV nodal reentry in >95% of patients with a single procedure. The risk of AV block requiring a permanent pacemaker is ~1% with the ablation procedure.
AV junctional tachycardias
These can also occur in the setting of enhanced normal automaticity, abnormal automaticity, or triggered activity. These tachycardias may or may not be associated with retrograde conduction to the atria, and the P waves may appear dissociated or produce intermittent conduction and early activation of the junction. These arrhythmias may occur as a manifestation of increased adrenergic tone or drug effect in patients with sinus node dysfunction or after surgical or catheter ablation. The arrhythmia may also be a manifestation of digoxin toxicity. The most common manifestation of digoxin intoxication is the sudden regularization of the response to AF. A junctional tachycardia due to digoxin toxicity typically does not manifest retrograde conduction. Sinus activity may appear dissociated or result in intermittent capture beats with a long PR interval. If the rate is >50 beats per minute and <100 beats per minute, the term accelerated junctional rhythm applies. Occasionally, automatic rhythms are mimicked by AVNRT that fails to conduct to the atrium. The triggering events associated with the onset of the tachycardia may provide a clue to the appropriate diagnosis. Initiation of the tachycardia without an atrial premature beat with a gradual acceleration in rate suggests an automatic focus.
TREATMENT Atrioventricular Junctional Tachycardias
Treatment of automatic/triggered junctional tachycardias is directed at decreasing adrenergic stimulation and reversing digoxin toxicity, if present. Digoxin therapy can be withheld if toxicity is suspected, and the administration of digoxin-specific antibody fragments can rapidly reverse digoxin toxicity if the tachycardia is producing significant symptoms and rapid termination is indicated. Junctional tachycardia due to abnormal automaticity can be treated pharmacologically with beta blockers. A trial of class IA or IC drugs may also be attempted. For incessant automatic junctional tachycardia, focal catheter ablation can be performed but is associated with an increased risk of AV block.
TACHYCARDIAS ASSOCIATED WITH ACCESSORY AV PATHWAYS
Tachycardias that involve accessory pathways (APs) between atria and ventricles commonly manifest a normal QRS complex with a short or long RP interval. They must be considered in the differential diagnosis of other narrow-complex tachycardias. Importantly, most tachycardias associated with APs involve a large macroreentrant circuit that includes the ventricles (Fig. 16-7). Thus, identifying these arrhythmias as “supraventricular” is actually a misnomer, and they deserve separate consideration.
Accessory pathways are typically capable of conducting rapidly in both an antegrade and a retrograde direction. In the absence of an AP, the sinus impulse normally activates the ventricles via the AV node and His-Purkinje system, resulting in a PR interval of 120–200 ms. When an antegradely conducting AP is present, the sinus impulse bypasses the AV node and can activate the ventricles rapidly, resulting in ventricular preexcitation. The resulting PR interval is shorter than anticipated. In addition, because the initial ventricular activation is due to muscle-to-muscle conduction, as opposed to rapid spread of activation via the His-Purkinje system, the initial portion of the QRS complex is slurred, creating the characteristic “delta wave.” The remaining portion of the QRS complex in sinus rhythm is created by a fusion of the ventricular activation wave front originating from the Purkinje network and the continued spread of activation from the site of insertion of the AP (Fig. 16-8). Evidence of ventricular preexcitation includes evidence in sinus rhythm of a short PR interval and a delta wave.
A. Sinus rhythm tracing of leads V1–V3 showing evidence of Wolff-Parkinson-White syndrome with short PR interval and delta wave. B. During atrial fibrillation, rapid conduction to the ventricles is observed producing a wide QRS complex tachycardia with marked irregularity of the ventricular response and morphology of the QRS complex.
The most common AP connects the left atrium to the left ventricle, followed by posterior septal, right free wall, and anterior septal APs. APs typically insert from the atrium into the adjacent ventricular myocardium. However, occasionally pathways, particularly those originating from the right atrium, can have a ventricular insertion at a site distant from the AV groove in the fascicles. These pathways conduct more slowly and are referred to as atriofascicular accessory pathways. Atriofascicular APs are unique in their tendency to demonstrate decremental antegrade conduction.
Other accessory pathway connections from the AV node to the fascicles may exist. These pathways are referred to as Mahaim fibers and typically manifest a normal PR interval with a delta wave.
Patients with manifest preexcitation and WPW syndrome are typically subject to both macroreentrant tachycardias and a rapid response to AF (Fig. 16-8). The most common macroreentrant tachycardia associated with WPW syndrome is referred to as orthodromic AV reentry. Ventricular activation occurs via the AV node and the His-Purkinje system. Conduction then returns or reenters the atria via retrograde conduction over the AP. The reentrant circuit develops because of the inhomogeneity in conduction and refractoriness in the AP and the normal AV node.
Characteristically, the AP has more rapid conduction but a longer refractory period than that of the AV node. Typical APs do not show evidence of antegrade decremental conduction. An APC can block in the AP and conduct sufficiently slowly or with decrement via the AV node to allow for retrograde recovery of activation of the AP and, in turn, of the atria (Fig. 16-7). This retrograde activation of the atria via the AP is referred to as an echo beat. If the pattern repeats itself, a tachycardia develops. Uncommonly, the reentrant circuit can be reversed so that the impulse reaches the ventricle via the AP and conducts retrogradely through to the atria via the His-Purkinje system and the AV node; this is referred to as antidromic AV reentry and/or preexcitation macroreentry, with the entire activation of the ventricle originating from the site of insertion of the AP. Although it is uncommon, it is important to recognize antidromic SVT. The ECG pattern during the tachycardia mimics VT originating from the site of ventricular insertion of the AP. The presence of manifest preexcitation in sinus rhythm provides a valuable clue to the diagnosis.
The second most common and potentially more serious arrhythmia associated with WPW syndrome is rapidly conducting AF. Nearly 50% of patients with evidence of APs are predisposed to episodes of AF. In patients who have rapid antegrade conduction from the atria to the ventricles over the AP, the AP can conduct rapidly in response to AF, resulting in a faster ventricular rate than would occur normally via the AV node. The rapid ventricular rates can result in hemodynamic compromise and even precipitate VF. The QRS pattern during AF in patients with manifest preexcitation can appear quite bizarre and change on a beat-to-beat basis due to the variability in the degree of fusion from activation over the AV node (Fig. 16-8).
In ~50% of patients with APs, there is no antegrade conduction over the AP; however, retrograde conduction is preserved. As a result, the AP is not manifest in sinus rhythm and is manifest only during the sustained tachycardia. The presence of a concealed AP is suggested by the timing and pattern of atrial activation during the tachycardia: the P wave typically follows ventricular activation with a short RP wave interval (Fig. 16-7). Because many APs connect the left ventricle to the left atrium, the pattern of atrial activation during the tachycardia frequently produces negative P waves in leads I and aVL. The tachycardia circuit and therefore its ECG manifestation during orthodromic tachycardia are identical both in patients with overt pre-excitation in sinus rhythm and in those with concealed APs. Patients with concealed APs, although prone to episodes of AF, are not at risk for developing a rapid ventricular response to the AF.
Occasionally, APs conduct extremely slowly in a retrograde fashion, resulting in longer retrograde conduction and the development of a long RP interval during the tachycardia (long RP tachycardia). Because of the presence of this dramatically slowed conduction, additional conduction slowing created by premature atrial complexes is not required for tachycardia to ensue. These patients are more prone to frequent episodes of tachycardia and can present with “incessant” tachycardias and tachycardia-induced LV cardiomyopathy. The correct diagnosis of a long RP tachycardia may be suggested by the pattern of initiation and the P-wave morphology. Frequently, however, an electrophysiologic evaluation is required to establish the diagnosis.
TREATMENT Accessory Pathway–Mediated Tachycardias
Acute treatment of AP-mediated macroreentrant orthodromic tachycardias is similar to that for AV nodal reentry and is directed at altering conduction in the AV node. Vagal stimulation with the Valsalva maneuver and carotid sinus pressure may create sufficient AV nodal slowing to terminate the AVRT. Intravenous administration of adenosine, 6–12 mg, is first-line pharmacologic therapy; IV, the calcium channel blockers verapamil and diltiazem or beta blockers may also be effective. In patients who manifest preexcitation and AF, therapy should be aimed at preventing a rapid ventricular response. In life-threatening situations, DC cardioversion should be used to terminate the AF. In nonlife-threatening situations, procainamide at a dose of 15 mg/kg administered IV over 20–30 min will slow the ventricular response and may organize and terminate AF. Ibutilide can also be used to facilitate termination of AF. During AF there may be rapid conduction over the AV node as well as the AP. Caution should be used in attempting to slow AV nodal conduction with the use of digoxin or verapamil; when administered IV, these drugs may actually result in an acute increase in rate over the AP, placing the patient at risk for development of VF. Digoxin appears to shorten the refractory period of the AP directly and thus increases the ventricular rate. Verapamil appears to shorten the refractory period indirectly by causing vasodilation and a reflex increase in sympathetic tone.
Chronic oral administration of beta blockers and/or verapamil or diltiazem may be used to prevent recurrent supraventricular reentrant tachycardias associated with APs. In patients with evidence of AF and a rapid ventricular response and in those with recurrences of SVT on AV nodal blocking drugs, strong consideration should be given to the administration of a class IA or IC antiarrhythmic drug such as quinidine, flecainide, or propafenone because these drugs slow conduction and increase refractoriness in the AP.
Patients with a history of recurrent symptomatic SVT episodes, incessant SVT, and heart rates >200 beats/min with SVT should be given strong consideration for undergoing catheter ablation. Patients who have demonstrated rapid antegrade conduction over their AP or the potential for rapid conduction should also be considered for catheter ablation. Catheter ablation therapy has been demonstrated to be successful in >95% of patients with documented WPW syndrome and appears effective regardless of age. The risk of catheter ablative therapy is low and is dictated primarily by the location of the AP. Ablation of parahisian APs is associated with a risk of heart block, and ablation in the left atrium is associated with a small but definite risk of thromboembolic phenomenon. These risks must be weighed against the potential serious complications associated with hemodynamic compromise, the risk of VF, and the severity of the patient’s symptoms with AP-mediated tachycardias.
Patients who demonstrate evidence of ventricular preexcitation in the absence of any prior arrhythmia history merit special consideration. The first arrhythmia manifestation can be a rapid SVT or, albeit of low risk (<1%), a life-threatening rapid response to AF. Patients who demonstrate intermittent preexcitation during ECG monitoring or an abrupt loss of AP conduction during exercise testing are at low risk of a life-threatening rapid response to AF. All other patients should be advised of their risks and therapeutic options in advance of a documented arrhythmia event.
VENTRICULAR PREMATURE COMPLEXES
The origin of premature beats in the ventricle at sites remote from the Purkinje network produces slow ventricular activation and a wide QRS complex that is typically >140 ms in duration. Ventricular premature complexes are common and increase with age and the presence of structural heart disease. VPCs can occur with a certain degree of periodicity that has become incorporated into the lexicon of electrocardiography. Ventricular premature complexes may occur in patterns of bigeminy, in which every sinus beat is followed by a VPC, or trigeminy, in which two sinus beats are followed by a VPC. VPCs may have different morphologies and are thus referred to as multiformed. Two successive VPCs are termed pairs or couplets. Three or more consecutive VPCs are termed VT when the rate is >100 beats per minute. If the repetitive VPCs terminate spontaneously and are more than three beats in duration, the arrhythmia is referred to as nonsustained VT.
APCs with aberrant ventricular conduction may also create a wide and early QRS complex. The premature P wave can occasionally be difficult to discern when it falls on the preceding T wave, and other clues must be used to make the diagnosis. The QRS pattern for a VPC does not appear to follow a typical right or left bundle branch block pattern as the QRS morphology is associated with aberrant atrial conduction and can be quite bizarre. On occasion, VPCs can arise from the Purkinje network of the ventricles, in which case the QRS pattern mimics aberration. The 12-lead ECG recording of the VPC may be required to identify subtle morphologic clues regarding the QRS complex to confirm its ventricular origin. Most commonly, VPCs are associated with a “fully compensatory pause” (i.e., the duration between the last QRS before the PVC and the next QRS complex is equal to twice the sinus rate [Fig. 16-3]). The VPC typically does not conduct to the atrium. If the VPC does conduct to the atrium, it may not be sufficiently early to reset the sinus node. As a result, sinus activity will occur and the antegrade wave front from the sinus node may encounter some delay in the AV node or His-Purkinje system from the blocked VPC wave front, or it may collide with the retrograde atrial wave front. Sinus activity will continue undisturbed, resulting in a delay to the next QRS complex (Fig. 16-3). Occasionally, the VPC can occur early enough and conduct retrograde to the atrium to reset the sinus node; the pause that results will be less than compensatory. VPCs that fail to influence the oncoming sinus impulse are termed interpolated VPCs. A ventricular focus that fires repetitively at a fixed interval may produce variably coupled VPCs, depending on the sinus rate. This type of focus is referred to as a parasystolic focusbecause its firing does not appear to be modulated by sinus activity and the conducted QRS complex. The ventricular ectopy will occur at a characteristic fixed integer or multiple of these intervals. The variability in coupling relative to the underlying QRS complex and a fixed interval between complexes of ventricular origin provide the diagnostic information necessary to identify a parasystolic focus.
TREATMENT Ventricular Premature Complexes
The threshold for treatment of VPCs is high, and the treatment is directed primarily at eliminating severe symptoms associated with palpitations. VPCs of sufficient frequency can cause a reversible cardiomyopathy. Depressed LV function in the setting of ventricular bigeminy and/or frequent nonsustained VT should raise the possibility of a cardiomyopathy that is reversible with control of the ventricular arrhythmia. In the absence of structural heart disease, VPCs do not appear to have prognostic significance. In patients with structural heart disease, frequent VPCs and runs of nonsustained VT have prognostic significance and may portend an increased risk of SCD. However, no study has documented that elimination of VPCs with antiarrhythmic drug therapy reduces the risk of arrhythmic death in patients with severe structural heart disease. In fact, drug therapies that slow myocardial conduction and/or enhance dispersion of refractoriness can actually increase the risk of life-threatening arrhythmias (drug-induced QT prolongation and TDP) despite being effective at eliminating VPCs.
ACCELERATED IDIOVENTRICULAR RHYTHM (AIVR)
AIVR refers to a ventricular rhythm that is characterized by three or more complexes at a rate >40 beats per minute and <120 beats per minute. The arrhythmia mechanism causing AIVR is thought to be due to abnormal automaticity. By definition, there is an overlap between AIVR and “slow” VT; both rhythms can manifest rates between 90 and 120 beats per minute. Because AIVR tends to be a benign rhythm with different therapeutic implications, it is worthwhile to attempt to distinguish it from “slow” VT. AIVR has a characteristic gradual onset and offset and more variability in cycle length. It is typically a brief, self-limiting arrhythmia. AIVR can be seen in the absence of any structural heart disease, but it is frequently present in the setting of acute myocardial infarction (MI), cocaine intoxication, acute myocarditis, digoxin intoxication, and postoperative cardiac surgery. Sustained forms of AIVR can exist, particularly in the setting of acute MI and postoperatively. In the setting of sustained AIVR, hemodynamic compromise can occur because of the loss of AV synchrony. Patients with RV infarction associated with proximal right coronary artery occlusion are most susceptible to associated bradyarrhythmias and the hemodynamic consequences of AIVR. In these patients, acceleration of the atrial rate either by the cautious administration of atropine or by atrial pacing may be an important treatment consideration.
VT originates below the bundle of His at a rate >100 beats per minute; most VT patients have rates >120 beats per minute. Sustained VT at rates <120 beats per minute and even <100 beats per minute can be observed, particularly in association with the administration of antiarrhythmic agents that can slow the rate. Because of the overlap in rates with AIVR, the arrhythmia ECG characteristics and the clinical circumstance sometimes can be used to distinguish the two forms of tachycardia. Slow sustained VT is less likely to show a marked warm-up in rate and the marked cycle-length oscillations seen with AIVR, and it is more likely to occur in the setting of chronic infarction or cardiomyopathy and less likely with acute infarction or myocarditis. Obviously, significant overlap may exist. Typically, slow VT will be initiated with programmed stimulation and is found to represent a large macroreentrant circuit in chronically diseased myocardium capable of supporting markedly slow conduction.
The QRS complex during VT may be uniform (monomorphic) or may vary from beat to beat (polymorphic). Polymorphic VT in patients who demonstrate a long QT interval during their baseline rhythm typically is referred to as torsades des pointes. The polymorphic VT associated with QT prolongation dramatically oscillates around the baseline on most of the monitored ECG leads, mimicking the “turning of the points” stitching pattern (Fig. 16-9).
Sinus rhythm with long QT interval and the polymorphic ventricular arrhythmia torsades des pointes. Dramatic T wave alternans are present in sinus rhythm.
Monomorphic VT suggests a stable tachycardia focus in the absence of structural heart disease or a fixed anatomic abnormality that can create the substrate for a stable reentrant VT circuit when structural disease is present. Monomorphic VT tends to be a reproducible and recurrent phenomenon and may be initiated with pacing and programmed ventricular stimulation. In contrast, polymorphic VT suggests a more dynamic and/or unstable process and, by its very nature, is less reproducible. Polymorphic VT may be produced by acute ischemia, myocarditis, or dynamic changes in the QT interval and enhanced dispersion of ventricular refractoriness. Polymorphic VTs are not reliably initiated with pacing or programmed stimulation.
A time duration of 30 s frequently is used to distinguish sustained from nonsustained VT. Hemodynamically unstable VT that requires termination before 30 s or VT that is terminated by therapy from an implantable defibrillator is also typically classified as sustained. Ventricular flutter appears as a sine wave on the ECG and has a rate of >250 beats per minute. A rapid rate coupled with the sine wave nature of the arrhythmia makes it impossible to identify a discrete QRS morphology. When antiarrhythmic drugs are being administered, a sine wave appearance of the QRS complex can be observed, even at rates as low as 200 beats per minute. VF is characterized by completely disorganized ventricular activation on the surface ECG. Polymorphic ventricular arrhythmias, ventricular flutter, and VF always produce hemodynamic collapse if allowed to continue. The hemodynamic stability of a unimorphic VT depends on the presence and severity of the underlying structural heart disease, the location of the site of origin of the arrhythmia, and the heart rate.
It is important to distinguish monomorphic VT from SVT with aberrant ventricular conduction due to right or left bundle branch block.
Importantly, the sinus or baseline 12-lead ECG tracing can provide important clues that help establish the correct diagnosis of a wide complex tachycardia. The presence of an aberrant QRS pattern that matches exactly that of the wide complex rhythm strongly supports the diagnosis of SVT. A right or left bundle branch block QRS pattern that does not match the QRS and/or that is wider in duration than the QRS during the wide complex tachycardia supports the diagnosis of VT. Most patients with VT have structural heart disease and show evidence of a prior Q wave MI during sinus rhythm. Important exceptions to this rule are discussed. Finally, the presence of a preexcited QRS pattern on the 12-lead ECG in sinus rhythm suggests that the wide complex rhythm represents an atrial arrhythmia, such as AFL or a focal AT, with rapid conduction over an AP or antidromic macroreentrant tachycardia (Fig. 16-8). If the arrhythmia is irregular with changing QRS complexes, the diagnosis of AF with ventricular preexcitation should be considered.
With the exception of some idiopathic outflow tract tachycardias, most VTs do not respond to vagal stimulation provoked by carotid sinus massage, the Valsalva maneuver, or adenosine administration. The IV administration of verapamil and/or adenosine is not recommended as a diagnostic test. Verapamil has been associated with hemodynamic collapse when administered to patients with structural heart disease and VT.
Patients with VT frequently demonstrate AV dissociation. Findings on physical examination of intermittent cannon a waves and variability of the first heart sound are consistent with AV dissociation. The presence of AV dissociation is characteristically marked by the presence of sinus capture or fusion beats. The presence of 1:1 ventriculoatrial conduction does not preclude a diagnosis of VT.
Additional characteristics of the 12-lead ECG during the tachycardia that suggest VT include (1) the presence of a QRS duration >140 ms in the absence of drug therapy, (2) a superior and rightward QRS frontal plane axis, (3) a bizarre QRS complex that does not mimic the characteristic QRS pattern associated with left or right bundle branch block, and (4) slurring of the initial portion of the QRS (Fig. 16-10). Table 16-6 provides a useful summary of ECG criteria that have evolved based on the described characteristics of VT.
Ventricular tachycardia. ECG showing AV dissociation (arrows mark P waves), wide QRS >200 ms, superior frontal plane axis, slurring of the initial portion of the QRS, and large S wave in V6—all clues to the diagnosis of ventricular tachycardia.
ECG CLUES SUPPORTING THE DIAGNOSIS OF VENTRICULAR TACHYCARDIA
TREATMENT Ventricular Tachycardia/Fibrillation
Sustained polymorphic VT, ventricular flutter, and VF all lead to immediate hemodynamic collapse. Emergency asynchronous defibrillation is therefore required, with at least 200-J monophasic or 100-J biphasic shock. The shock should be delivered asynchronously to avoid delays related to sensing of the QRS complex. If the arrhythmia persists, repeated shocks with the maximum energy output of the defibrillator are essential to optimize the chance of successful resuscitation. Intravenous lidocaine and/or amiodarone should be administered but should not delay repeated attempts at defibrillation.
For any monomorphic wide complex rhythm that results in hemodynamic compromise, a prompt R-wave synchronous shock is required. Conscious sedation should be provided if the hemodynamic status permits. For patients with a well-tolerated wide complex tachycardia, the appropriate diagnosis should be established on the basis of strict ECG criteria (Table 16-6). Pharmacologic treatment to terminate monomorphic VT is not typically successful (<30%). Intravenous procainamide, lidocaine, or amiodarone can be utilized. If the arrhythmia persists, synchronous R-wave cardioversion after the administration of conscious sedation is appropriate. Selected patients with focal outflow tract tachycardias who demonstrate triggered or automatic VT may respond to IV beta blocker administration. Idiopathic LV septal VT appears to respond uniquely to IV verapamil administration.
VT in patients with structural heart disease is now almost always treated with the implantation of an ICD to manage anticipated VT recurrence. The ICD can provide rapid pacing and shock therapy to treat most VTs effectively (Fig. 16-11).
Ventricular tachycardia (VT) (*) during atrial fibrillation stopped by pacing (#) from an implantable cardioverter defibrillator (ICD) from recording stored by ICD. The atrial electrogram shows characteristic fibrillatory waves through the tracing. The ventricular electrogram shows an irregularly irregular response consistent with atrial fibrillation at the beginning of the tracing. The ventricular electrogram suddenly changes in morphology (*) and becomes regular, consistent with the diagnosis of VT. Pacing transiently accelerates the rate and interrupts the rapid VT. The patient was unaware of the life-threatening event.
Prevention of VT remains important, and >50% of patients with a history of VT and an ICD may need to be treated with adjunctive antiarrhythmic drug therapy to prevent VT recurrences or to manage atrial arrhythmias. Because of the presence of an ICD, there is more flexibility with respect to the selection of anti-arrhythmic drug therapy. The use of sotalol or amiodarone represents first-line therapy for patients with a history of structural heart disease and life-threatening monomorphic or polymorphic VT not due to long QT syndrome. Importantly, sotalol has been associated with a decrease in the defibrillation threshold, which reflects the amount of energy necessary to terminate VF. Amiodarone may be better tolerated in patients with a more marginal hemodynamic status and systolic blood pressure. The risk of end-organ toxicity from amiodarone must be weighed against the ease of use and general efficacy. Antiarrhythmic drug therapy with agents such as quinidine, procainamide, and propafenone, which might not normally be used in patients with structural heart disease because of the risk of proarrhythmia, may be considered in patients with an ICD and recurrent VT.
Catheter ablative therapy for VT in patients without structural heart disease results in cure rates >90%. In patients with structural heart disease, catheter ablation that includes a strategy for eliminating unmappable/rapid VT and one that incorporates endocardial as well as epicardial mapping and ablation should be employed. In most patients, catheter ablation can reduce or eliminate the requirement for toxic drug therapy and should be considered in any patient with recurrent VT. The utilization of ablative therapy to reduce the incidence of ICD shocks for VT in patients who receive the ICD as part of primary prevention for VT is being actively investigated.
MANAGEMENT OF VT STORM Repeated VT episodes requiring external cardioversion/defibrillation or repeated appropriate ICD shock therapy are referred to as VT storm. Although a definition of more than two episodes in 24 h is used, most patients with VT storm will experience many more episodes. In the extreme form of VT storm, the tachycardia becomes incessant and the baseline rhythm cannot be restored for any extended period. In patients with recurrent polymorphic VT in the absence of the long QT interval, one should have a high suspicion of active ischemic disease or fulminant myocarditis. Intravenous lidocaine or amiodarone administration should be coupled with prompt assessment of the status of the coronary anatomy. Endomyocardial biopsy, if indicated by clinical circumstances, may be used to confirm the diagnosis of myocarditis, although the diagnostic yield is low. In patients who demonstrate QT prolongation and recurrent pause-dependent polymorphic VT (TDP), removal of an offending QT-prolonging drug, correction of potassium or magnesium deficiencies, and emergency pacing to prevent pauses should be considered. Intravenous beta blockade therapy should be considered for polymorphic VT storm. A targeted treatment strategy should be employed if the diagnosis of the polymorphic VT syndrome can be established. For example, quinidine or isoproterenol can be used in the treatment of Brugada syndrome. Intraaortic balloon counterpulsation or acute coronary angioplasty may be needed to stop recurrent polymorphic VT precipitated by acute ischemia. In selected patients with a repeating VPC trigger for their polymorphic VT, the VPC can be targeted for ablation to prevent recurrent VT.
In patients with recurrent monomorphic VT, acute IV administration of lidocaine, procainamide, or amiodarone can prevent recurrences. The use of such therapy is empirical, and a clinical response is not certain. Procainamide and amiodarone are more likely to slow the tachycardia and make it hemodynamically tolerated. Unfortunately, antiarrhythmic drugs, especially those that slow conduction (e.g., amiodarone, procainamide), can also facilitate recurrent VT or even result in incessant VT. VT catheter ablation can eliminate frequent recurrent or incessant VT and frequent ICD shocks. Such therapy should be deployed earlier in the course of arrhythmia events to prevent adverse consequences of recurrent VT episodes and adverse effects from antiarrhythmic drugs.
UNIQUE VT SYNDROMES
Although most ventricular arrhythmias occur in the setting of CAD with prior MI, a significant number of patients develop VT in other settings. A brief discussion of each unique VT syndrome is warranted. Information that illustrates a unique pathogenesis and enhances the ability to make the correct diagnosis and institute appropriate therapy will be highlighted.
Idiopathic outflow tract VT
VT in the absence of structural heart disease is referred to as idiopathic VT. There are two major varieties of these VTs. Outflow tachycardias originate in the RV and LV outflow tract regions. Approximately 80% of outflow tract VTs originate in the RV and ~20% in the LV outflow tract regions. Outflow tract VTs appear to originate from anatomic sites that form an arc that begins just above the tricuspid valve and extends along the roof of the outflow tract region to include the free wall and septal aspect of the right ventricle just beneath the pulmonic valve, the aortic valve region, and then the anterior/superior margin of the mitral valve annulus. These arrhythmias appear more commonly in women. Importantly, these ventricular arrhythmias are rarely associated with SCD unless manifest by very short coupled premature complexes that trigger VF. Patients manifest symptoms of palpitations with exercise, stress, and caffeine ingestion. In women, the arrhythmia is more commonly associated with hormonal triggers and can frequently be timed to the premenstrual period, gestation, and menopause. Uncommonly, the VPCs and VTs can be of sufficient frequency and duration to cause a tachycardia-induced cardiomyopathy.
The pathogenesis of outflow tract VT remains unknown, and there is no definite anatomic abnormality identified with these VTs. Vagal maneuvers, adenosine, and beta blockers tend to terminate the VTs, whereas catecholamine infusion, exercise, and stress tend to potentiate the outflow tract VTs. Based on these observations, the mechanism of the arrhythmia is most likely calcium-dependent triggered activity. Preliminary data suggest that at least in some patients, a somatic mutation of the inhibitory G protein (Gαi2) may serve as the genetic basis for the VT. In contrast to VT in patients with CAD, outflow tract VTs are uncommonly initiated with programmed stimulation but are able to be initiated by rapid-burst atrial or ventricular pacing, particularly when coupled with the infusion of isoproterenol.
Outflow tract VT typically produces large monophasic R waves in the inferior frontal plane leads II, III, and aVF, and typically occurs as nonsustained bursts of VT and/or frequent premature beats. Cycle length oscillations during the tachycardia are common. Since most VT originates in the RV outflow tract, the VT typically has a left bundle branch block (LBBB) pattern in lead V1 (negative QRS vector) (Fig. 16-12). Outflow tract VTs, originating in the left ventricle, particularly those associated with an origin from the mitral valve annulus, have a right bundle branch block (RBBB) pattern in lead V1 (positive QRS vector).
Common idiopathic ventricular tachycardia (VT) ECG patterns. Right ventricular outflow tract (RVOT) VT with typical left bundle QRS pattern in V1 and inferiorly directed frontal plane axis, and left ventricular septal VT from the inferior septum with a narrow QRS RBBB pattern in V1 and superior and leftward front plane QRS axis.
TREATMENT Idiopathic Outflow Tract Ventricular Tachycardia
Acute medical therapy for idiopathic outflow tract VT is rarely required because the VT is hemodynamically tolerated and is typically nonsustained. Intravenous beta blockers frequently terminate the tachycardia. Chronic therapy with beta or calcium channel blockers frequently prevents recurrent episodes of the tachycardia. The arrhythmia also appears to respond to treatment with class IA or IC agents or with sotalol. Catheter ablative therapy has been utilized successfully to eliminate the tachycardia with success rates >90%. Because of the absence of structural heart disease and the focal nature of these arrhythmias, the 12-lead ECG pattern during VT can help localize the site of origin of the arrhythmia and help facilitate catheter ablation. Efficacy of therapy is assessed with treadmill testing and/or ECG monitoring, and electrophysiologic study is performed only when the diagnosis is in question or to perform catheter ablation.
Idiopathic LV septal/fascicular VT
The second most common idiopathic VT is linked anatomically to the Purkinje system in the left ventricle. The arrhythmia mechanism appears to be macroreentry involving calcium-dependent slow response fibers that are part of the Purkinje network, although automatic tachycardias have also been observed. A 12-lead ECG morphology of the VT shows a narrow RBBB pattern and a superior leftward axis or an inferior rightward axis, depending on whether the VT originates from the posterior or anterior fascicles (Fig. 16-12). Idiopathic LV septal VT is unique in its suppression with verapamil. Beta blockers also have been used with some success as primary or effective adjunctive therapy. Catheter ablation is very effective therapy for VT resistant to drug therapy or in patients reluctant to take daily therapy, with anticipated successful elimination of VT in >90% of patients.
VT associated with LV dilated cardiomyopathy
Monomorphic and polymorphic VTs may occur in patients with nonischemic dilated cardiomyopathy (Chap. 21). Although the myopathic process may be diffuse, there appears to be a predilection for the development of fibrosis around the mitral and aortic valvular regions. Most uniform sustained VT can be mapped to these regions of fibrosis. Drug therapy is usually ineffective in preventing VT, and empirical trials of sotalol or amiodarone are usually initiated only for recurrent VT episodes after ICD implantation. VT associated with nonischemic dilated cardiomyopathy appears to be less amenable to catheter ablative therapy from the endocardium; frequently, the VT originates from epicardial areas of fibrosis and catheter access to the epicardium can be gained via a percutaneous pericardial puncture to improve the outcome of ablation techniques. In patients with a history of depressed myocardial dysfunction due to a nonischemic cardiomyopathy with an LV ejection fraction <30%, data now support the implantation of a prophylactic ICD device to reduce the risk of SCD from the first VT/VF episode effectively.
Bundle branch reentrant VT
Monomorphic VT in patients with idiopathic nonischemic cardiomyopathy or valvular cardiomyopathy is frequently due to a large macroreentrant circuit involving the various elements of the His-Purkinje network. The arrhythmia usually occurs in the presence of underlying disease of the His-Purkinje system. In sinus rhythm, an incomplete left bundle block is typically present and the time it takes to traverse the His-Purkinje network is delayed; this slow conduction serves as the substrate for reentry. Characteristically, the VT circuit rotates in an antegrade direction down the right bundle and retrograde up the left posterior or anterior fascicles and left bundle branch. As a result, bundle branch reentrant VT typically has a QRS morphology with a left bundle branch block type of pattern and a leftward superior axis (Fig. 16-13). The circuit for bundle branch (LBB) reentrant VT can occasionally rotate in the opposite direction, antegrade through the left bundle and retrograde through the right bundle, in which case an RBBB pattern during VT will be manifest.
Bundle branch reentrant ventricular tachycardia (VT) showing typical QRS morphologies when VT is initiated with stimulation from the right ventricle (left bundle branch block [LBBB] VT pattern) or left ventricle (right bundle branch block [RBBB] VT pattern) and schema for circuit involving the His-Purkinje network.
It is important to recognize bundle branch reentrant VT because it is readily amenable to ablative therapy that targets a component of the His-Purkinje system, typically the right bundle, to block the VT circuit. Less commonly, bundle branch reentry may occur in the absence of structural heart disease or in the setting of CAD. The use of adjunctive ICD therapy is dictated by the ability to eliminate the VT successfully and the severity of the LV dysfunction.
VT associated with hypertrophic cardiomyopathy
(See also Chap. 21) VT and VF have also been associated with hypertrophic cardiomyopathy. In patients with hypertrophic cardiomyopathy and a history of sustained VT/VF, unexplained syncope, a strong family history of SCD, LV septal thickness >30 mm, or nonsustained spontaneous VT, the risk of SCD is high and ICD implantation is usually indicated. Amiodarone, sotalol, and beta blockers have been used to control recurrent VT. Experience with ablative therapy is limited because of the infrequency with which the VT is tolerated hemodynamically. Ablation procedures that target the substrate for VT/VF and ablate areas of low voltage consistent with fibrosis, which frequently are located in apical aneurysms, appear to have promise in this setting. WPW syndrome has been observed in patients with hypertrophic cardiomyopathy associated with PRKAG2 mutations.
VT associated with other infiltrative cardiomyopathies and neuromuscular disorders
An increased arrhythmia risk has been identified when cardiac involvement occurs in a variety of infiltrative diseases and neuromuscular disorders (Table 16-7). Many patients manifest AV conduction disturbances and may require permanent pacemaker insertion. The decision to implant an ICD device should follow current established guidelines for patients with nonischemic cardiomyopathy, which include an LV ejection fraction <35% or a history of unexplained syncope with significant LV dysfunction. A recent report identified AF, PR interval >240 ms, QRS 120 ms, or heart block and type 1 myotonic dystrophy as predicting a risk of sudden death. Additional study will be required to determine if patients with lesser degrees of LV dysfunction or other more diffuse myopathic disease processes also have identifiable risk and warrant primary ICD implantation. A potential proarrhythmic risk of antiarrhythmic drug therapy should be acknowledged, and drug therapy should be reserved for symptomatic arrhythmias and limited to amiodarone or sotalol if an ICD is not present.
INFILTRATIVE/INFLAMMATORY AND NEUROMUSCULAR DISORDERS ASSOCIATED WITH AN INCREASED VENTRICULAR ARRHYTHMIA RISK
Arrhythmogenic RV cardiomyopathy/dysplasia (ARVCM/D)
(See also Chap. 21) ARVCM/D due to a genetically determined dysplastic process or after a suspected viral myocarditis is also associated with VT/VF. The sporadic nonfamilial/nondysplastic form of RV cardiomyopathy appears to be more common; however, this may vary with ethnicity. In patients predisposed to VT, there appears to be a predominance of perivalvular fibrosis involving mostly the free wall of the right ventricle in proximity to the tricuspid and pulmonic valves. The surface ECG leads that reflect RV activation, including V1–V3, may show terminal notching of the QRS complex and inverted T waves in sinus rhythm. When the terminal notching is distinct and appears separated from the QRS complex, it is referred to as an epsilon wave(Fig. 16-14). Epsilon waves are consistent with markedly delayed ventricular activation in the region of the RV free wall near the base of the tricuspid and pulmonic valves in areas of extensive fibrosis.
Leads V1 to V3 in sinus rhythm from a normal subject (A), from a patient with arrhythmogenic right ventricular cardiomyopathy showing epsilon waves (arrow) and T-wave inversion (B), and from a patient with Brugada syndrome with ST-segment elevation in V1 and V2 (C).
In patients with ARVCM/D, echocardiography demonstrates RV enlargement with RV wall motion abnormalities and RV apical aneurysm formation. MRI may show fatty replacement of the ventricle, thinning of the RV free wall with increased fibrosis, and associated wall motion abnormalities. Because of the presence of extensive amounts of fat normally covering the epicardium in the region of the RV, caution must be used to avoid overinterpreting the MRI in trying to determine the appropriate diagnosis. Patients tend to have multiple VT morphologies. The VT will typically have an LBBB type QRS pattern in V1 and tend to have poor R-wave progression in V1 through V6, consistent with an RV free-wall origin. Areas of low electrogram voltage that are identified during RV catheter endocardial sinus rhythm voltage mapping may be helpful in confirming the diagnosis. Importantly, endocardial biopsy may not identify the presence of fatty replacement or fibrosis unless directed to the basal RV free wall. The familial forms of this syndrome have been linked to a number of desmosomal protein mutations. A distinct genetic form of this syndrome, Naxos disease, consists of arrhythmogenic RV dysplasia coupled with palmar-plantar keratosis and woolly hair and is associated with a high risk of SCD in adolescents and young adults.
TREATMENT Arrhythmogenic Right Ventricular Cardiomyopathy/Dysplasia
The threshold for ICD implantation in patients with an established diagnosis of ARVCM/D is low. An ICD typically is implanted in patients deemed to have a persistent VT risk, those who have had spontaneous or inducible rapid VTs, and those who show concomitant LV cardiomyopathy. Treatment options for recurrent VT in patients with ARVCM/D include the use of the antiarrhythmic agent sotalol. Beta blockers serve as useful adjunctive therapy when coupled with other antiarrhythmic agents. Catheter ablative therapy directed at mappable sustained ventricular arrhythmias is also highly successful in controlling recurrent VT. In selected patients with multiple VT morphologies and unstable VT, linear ablation lesions directed at endocardial scars and, if required, targeting late potentials in epicardial scars, defined by catheter-based bipolar voltage mapping, provide significant amelioration of the recurrent VT episodes.
VT after operative tetralogy of Fallot repair
VT may also occur after surgical repair of tetralogy of Fallot. Patients typically develop VT many years after the surgery. VT tends to occur in patients with evidence of RV systolic dysfunction. The VT mechanism and location are typically a macroreentrant circuit around the right ventriculotomy scar to the valve annuli. Catheter ablation creating linear lesions that extend from either the pulmonic or the tricuspid annulus to the ventriculotomy scar is typically effective in preventing arrhythmia recurrences. An ICD is usually implanted in patients who manifest rapid VT, have persistent inducible VT after ablation, or have concomitant LV dysfunction.
Fascicular tachycardia caused by digoxin toxicity
Digoxin toxicity can produce increased ventricular ectopy and, when coupled with bradyarrhythmias caused by digoxin toxicity, may predispose to sustained polymorphic ventricular arrhythmias and VF. The signature VT associated with digoxin toxicity is bidirectional VT (Fig. 16-15). This unique VT is due to triggered activity associated with calcium overload resulting from the inhibition of Na+, K+-ATPase by digoxin. Bidirectional VT originates from the left anterior and posterior fascicles, creating a relatively narrow QRS right bundle branch (RBB) configuration with a beat-to-beat alternating right and left frontal plane QRS axis. This VT seldom is observed in the absence of digoxin toxicity. Treatment for bidirectional VT or other hemodynamically significant arrhythmias due to digoxin excess includes correction of electrolyte disorders and IV infusion of digoxin-specific Fab fragments. The antibody fragments will, over the course of 1 h, bind digoxin and eliminate toxic effects. In the setting of normal renal function, the bound complex is secreted.
Digoxin toxic bidirectional fascicular tachycardia.
GENETICALLY DETERMINED ABNORMALITIES THAT PREDISPOSE TO POLYMORPHIC VENTRICULAR ARRHYTHMIAS
Ion channel defects that affect cardiac depolarization and repolarization may predispose to life-threatening polymorphic VT and SCD. These defects frequently produce unique ECG characteristics during sinus rhythm that facilitate the diagnosis.
Long QT syndrome
The congenital form of LQTS consists of defects in cardiac ion channels that are responsible for cardiac repolarization. Defects that enhance sodium or calcium inward currents or inhibit outward potassium currents during the plateau phase of the action potential lengthen action potential duration and, hence, the QT interval. Of the eight genetic mutations identified to date, five affect the α or β subunits of the three different potassium channels involved with repolarization (Table 16-8). Since many patients with QT prolongation do not have one of the defined mutations, it is anticipated that other genetic abnormalities affecting repolarization channel function will be identified.
INHERITED ARRHYTHMIA DISORDERS: “CHANNELOPATHIES” WITH HIGH RISK OF VENTRICULAR ARRHYTHMIAS
The triggers for the ventricular arrhythmias are thought to be due to early afterdepolarizations potentiated by intracellular calcium accumulation from a prolonged action potential plateau. Heterogeneity of myocardial repolarization indexed by a longer QT interval predisposes to polymorphic ventricular arrhythmias in response to the triggers (Fig. 16-9).
In most patients with LQTS, the QT interval corrected for heart rate using Bazett’s formula is >460 ms in men and >480 ms in women with LQTS. Marked lengthening of the QT interval to >500 ms is clearly associated with a greater arrhythmia risk in patients with LQTS. Many affected individuals may have QT intervals that intermittently measure within a normal range or fail to shorten appropriately with exercise. Some individuals manifest the syndrome only when exposed to a drug, such as sotalol, that alters channel function.
The genotype associated with LQTS appears to influence prognosis, and identification of the genotype appears to help optimize clinical management. The first three genotypic designations of the mutations identified, LQT1, LQT2, and LQT3, appear to account for >99% of patients with clinically relevant LQTSs. Surface ECG characteristics may be helpful in distinguishing the three most common genotypes, with genetic testing being definitive.
LQT1 represents the most common genotypic abnormality. Patients with LQT1 fail to shorten or actually prolong their QT interval with exercise. The T wave in patients with LQT1 tends to be broad and constitutes the majority of the prolonged QT interval. The most common trigger for potentiating cardiac arrhythmias in patients with LQT1 is exercise, followed by emotional stress.
More than 80% of male patients have their first cardiac event by age 20, so competitive exercise should be restricted and swimming avoided for these patients. Patients tend to respond to beta blocker therapy. Patients with two LQT1 alleles have Jervell and Lange-Nielsen syndrome, with more dramatic QT prolongation and deafness and a worse arrhythmia prognosis.
LQT2 is the second most common genotypic abnormality. The T wave tends to be notched and bifid. In LQT2 patients, the most common precipitant is emotional stress, followed by sleep or auditory stimulation. Despite the occurrence during sleep, patients typically respond to beta blocker therapy.
LQT3 is due to a mutation in the gene that encodes the cardiac sodium channel on chromosome 3. Prolongation of the action potential duration occurs because of failure to inactivate this channel. LQT3 patients have either late-onset peaked biphasic T waves or asymmetric peaked T waves. The arrhythmia events tend to be more life threatening, and thus the prognosis for LQT3 is the poorest of all the LQTs. Male patients appear to have the worst prognosis among patients with LQT3. Most events in LQT3 patients occur during sleep, suggesting that they are at higher risk during periods of slow heart rates. Beta blockers are not recommended, and exercise is not restricted in LQT3.
TREATMENT Long QT Syndrome
The institution of ICD therapy should be strongly considered in any patient with LQTS who has demonstrated any life-threatening arrhythmia. Patients with syncope with a confirmed diagnosis based on unequivocal ECG criteria or positive genetic testing should also be given the same strong consideration. Primary prevention with prophylactic ICD implantation should be considered in male patients with LQT3 and in all patients with marked QT prolongation (>500 ms), particularly when coupled with an immediate family history of SCD. Future epidemiologic investigation may provide firmer guidelines to sort patients further on the basis of risks such as age, gender, arrhythmia history, and genetic characteristics. In all patients with documented or suspected LQTS, drugs that prolong the QT interval must be avoided. For an updated list of drugs, see www.qtdrugs.org.
Patients with a genetic predisposition related to what appear to be sporadic mutations and/or single nucleotide polymorphisms can develop marked QT prolongation in response to drugs that alter repolarization currents. The QT prolongation and associated polymorphic ventricular tachycardia (TDP) are seen more frequently in women and may be a manifestation of subclinical LQTS. Drug-induced long QT and TDP frequently are potentiated by the development of hypokalemia and bradycardia. The offending drugs typically block the potassium IKr channel (Table 16-5). Since most drug effects are dose dependent, important drug-drug interactions that alter metabolism and/or alterations in elimination kinetics because of hepatic or renal dysfunction frequently contribute to the arrhythmias.
TREATMENT Acquired Long QT Syndrome
Acute therapy for acquired LQTS is directed at eliminating the offending drug therapy, reversing metabolic abnormalities by the infusion of magnesium and/or potassium, and preventing pause-dependent arrhythmias by temporary pacing or the cautious infusion of isoproterenol. Class IB antiarrhythmic agents (e.g., lidocaine) that do not cause QT prolongation may also be used, though they are frequently ineffective. Supportive therapy to allay anxiety and prevent pain with required DC shock therapy for sustained arrhythmias and efforts to facilitate drug elimination are important.
Short QT syndrome
A gain in function of repolarization currents can result in a shortening of atrial and ventricular refractoriness and marked QT shortening on the surface ECG (Table 16-8). The T wave tends to be tall and peaked. A QT interval <320 ms is required to establish the diagnosis of this uncommon syndrome. Mutations in the HERG, KvLQT1, and KCNJ2 genes have been identified. Patients with the syndrome are predisposed to both AF and VF. ICD implantation is recommended. Double counting of QRS and T waves may lead to inappropriate ICD shocks. Drug therapy with quinidine has been used to lengthen the QT interval and reduce the amplitude of the T wave. This therapy is being evaluated to determine long-term efficacy in preventing arrhythmias in this syndrome.
The major clinical features of Brugada syndrome include manifest, transient, or concealed ST segment elevation in V1 to V3 that typically can be provoked with the sodium channel-blocking drugs ajmaline, flecainide, and procainamide and a risk of polymorphic ventricular arrhythmias. It appears that a diminished inward sodium current in the region of the RV out-flow tract epicardium is responsible for Brugada syndrome (Table 16-8). A loss of the action potential dome in the RV epicardium due to unopposed ITo potassium outward current results in dramatic shortening of the action potential. The large potential difference between the normal endocardium and rapidly depolarized RV outflow epicardium gives rise to ST-segment elevation in V1–V3 in sinus rhythm and predisposes to local ventricular reentry (Fig. 16-14). The majority of genetic abnormalities responsible for the syndrome have not been described; however, in ~20% of patients, mutations of SCN5A genes have been identified. Although identified in both genders and all races with an autosomal dominant inheritance pattern, the arrhythmia syndrome is most common in young male patients (~75%) and is thought to be responsible for the sudden and unexpected nocturnal death syndrome (SUDS) described in Southeast Asian men. The ventricular arrhythmia characteristically occurs with rest or during sleep. Fever and other sodium channel-blocking drugs have also precipitated ventricular arrhythmias.
The presence of spontaneous coved-type ST elevation in the right precordial leads and a history of syncope or aborted sudden cardiac death are predictors of an adverse outcome. Because of the overlap in SCN5A mutations, the association of Brugada syndrome with phenotypic LQT3 and conduction disturbances has been noted.
TREATMENT Brugada Syndrome
A drug challenge with procainamide may be important to establish the diagnosis and the probable cause of unexplained syncope when the surface ECG is equivocal (saddleback ST elevation pattern). Ajmaline and intravenous flecainide, which are not available in the United States, may have higher sensitivities for identifying the syndrome. Successful acute management of recurrent VT has been reported with isoproterenol or quinidine administration, although experience has been limited. Patients who do not benefit from beta blockers and chronic suppression with quinidine, which may lengthen epicardial action potential duration by blocking ITO current, may be considered for ICD implantation. ICD treatment to manage recurrences and prevent sudden death is recommended for all patients who have had documented arrhythmia episodes and patients with syncope and positive spontaneous or provoked coved-type ECG ST-segment changes in V1–V3. Family members should undergo ECG screening for the presence of the abnormality. The role of programmed cardiac stimulation and the use of ICD therapy in asymptomatic patients with the Brugada-type ECG pattern remain somewhat controversial, as is provocative drug infusion and programmed stimulation in patients with borderline abnormalities and no arrhythmia symptoms. Longer-term follow-up in a larger group of these relatively low-risk patients may be required before definitive recommendations can be provided. Counseling on controversies that exist, the potential risk of fever, and inadvertent administration of tricyclic antidepressants should be considered. Genetic testing may be helpful in confirming the presence of the genetic abnormality in family members of patients who manifest the arrhythmia syndrome.
Catecholaminergic polymorphic VT
A mutation of the myocardial ryanodine release channel, which effectively creates a “leak” in calcium from the sarcoplasmic reticulum, has been identified in patients with catecholaminergic VT (Table 16-8). The accumulation of intracellular calcium potentiates delayed afterdepolarizations and triggered activity. Patients can manifest bidirectional VT, nonsustained polymorphic VT, or recurrent VF. Both an autosomal dominant familial form and sporadic forms of the disease have been described. More recently, an autosomal recessive variant associated with a mutation in the sarcoplasmic reticulum calcium-buffering protein calsequestrin has also been identified. The arrhythmias are precipitated by exercise and emotional stress (Fig. 16-16). Exercise restriction is warranted. Treatment with beta blockers and ICD implantation has been recommended. Prevention of inappropriate or easily triggered ICD shocks by proper ICD programming is essential to prevent VT storm from endogenous catecholamine release.
Catecholaminergic polymorphic ventricular tachycardia noted during an exercise stress test.
SPECIAL CONSIDERATION: APPROACH TO TACHYARRHYTHMIAS IN ATHLETES
The first manifestation of a tachyarrhythmia, whether benign or malignant, may occur during athletic activity. Fortunately, successful cardiopulmonary resuscitation of life-threatening ventricular arrhythmias has increased with the use of automatic external defibrillators at major sporting events and schools. Rarely, VF may be precipitated by blunt precordial blows without structural injury to the heart or chest wall (commotio cordis).
The approach to the athlete should begin with an assessment of the severity and significance of symptoms. Syncope with exertion should be assumed to be caused by a potentially lethal arrhythmia. A thorough cardiac evaluation and restricted participation in competitive sports are in order until a less life-threatening diagnosis can be established. ECG recording at the time of the symptomatic events usually can establish the diagnosis, although it may be difficult to obtain.
In patients with syncope and no ECG-documented arrhythmia, a systematic attempt to define a cardiac structural or primarily electrical abnormality with a routine ECG and a transthoracic echocardiogram is in order. Common structural abnormalities associated with fatal or life-threatening ventricular arrhythmias include hypertrophic cardiomyopathy, arrhythmogenic cardiomyopathy, and acute myocarditis. Coronary anomalies should be suspected if the arrhythmia symptoms are preceded in onset by chest pain. The 12-lead ECG should be screened for the presence of preexcitation, QT prolongation, a Brugada-type ECG pattern or epsilon waves, and T-wave inversions consistent with a nonischemic RV or LV cardiomyopathy or myocarditis. Additional ECG monitoring may be required. A stress test may be appropriate to provoke arrhythmias, especially if there are recurrent symptoms. It is critical to achieve the level of exercise that precipitated the arrhythmia, which for some athletes may be a challenge for the exercise lab.
Management of an athlete with cardiac arrhythmias may be a challenge, with a tendency to discourage participation in competitive sports and institute treatment whenever there is a perception of increased risk. Guidelines for restricting athletic activity have been promulgated on the basis of expert consensus and evidence-based data and can facilitate management once a diagnosis has been established (Table 16-9). Treatment should be based on standards established for each arrhythmia syndrome. Curative catheter ablative therapy should be applied if indicated. ICD therapy, if required, is incompatible with contact sports because of the potential for blunt trauma and consequent damage to the device. Although ICDs are effective, their psychosocial impact, the potential for inappropriate shocks for sinus tachycardia, and lead-related complications must be recognized.
RECOMMENDATIONS FOR COMPETITIVE ATHLETES WITH SELECTED CARDIOVASCULAR ABNORMALITIES