The frequency and clinical significance of arrhythmias are different in children compared with adults. Although arrhythmias are relatively infrequent in infants and children, the common practice of monitoring cardiac rhythm in children requires primary care physicians, emergency department physicians, and intensive care physicians to be able to recognize and manage basic arrhythmias.
The normal heart rate varies with age: The younger the child, the faster the heart rate. Therefore, the definitions of bradycardia (<60 beats/min) and tachycardia (>100 beats/min) used for adults do not apply to infants and children. Tachycardia is defined as a heart rate beyond the upper limit of normal for the patient’s age, and bradycardia is defined as a heart rate slower than the lower limit of normal. Normal resting heart rates by age are presented in Table 24-1.
This chapter discusses basic arrhythmias according to the origin of their impulse. Each arrhythmia is described along with its causes, significance, and treatment.
Rhythms Originating in the Sinus Node
All rhythms that originate in the sinoatrial (SA) node (sinus rhythm) have two important characteristics (Fig. 24-1). Both are required for a rhythm to be called sinus rhythm.
1. P waves precede each QRS complex with a regular PR interval. (The PR interval may be prolonged, as in first-degree atrioventricular [AV] block); in this case, the rhythm is sinus with first-degree AV block).
2. The P axis falls between 0 and +90 degrees, an often neglected criterion. This produces upright P waves in lead II and inverted P waves in aVR. (See Chapter 3 for a detailed discussion of sinus rhythm.)
Regular Sinus Rhythm
Description. The rhythm is regular, and the rate is normal for age. The two characteristics of sinus rhythm described previously are present (see Fig. 24-1).
Significance. This rhythm is normal at any age.
Management. No treatment is required.
Description. Characteristics of sinus rhythm are present (see previous description). The rate is faster than the upper limit of normal for age (see Table 24-1). A rate above 140 beats/min in children and above 170 beats/min in infants may be significant. The heart rate usually is below 200 beats/min in sinus tachycardia (see Fig. 24-1).
Causes. Anxiety, fever, hypovolemia or circulatory shock, anemia, congestive heart failure (CHF), administration of catecholamines, thyrotoxicosis, and myocardial disease are possible causes.
Significance. Increased cardiac work is well tolerated by healthy myocardium.
Management. The underlying cause is treated.
Description. The characteristics of sinus rhythm are present (see previous description), but the heart rate is slower than the lower limit of normal for the age (see Table 24-1). A rate slower than 80 beats/min in newborn infants and slower than 60 beats/min in older children may be significant (see Fig. 24-1).
FIGURE 24-1 Normal and abnormal rhythms originating in the sinoatrial node. (From Park MK, Guntheroth WG: How to Read Pediatric ECGs, 4th ed. Philadelphia, Mosby, 1992.)
NORMAL RANGES OF RESTING HEART RATE
From Davignon A, Rautaharju P, Boisselle E, Soumis F, Megelas M, Choquette A. Normal ECG standards for infants and children. Pediatr Cardiol 1:123-131, 1979/1980.
Causes. Sinus bradycardia may occur in normal individuals and trained athletes. It may occur with vagal stimulation, increased intracranial pressure, hypothyroidism, hypothermia, hypoxia, hyperkalemia, and administration of drugs such as β-adrenergic blockers.
Significance. In some patients, marked bradycardia may not maintain normal cardiac output.
Management. The underlying cause is treated.
Description. There is a phasic variation in the heart rate caused by respiratory influences on the autonomic nervous system, increasing during inspiration and decreasing during expiration. The arrhythmia occurs, with maintenance of characteristics of sinus rhythm (see Fig. 24-1).
Causes. This is a normal phenomenon and is caused by phasic variation in the firing rate of cardiac autonomic nerves with the phases of respiration.
Significance. Sinus arrhythmia has no significance because it is a normal finding in children and a sign of good cardiac reserve.
Management. No treatment is indicated.
Description. In sinus pause, the sinus node pacemaker momentarily ceases activity, resulting in absence of the P wave and QRS complex for a relatively short time (see Fig. 24-1). Sinus arrest is of longer duration and usually results in an escape beat (see later discussion) by other pacemakers, such as the AV junctional or nodal tissue (junctional or nodal escape beat).
Causes. Increased vagal tone, hypoxia, sick sinus syndrome, and digitalis toxicity are possible causes. Well-conditioned athletes may have bradycardia and sinus pause of greater than 2 seconds because of prominent vagal influence.
Significance. Sinus pause of less than 2 seconds are normal in young children and adolescents. It usually has no hemodynamic significance but may reduce cardiac output in patients with frequent and long period of sinus pause.
Management. Treatment is rarely indicated except in sinus node dysfunction (or sick sinus syndrome; see later discussion).
Sinoatrial Exit Block
Description. A P wave is absent from the normally expected P wave, resulting in a long RR interval. The duration of the pause is a multiple of the basic PP interval. An impulse formed within the sinus node fails to propagate normally to the atrium.
Causes. Excessive vagal stimulation, myocarditis or fibrosis involving the atrium, and drugs such as quinidine, procainamide or digitalis.
Significance. It is usually transient and has no hemodynamic significance. Rarely, the patient may have syncope.
Management. The underlying cause is treated.
Sinus Node Dysfunction (Sick Sinus Syndrome)
Description. In sinus node dysfunction, the sinus node fails to function as the dominant pacemaker of the heart or performs abnormally slowly, resulting in a variety of arrhythmias. These arrhythmias may include profound sinus bradycardia, sinus pause or arrest, sinus node exit block, slow junctional escape beats, and ectopic atrial or nodal rhythm. Clear documentation is not always possible. Long-term recording with Holter is better in documenting overall heart rate variation and the prevalence of abnormally slow and fast rhythm.
Bradytachyarrhythmia occurs when bradycardia and tachycardia alternate. Whereas bradycardia may originate in the sinus node, atria, AV junction, or ventricle, tachycardia is usually caused by atrial flutter or fibrillation and less commonly by reentrant supraventricular tachycardia (SVT). When these arrhythmias are accompanied by symptoms such as dizziness or syncope, sinus node dysfunction is referred to as sick sinus syndrome.
1. Injury to the sinus node caused by extensive cardiac surgery, particularly involving the atria (e.g., the Senning operation, Fontan procedure, or surgery for partial or total anomalous pulmonary venous return or endocardial cushion defect) are possible causes.
2. Some cases of sick sinus syndrome are idiopathic, involving an otherwise normal heart without structural defect.
3. Rarely, myocarditis, pericarditis, or rheumatic fever is a cause.
4. Congenital heart defects (CHDs) (e.g., sinus venosus atrial septal defect [ASD], Ebstein’s anomaly, left atrial isomerism [polysplenia syndrome])
5. Secondary to antiarrhythmic drugs (e.g., digitalis, propranolol, verapamil, quinidine
Bradytachyarrhythmia is the most worrisome. Profound bradycardia after a period of tachycardia (overdrive suppression) can cause presyncope, syncope, and even death.
1. For severe bradycardia:
a. Acute symptomatic bradycardia is treated with intravenous (IV) atropine (0.04 mg/kg IV every 2–4 hours) or isoproterenol (0.05–0.5 μg/kg IV) or transcutaneous pacing. Temporary transvenous or transesophageal pacing can be used until a permanent pacing system can be implanted.
b. Chronic medical treatment using drugs has not been uniformly successful and is not accepted as standard treatment of sinus node dysfunction.
c. Symptomatic bradycardia is treated with permanent pacing. Asymptomatic patients with heart rate under 40 beats/min or pauses longer than 3 seconds are less clear indications for permanent pacing.
d. Permanent implantation is the treatment of choice in symptomatic patients, especially those with syncope. Most patients receive atrial demand pacing. Patients with any degree of AV nodal dysfunction receive dual chamber pacemakers. Ventricular demand pacemakers may be used.
2. For symptomatic tachycardia:
a. Antiarrhythmic drugs, such as propranolol or quinidine, may be given to suppress tachycardia, but they are often unsuccessful.
b. Digoxin may help to decrease AV conduction of rapid tachycardia.
c. Catheter ablation of arrhythmia substrates (often requiring concomitant surgical revision of previous surgeries) may be indicated.
e. Patients with tachycardia–bradycardia syndrome may benefit from antitachycardia pacemakers.
Rhythms Originating in the Atrium
Rhythms that originate in the atrium (ectopic atrial rhythm) are characterized by the following (Fig. 24-2):
1. P waves have an unusual contour, which is caused by an abnormal P axis or an abnormal number of P waves per QRS complex.
2. QRS complexes are usually of normal configuration, but occasional bizarre QRS complexes caused by aberrancy may occur (see later discussion).
Premature Atrial Contraction
Description. The QRS complex appears prematurely. The P wave may be upright in lead II when the ectopic focus is high in the atrium. The P wave is inverted when the ectopic focus is low in the atrium (so-called coronary sinus rhythm). The compensatory pause is incomplete; that is, the length of two cycles, including one premature beat, is less than the length of two normal cycles (see Fig. 24-2).
An occasional premature atrial contraction (PAC) is not followed by a QRS complex (i.e., a nonconducted PAC; see Fig. 24-2). A nonconducted PAC is differentiated from a second-degree AV block by the prematurity of the nonconducted P wave (P′ in Fig. 24-2). The P′ wave occurs earlier than the anticipated normal P wave, and the resulting PP′ interval is shorter than the normal PP interval for that individual. In second-degree AV block, the P wave that is not followed by the QRS complex occurs at the anticipated time, maintaining a regular PP interval.
Causes. PAC appears in healthy children, including newborns. It also may appear after cardiac surgery and with digitalis toxicity.
Significance. PAC has no hemodynamic significance.
Management. Usually no treatment is indicated except in cases of digitalis toxicity.
Wandering Atrial Pacemaker
Description. Wandering atrial pacemaker is characterized by gradual changes in the shape of P waves and PR intervals (see Fig. 24-2). The QRS complex is normal.
FIGURE 24-2 Arrhythmias originating in the atrium. (From Park MK, Guntheroth WG: How to Read Pediatric ECGs, 4th ed. Philadelphia, Mosby, 1992.)
Causes. Wandering atrial pacemaker is seen in otherwise healthy children. It is the result of a gradual shift of the site of impulse formation in the atria through several cardiac cycles.
Significance. Wandering atrial pacemaker is a benign arrhythmia and has no clinical significance.
Management. No treatment is indicated.
Ectopic (or Autonomic) Atrial Tachycardia
Description. There is a narrow QRS complex tachycardia (in the absence of aberrancy or preexisting bundle branch block) with visible P waves at an inappropriately rapid rate. The P axis is different from that of sinus rhythm. When the ectopic focus is near the sinus node, the P axis may be the same as in sinus rhythm. The usual heart rate in older children is between 110 and 160 beats/min, but the tachycardia rate varies substantially during the course of a day, reaching 200 beats/min with sympathetic stimuli. This arrhythmia is sometimes difficult to distinguish from the re-entrant AV tachycardia, and thus it is included under SVT. It represented 18% of SVT in one study.
Holter monitoring may demonstrate a characteristic gradual acceleration of the heart rate, the so-called warming up period, rather than abrupt onset and termination seen with reentrant AV tachycardia. The P waves of ectopic atrial tachycardia may not conduct to the ventricle, especially during sleep, when parasympathetic tone is heightened.
Causes. Ectopic atrial tachycardia originates from a single focus in the atrium. This arrhythmia is believed to be secondary to increased automaticity of nonsinus atrial focus.
1. Most patients have structurally normal heart (idiopathic).
2. Myocarditis, chronic cardiomyopathy, AV valve regurgitation, atrial dilatation, atrial tumors, and previous cardiac surgery involving atria (e.g., Senning operation, Fontan procedure) may be the cause.
3. Occasionally, respiratory infections caused by mycoplasma or viruses may trigger the arrhythmia.
Significance. The chronic nature of the arrhythmia at a relatively low heart rate (<150 beats/min) can escape detection, and CHF is common at presentation. There is a high association of this tachycardia with tachycardia-induced cardiomyopathy.
Management. It is refractory to medical therapy and cardioversion. Drugs that are effective in reentrant atrial tachycardia (e.g., adenosine) do not terminate the tachycardia. Cardioversion is ineffective because the ectopic rhythm resumes immediately.
1. Drugs to slow the ventricular rate are not usually successful. There are conflicting reports regarding the efficacy of digoxin and beta-blockers in slowing down the ventricular rate. Class IC (e.g., flecainide) and class III (e.g., amiodarone) antiarrhythmic agents are generally most effective (up to 75%).
2. Radiofrequency ablation may prove to be effective in 95% to 100%. In children, a majority of the foci are found in the left atrium near the pulmonary veins and the atrial appendage in contrast to the right atrium found in adults.
Chaotic (or Multifocal) Atrial Tachycardia
Description. This is an uncommon tachycardia characterized by three or more distinct P-wave morphologies. The PP and RR intervals are irregular with variable PR intervals. The arrhythmia may be misdiagnosed as atrial fibrillation (AF).
Causes. Most patients with the condition are infants; it is very rare after 5 years of age. About 30% to 50% have respiratory illness. Myocarditis and birth asphyxia have been described. Structural heart disease may or may not be present.
Significance. The mechanism of this arrhythmia has been poorly defined. Cardiac enlargement or reduced left ventricular (LV) systolic function may be present at the diagnosis. Sudden death has been reported in up to 17% while on therapy. Spontaneous resolution frequently occurs.
Management. This arrhythmia is refractory to cardiac pacing, cardioversion, and adenosine.
1. When there is no evidence of cardiac dysfunction, observation on a regular basis may be reasonable.
2. Drugs that slow AV conduction (propranolol or digoxin) and those that decrease automaticity (e.g., class IA or IC or class III) have not been very effective.
3. Concurrent illness should be treated.
4. If the patient has cardiac dysfunction, medical therapy with amiodarone should be begun. Amiodarone appears to be the current treatment of choice.
Description. The pacemaker lies in an ectopic focus, and “circus movement” in the atrium is the mechanism of this arrhythmia.
1. Typical atrial flutter is characterized by an atrial rate (F wave with “sawtooth” configuration) of about 300 (ranges, 240–360) beats/min, a ventricular response with varying degrees of block (e.g., 2:1, 3:1, 4:1), and normal QRS complexes (see Fig. 24-2).
2. Another form of atrial flutter may be seen in children who have undergone atrial surgery with multiple suture lines. Atrial flutter is secondary to a reentry mechanism within the scarred atrial muscle (called incisional intraatrial reentrant tachycardia). In this situation, the atrial rates are commonly 250 beats/min or slower, and the P-wave morphology is variable without the usual sawtooth F waves, and the P wave is often difficult to detect. Either 2:1 or 1:1 AV conduction is present.
Causes. Atrial flutter usually suggests a significant cardiac pathology, although most fetuses and neonates with atrial flutter have normal hearts, and spontaneous conversion is common. Structural heart disease with dilated atria, acute infectious illness, myocarditis or pericarditis, digitalis toxicity, and thyrotoxicosis are possible causes. Previous surgical procedures involving atria (e.g., Senning operation for transposition, Fontan operation, and other CHDs) may cause incisional intraatrial reentrant tachycardia.
Significance. The ventricular rate determines eventual cardiac output. With a reasonable ventricular rate, the arrhythmia is well tolerated for a long time. A too-rapid ventricular rate may decrease cardiac output and result in heart failure. Thrombus formation may lead to embolic events. Uncontrolled atrial flutter may precipitate heart failure. The flutter may be associated with syncope, presyncope, or chest pain.
Management. Management of atrial flutter is divided into acute conversion, chronic suppression of the arrhythmia, control of ventricular rate, prevention of recurrences, and for refractory cases.
1. Acute situation
a. Adenosine does not convert the arrhythmia to sinus rhythm, although it may be helpful in confirming the diagnosis of atrial flutter by temporarily blocking AV conduction.
b. Immediate synchronized DC cardioversion is the treatment of choice for atrial flutter of short duration if the infant or child is in severe CHF.
c. Transesophageal atrial overdrive pacing may be used for the same purpose. Pacing stimuli are delivered at rates 20% to 25% faster than the flutter rate until the flutter circuit is captured.
d. In children, IV amiodarone (class III) or IV procainamide (class IA) may be effective.
2. For chronic cases: For long-standing atrial flutter or fibrillation (of 24–48 hours) or those with an unknown duration, thrombus formation may lead to cerebral embolic events, especially when the atrial arrhythmia is terminated.
a. It is essential to rule out atrial thrombi, preferably by echocardiography. Transesophageal echocardiography may define atrial thrombi better than transthoracic echocardiography.
b. If a thrombus is found or its absence is uncertain, anticoagulation with warfarin (with an international normalized ratio between 2.0 and 3.0) is started and cardioversion delayed for 2 to 3 weeks. After conversion to sinus rhythm, anticoagulation is continued for an additional 3 to 4 weeks.
3. For rate control: For control of ventricular rate, calcium channel blockers appear to be the drug of choice. Propranolol may be equally effective. In the past, digoxin was popular for this purpose.
4. For prevention of recurrences: Class I and class III antiarrhythmic drugs have been shown to be successful in preventing recurrences in some cases. However, class IA drugs (procainamide, quinidine, disopyramide) also have anticholinergic effects that may produce a faster conduction through the AV node, worsening the situation. Therefore, they should be used with drugs that offset the anticholinergic effect, such as digoxin, beta-blockers, or diltiazem. Amiodarone and ibutilide (class III) have also been shown to be effective in treating atrial flutter. For a quick review of antiarrhythmic drugs, readers should see Tables A-4 and A-5 in Appendix A.
5. For refractory cases: For incisional intraatrial reentry tachycardia, radiofrequency ablation to interrupt the flutter circuit may be indicated. The success rate for this condition is not as high as in typical atrial flutter (with an acute success rate of 75% and a recurrence rate as high as 50%).
Description. Atrial fibrillation (AF) is the most common arrhythmia seen in adults, but it is rare in children and is less common than atrial flutter in children. The mechanism of this arrhythmia is “circus movement,” as in atrial flutter. AF is characterized by an extremely fast atrial rate (f wave at a rate of 350–600 beats/min) and an “irregularly irregular” ventricular response with narrow QRS complexes (see Fig. 24-2).
Causes. AF usually is associated with structural heart diseases with dilated atria, such as seen with mitral stenosis and regurgitation, Ebstein’s anomaly, tricuspid atresia, ASD, or previous intraatrial surgery. Thyrotoxicosis, pulmonary emboli, and pericarditis should be suspected in a previously normal child who develops AF.
Significance. The rapid ventricular rate, in addition to the loss of coordinated contraction of the atria and ventricles, decreases the cardiac output, as occurs in atrial tachycardia. AF usually suggests a significant cardiac pathology.
Management. Some part of medical management for AF is similar to that described under a trial flutter (see previous discussion).
1. If AF has been present for more than 48 hours, anticoagulation with warfarin for 2 to 3 weeks is recommended to prevent systemic embolization of atrial thrombus if the conversion can be delayed. Anticoagulation is continued for 3 to 4 weeks after the restoration of sinus rhythm. If cardioversion cannot be delayed, IV heparin should be started and cardioversion performed when activated partial thromboplastin time (aPTT) reaches 1.5 to 2.5 times control levels (in 5–10 days), with subsequent oral anticoagulation with warfarin. An alternative to anticoagulation is transesophageal echocardiography to rule out atrial thrombus.
2. Propranolol, verapamil, or digoxin may be used to slow the ventricular rate.
3. Class I antiarrhythmic agents (e.g., quinidine, procainamide, flecainide) and the class III agent amiodarone may be used, but the success rate in rhythm conversion is disappointingly low. Tables A-4 and A-5 in Appendix A provide a quick review of antiarrhythmic drugs.
4. In patients with chronic AF, anticoagulation with warfarin should be considered to reduce the incidence of thromboembolism. In chronic cases, rate control, rather than conversion, is increasingly used.
5. In the Cox maze procedure (or the “cut-and-sew-maze”), multiple surgical incisions are made in the right and left atria that are then repaired in an attempt to minimize the formation of reentrant loop. The procedure showed greater than a 96% cure rate 10 years after the surgery in adult patients. Freedom from stroke has generally been reported as exceeding 99% for Cox maze procedures.
6. Radiofrequency ablation to electrically isolate the pulmonary veins from the left atrium or directly ablating the ectopic focus within the pulmonary veins has shown better results than pharmacologic agents in rhythm control in adults.
Supraventricular tachycardia (SVT) is a general term that refers to any rapid heart rhythm originating above the ventricular tissue. In general, SVTs are caused by two separate mechanisms. The first mechanism is reentry, and the second is automaticity. The majority of SVTs are caused by reentrant (or reciprocating) AV tachycardia rather than rapid firing of a single focus in the atria. Examples of reentry (reciprocating) SVT include AV reentrant (or reciprocating) tachycardia and nodal reentrant AV tachycardia. Examples of automatic types of SVTs are atrial ectopic tachycardia and junctional ectopic tachycardia (JET). These arrhythmias share a number of clinical, electrocardiographic, and therapeutic similarities. Most of the discussion here focuses on the reentry type of SVTs; others are discussed under specific headings.
Reentrant (Reciprocating) Type of SVT
In the reentry type of SVT, the heart rate is extremely rapid and regular (usually 240 ± 40 beats/min). The P wave usually is invisible. When visible, the P wave has an abnormal P axis and either precedes or follows the QRS complex (see Figs. 24-2 and 24-3). The QRS complex is usually normal (narrow), but occasionally, aberrancy increases the QRS duration, making differentiation from ventricular tachycardia (VT) difficult (see later discussion).
FIGURE 24-3 Diagram showing the mechanism of reciprocating atrioventricular tachycardia (RAVT) in relation to electrocardiographic (ECG) findings. A, Orthodromic accessory RAVT is the most common mechanism of supraventricular tachycardia in patients with Wolff-Parkinson-White syndrome. Antegrade conduction through the normal, slow atrioventricular (AV) node produces a normal QRS complex, and the retrograde conduction through the bypass tract creates inverted P waves after the QRS complex (with a short RP interval). B,In antidromic accessory RAVT, the antegrade conduction through the bypass tract produces a wide QRS complex. Retrograde P waves precede the wide QRS complex with a short PR interval (and a long RP interval). C, In orthodromic nodal RAVT (common form), the retrograde P waves are usually concealed in the QRS complex of normal duration. The ECG is similar to that of orthodromic accessory RAVT, and differentiation between these two is possible only when the tachyarrhythmia terminates by the presence of preexcitation in the accessory RAVT. D, In antidromic nodal RAVT (uncommon), narrow QRS complexes are preceded by retrograde P waves, with a short PR interval. The ECG is similar to that of ectopic atrial tachycardia. LV, left ventricle; RV, right ventricle. (From Park MK, Guntheroth WG: How to Read Pediatric ECGs, 4th ed. Philadelphia, Mosby, 1992.)
AV reentrant (or reciprocating) tachycardia is not only the most common mechanism of SVT but also the most common tachyarrhythmia seen in the pediatric age group. This arrhythmia was formerly called paroxysmal atrial tachycardia (PAT) because the onset and termination of this arrhythmia were characteristically abrupt.
In SVT caused by reentry, two pathways are involved; at least one of these is the AV node, and the other is an accessory pathway. The accessory pathway may be an anatomically separate bypass tract, such as the bundle of Kent (which produces accessory reciprocating AV tachycardia [RAVT]; see Fig. 24-3, A and B), or only a functionally separate bypass tract, such as in a dual AV node pathway (which produces nodal RAVT; see Fig. 24-3, C and D). Patients with accessory pathways frequently have Wolff-Parkinson-White (WPW) preexcitation.
Figure 24-3 shows the mechanism of RAVT in relation to electrocardiographic (ECG) findings. If a PAC occurs, the prematurity of the extrasystole may find the accessory bundle refractory, but the AV node may conduct, producing a normal QRS complex; when the impulse reaches the bundle of Kent from the ventricular side, the bundle will have recovered and allows reentry into the atrium, producing a superiorly directed P wave that is difficult to detect. In turn, the cycle is maintained by reentry into the AV node, with a very fast heart rate. When there is an antegrade conduction through the AV node (slow pathway); the rhythm is called orthodromic reciprocating atrioventricular tachycardia (see Fig. 24-3, A).
Less common is a widened QRS complex with antegrade conduction into the ventricle via the accessory (fast) pathway and retrograde conduction through the (slower) AV node (antidromic reciprocating AV tachycardia; see Fig. 24-3, B). A premature ventricular contraction (PVC) could initiate this arrhythmia if the recovery time of the two limbs is ideal for the initiation of the reentry.
Dual pathways in the AV node are more common than accessory bundles, at least as functional entities. For SVT to occur, the two pathways would have to have, at least temporarily, different conduction and recovery rates, creating the substrate for a reentry tachycardia. When the normal, slow pathway through the AV node is used in antegrade conduction to the bundle of His (orthodromic), the resulting QRS complex is normal with an abnormal P vector, but the latter is unrecognizable because it is superimposed on the QRS complex (see Fig. 24-3, C). The resulting tachycardia could be the same as that seen with SVT associated with WPW syndrome. The two can be differentiated only after conversion from the SVT; after conversion, the patient with accessory bundle would have WPW preexcitation. In antidromic nodal reentrant AV tachycardia (see Fig. 24-3, D), which is uncommon, the fast tract of the AV node transmits the antegrade impulse to the bundle of His, and the normal, slow pathway of the AV node transmits the impulse retrogradely. The resulting SVT demonstrates normal QRS duration, a short PR interval, and an inverted P wave.
In general, nodal RAVT is more influenced by increased sympathetic tone than accessory RAVT. Nodal RAVT is more likely triggered by physical activity, emotional stress, and abrupt changes in body position. In addition, nodal RAVT is less likely to be incessant (and therefore rarely causing tachycardia-induced cardiomyopathy). SVT, seen in the first year of life but few afterward, is more likely to have accessory RAVT, and an adolescent who has first SVT is more likely to have nodal RAVT.
Any type of AV block is incompatible with reentrant tachycardia; AV block would abruptly terminate the tachycardia, at least temporarily. This is the reason that adenosine, which transiently blocks AV conduction, works well for this type of arrhythmia.
Automatic Type of SVT
Ectopic (or nonreciprocating) atrial tachycardia is a rare mechanism of SVT in which rapid firing of a single focus in the atrium is responsible for the tachycardia (see previous section). Unlike in reciprocating atrial tachycardia, in ectopic atrial tachycardia, the heart rate varies substantially during the course of a day, and second-degree AV block may develop. In contrast, in reentrant tachycardia, second-degree AV block terminates the SVT. Nodal ectopic tachycardia may superficially resemble atrial tachycardia because the P wave is buried in the T waves of the preceding beat and becomes invisible but the rate of nodal tachycardia is relatively slower (120–200 beats/min) than the rate of ectopic atrial tachycardia. These two arrhythmias are further discussed under separate headings.
1. WPW preexcitation is present in 10% to 20% of cases, which is evident only after conversion to sinus rhythm. Approximately 10% of WPW patients have multiple (two to four) accessory pathways.
2. No heart disease is found in about half of patients. This idiopathic type of SVT occurs more commonly in young infants than in older children.
3. Patients with some CHDs (e.g., Ebstein’s anomaly, single ventricle, congenitally corrected transposition of the great arteries) are more prone to this arrhythmia.
4. SVT may occur after cardiac surgeries.
1. Many infants tolerate SVT well. If the tachycardia is sustained for 6 to 12 hours, signs of CHF usually develop in infants. Clinical manifestations of CHF include irritability, tachypnea, poor feeding, and pallor. When CHF develops, the infant’s condition can deteriorate rapidly.
2. Older children may complain of chest pain, palpitation, shortness of breath, lightheadedness, and fatigue. A pounding sensation in the neck (i.e., neck pulsation) is fairly unique to the reentrant-type SVT and considered to be the result of cannon waves when the atrium contracts against a simultaneously contracting ventricle.
Acute Treatment of SVT
1. Vagal stimulatory maneuvers (unilateral carotid sinus massage, gagging, pressure on an eyeball) may be effective in older children but rarely effective in infants. Placing an ice-water bag on the face (for up to 10 seconds) is often effective in infants (by diving reflex). In children, a headstand often successfully interrupts the SVT.
FIGURE 24-4 Adenosine can uncover the mechanism of supraventricular tachycardia. A 3-month-old infant developed an extremely fast, narrow QRS complex tachycardia and a heart rate of 220 beats/min after insertion of a central line through a jugular vein. Adenosine produced a transient atrioventricular block and unmasked very rapid atrial fibrillation waves (570 beats/min).
2. If the vagal maneuver is ineffective, adenosine is considered the drug of choice. It has negative chronotropic, dromotropic, and inotropic actions with a very short duration of action (half-life <10 seconds) and minimal hemodynamic consequences. Adenosine is effective for almost all reciprocating SVT (in which the AV node forms part of the reentry circuit) of both narrow- and wide-complex regular tachycardia. It is not effective for irregular tachycardia. It is not effective for non-reciprocating atrial tachycardia, atrial flutter or AF, and VT, but it has differential diagnostic ability. Its transient AV block may unmask atrial activities by slowing the ventricular rate and thus help clarify the mechanism of certain supraventricular arrhythmias (see Fig. 24-4).
Adenosine is given by rapid IV bolus followed by a saline flush, starting at 50 μg/kg, increasing in increment of 50 μg/kg, every 1 to 2 minutes. The usual effective dose is 100 to 150 μg/kg with maximum dose of 250 μg/kg. Adenosine is 90% to 100% effective.
3. If the infant is in severe CHF and adenosine is not readily available, emergency treatment is directed at immediate cardioversion. The initial dose of 0.5 joule/kg is increased in steps up to 2 joule/kg.
4. IV administration of propranolol may be used to treat SVT in the presence of WPW syndrome. IV verapamil should be avoided in infants younger than 12 months of age because it may produce extreme bradycardia and hypotension in infants. Esmolol, other beta-adrenergic blockers, verapamil, and digoxin also have been used with some success.
5. Overdrive suppression (by transesophageal pacing or by atrial pacing) may be effective in children who have been digitalized.
Prevention of Recurrence of SVT
1. In infants without WPW preexcitation, oral propranolol for 12 months is effective.
2. Verapamil can also be used, but it should be used with caution in patients with poor LV function and in young infants.
3. In infants or children with or without WPW preexcitation on the ECG, beta-blockers such as atenolol or nadolol are often the medication of choice in the long-term management. In the presence of WPW preexcitation, digoxin or verapamil may increase the rate of antegrade conduction of the impulse through the accessory pathway and therefore should be avoided.
4. For children who have infrequent episodes of SVT that result in little hemodynamic compromise, observation is indicated. They should be taught how to apply vagal maneuvers (e.g., gagging, headstands). If not effective, adenosine is used to correct the rhythm. Alternatively, the use of a beta-blocker or calcium channel blocker can be effective in slowing and terminating the SVT.
5. Radiofrequency catheter ablation or surgical interruption of accessory pathways should be considered if medical management fails or frequent recurrences occur. Ablation therapy is controversial for asymptomatic patients with WPW preexcitation. Ablation is not recommended in infants 1 to 2 years of age because of a possibility of spontaneous resolution of SVT.
Radiofrequency ablation can be carried out with a high degree of success and a low complication rate. The success rate of the procedure for accessory pathway ablation is between 90% and 95%; the highest success rates are found in patients with left-sided accessory pathways. Patients with para-Hisian pathways have the lowest success rate because of more cautious applications (fearing risk of AV block). A risk of heart block is 1.2% with a risk as high as 10.4% for patients with a midseptal ablation site. The overall risk of complication is 3% to 4%.
Rhythms Originating in the Atrioventricular Node
Rhythms that originate in the AV node are characterized by the following findings (Fig. 24-5):
1. The P wave may be absent, or inverted P waves may follow the QRS complex.
2. The QRS complex usually is normal in duration and configuration.
Only the lower part (NH region) of the AV node has pacemaker ability. The upper (AN region) and middle (N region) parts do not function as pacemakers but delay the conduction of an impulse, either antegrade or retrograde.
Junctional (or Nodal) Premature Beats
Description. A normal QRS complex occurs prematurely. P waves usually are absent, but inverted P waves may follow QRS complexes (see Fig. 24-5). The compensatory pause may be complete or incomplete.
Causes. Nodal premature beats usually are idiopathic in an otherwise normal heart; they may result from cardiac surgery and digitalis toxicity.
Significance. Nodal premature beats usually have no hemodynamic significance.
Management. Treatment is not indicated unless the cause is digitalis toxicity.
Junctional (or Nodal) Escape Beats
Description. When the SA node impulse fails to reach the AV node, the NH region of the AV node initiates an impulse (junctional or nodal escape beat). The resulting QRS complex occurs later than the anticipated normal beat. The P wave may be absent, or an inverted P wave follows the QRS complex (see Fig. 24-5). The duration and configuration of QRS complexes are normal.
FIGURE 24-5 Arrhythmias originating in the atrioventricular node.
Causes. Nodal escape beats may occur after cardiac surgery involving the atria (the Senning procedure or Fontan operation) or in otherwise healthy children.
Significance. Nodal escape beats have little hemodynamic significance.
Management. Generally, no specific treatment is required.
Nodal (or Junctional) Rhythm
Description. If the SA node consistently fails, the AV node may function as the main pacemaker of the heart, producing a relatively slow rate (40–60 beats/min). Nodal rhythm is characterized by no P waves or inverted P waves after QRS complexes and normal (narrow) QRS complexes with a rate of 40 to 60 beats/min (see Fig. 24-5).
Causes. Nodal or junctional rhythm may occur in an otherwise normal heart, increased vagal tone (increased intracranial pressure, pharyngeal stimulation), and digitalis toxicity or as a result of cardiac surgery. Rarely, it may be seen in children with polysplenia syndrome (left atrial isomerism).
Significance. The slow heart rate may significantly decrease cardiac output and produce symptoms.
Management. No treatment is indicated if the patient is asymptomatic. Known causes such as digitalis toxicity should be treated. Atropine or electric pacing is indicated if the patient is symptomatic from bradycardia.
Accelerated Nodal Rhythm
Description. When the patient has a normal sinus rate and AV conduction and the AV node (NH region) has enhanced automaticity and captures the pacemaker function at a faster rate (60–120 beats/min) than the normal junctional rate (40–60 beats/min), the rhythm is called accelerated nodal (or AV junctional) rhythm. P waves are either absent or inverted P waves follow the normal QRS complexes.
Causes. Accelerated nodal rhythm may be idiopathic, may result from digitalis toxicity or myocarditis, or may occur after cardiac surgery.
Significance. Accelerated nodal rhythm has little hemodynamic significance.
Management. No treatment is necessary unless caused by digitalis toxicity.
Junctional Ectopic Tachycardia (Nodal Tachycardia)
Description. Either the P waves are absent or inverted P waves follow QRS complexes (see Fig. 24-5). The ventricular rate varies from 140 to 240 beats/min. The QRS complex is usually normal, but aberration may occur on rare occasions, as in atrial tachycardia. JET is sometimes difficult to separate from other types of SVTs.
Causes. Enhanced automaticity of the junctional area is the suspected mechanism of the arrhythmia. There are two types, postoperative and congenital.
The postoperative type is the most common form of junctional autonomic rhythm in children. It is a transient disorder seen immediately after open heart surgery lasting 24 to 48 hours. Trauma, stretch, or ischemia to the AV node and electrolyte imbalances resulting from surgical procedures may be responsible for the rhythm abnormality.
The rare congenital form may occur without a heart defect or may be associated with a cardiac malformation (up to 50%). Developmental abnormalities of the AV node and superimposed fibrosis, inflammation, and focal degeneration may be the underlying causes.
Significance. In the postoperative type, there is a loss of AV synchrony in the presence of a fast rate (nearly 200 beats/min), which compromises cardiac output, leading to a fall in blood pressure. Increased endogenous catecholamine levels and administered inotropic support (to maintain adequate blood pressure and renal perfusion) may result in peripheral vasoconstriction leading to a raise in the core temperature. The rising core temperature exacerbates the tachycardia, worsening ventricular performance.
In the congenital form, most patients present before 6 months of age, usually with congestive heart failure. The overall mortality rate is about 35% for this form of tachycardia.
Management. A number of complementary measures are used to treat postoperative JET. They are aimed at correcting pathophysiology of the postoperative tachycardia.
1. A heart rate less than 170 beats/min is well tolerated, but the rate faster than 170 to 190 beats/min needs to be slowed. Atrial overdrive pacing (typically 10 beats/min higher than the rate) often restores AV synchrony.
2. Mild systemic hypothermia is induced, usually a core temperature of 34° to 35°C. At a core temperature below 32°C, ventricular function may be impaired.
3. Cardiac output should be maximized by carefully titrating fluid and electrolyte balance, inotropic support, and pain management.
4. IV amiodarone appears to be the drug of choice as antiarrhythmic therapy. Digoxin, used in the past, has little place in treatment.
5. As an alternative, extracorporeal membrane oxygenation can be used for this arrhythmia.
FIGURE 24-6 Ventricular arrhythmias. (From Park MK, Guntheroth WG: How to Read Pediatric ECGs, 4th ed. Philadelphia, Mosby, 1992.)
For the congenital type, amiodarone appears to be the drug of choice. Amiodarone in a high dose was effective in 85% of the patients with almost a 75% survival rate. If amiodarone is not effective, ablation therapy may be tried.
Rhythms Originating in the Ventricle
Rhythms that originate in the ventricle (ventricular arrhythmias) are characterized by the following (Fig. 24-6):
1. Bizarre and wide QRS complexes
2. T waves pointing in directions opposite of QRS complexes
3. QRS complexes randomly related to P waves, if visible
Premature Ventricular Contraction
A bizarre, wide QRS complex appears earlier than anticipated, and the T wave points in the opposite direction. A full compensatory pause usually appears; that is, the length of two cycles, including the premature beat, is the same as that of two normal cycles (see Fig. 24-6). The presence of a full compensatory pause indicates that the sinus node is not prematurely discharged by the PVC. If the retrograde impulse discharges and resets the sinus node prematurely, it produces a pause that is not fully compensatory.
PVCs may be classified into several types, depending on their interrelationship, similarities, timing, and coupling intervals.
1. Ventricular bigeminy or coupling: Each abnormal QRS complex regularly alternates with a normal QRS complex.
2. Ventricular trigeminy: Each abnormal QRS complex regularly follows two normal QRS complexes.
3. Couplets: Two abnormal QRS complexes appear in sequence.
4. Triplets: Three abnormal QRS complexes appear in sequence. Three or more successive PVCs arbitrarily are termed ventricular tachycardia.
FIGURE 24-7 Types of premature ventricular contractions (PVCs) according to timing in the cardiac cycle. A, Regular sinus rhythm. B, Interpolated PVC followed by a slightly prolonged PR interval. C, Early PVC, which results in a retrogradely conducted P wave (P′) with a less than full compensatory pause. The first postectopic beat is a ventricular escape beat (E). D, Early PVC with a retrogradely conducted P wave (P′) with a less than full compensatory pause. A ventricular fusion beat (F) resumes the cardiac cycle. E, Late PVC, which results in a full compensatory pause; presumably, retrograde discharge of the sinus node did not occur. F, Ventricular fusion beat with a full compensatory pause.
Similarities among PVCs
Depending on the similarities of the bizarre QRS complex, PVCs may be classified into the following types:
1. Uniform (monomorphic or unifocal) PVCs: Abnormal QRS complexes have the same configuration in a single lead. They are assumed to originate from a single focus.
2. Multiform (polymorphic or multifocal) PVCs: Abnormal QRS complexes have different configurations in a single lead. They are assumed to originate from different foci.
Timing in the cardiac cycle
Depending on their timing in the cardiac cycle, PVCs may be classified into several types (Fig. 24-7):
1. Interpolated PVC: The PVC appears between two conducted sinus beats. Sinus rhythm is not interrupted, and there is no compensatory pause after the PVC. The PR interval after the PVC is slightly increased (see Fig. 24-7, B).
2. Early PVC: The PVC appears shortly after the normal T wave of the preceding beat. A compensatory pause may appear. If the sinus rate is slow and a retrograde atrial conduction prematurely discharges the sinus node, a noncompensatory pause results. Either a ventricular escape beat or a fusion beat resumes the cardiac cycle (see Fig. 24-7, C and D).
3. Late PVC: The PVC appears slightly before the normal P wave of the next beat. A full compensatory pause results (see Fig. 24-7, E).
4. Fusion beats: The PVC occurs so late in the cardiac cycle that a normal sinus pacemaker impulse has already penetrated the AV node and started to depolarize the ventricle. The resulting QRS complex appears midway between the patient’s normal conducted beat and the pure ectopic ventricular beat because it is produced partly by a normally conducted supraventricular impulse and partly by an ectopic ventricular impulse (see Fig. 24-7, F). The presence of a “fusion” complex is a reliable sign of PVC and helps differentiate VT from a supraventricular arrhythmia with aberrant ventricular conduction (see later discussion).
1. Fixed coupling. PVCs appear at a constant interval after the QRS complex of the previous cardiac cycle. This suggests ventricular reentry within the Purkinje system as the underlying mechanism. Most PVCs in children have a fixed coupling interval and a uniform left bundle branch block (LBBB) morphology.
FIGURE 24-8 Apical four-chamber view of an echocardiogram showing a false tendon (solid arrows) in the left ventricle (LV) in a 13-year-old boy who had surgical repair of a ventricular septal defect (open arrow). This patient had a “twanging string” type of systolic murmur and occasional uniform premature ventricular contractions. LA, left atrium; RA, right atrium; RV, right ventricle.
2. Varying coupling. When coupling intervals vary by more than 80 msec, the PVCs may result from parasystole. If the intervals between ectopic beats can be factored so that each interval is a multiple of a single basic interval (within 0.08 second), ventricular parasystole is diagnosed. Ventricular parasystole consists of an impulse-forming focus in the ventricle that is independent of the sinus node-generated impulse and is protected from depolarization (entrance block) by sinus impulses.
1. PVC may appear in otherwise healthy children. Up to 50% to 70% of normal children may show PVCs on 24-hour ambulatory ECGs.
2. A link has been found between LV false tendon and PVCs. False tendons are thin, chordal strands that extend from the ventricular septum to either the LV free wall or an LV papillary muscle; they are detectable by two-dimensional echocardiography (Fig. 24-8). False tendons contain Purkinje fibers, which may be the source of the arrhythmia.
3. Myocarditis, myocardial injury or infarction, cardiomyopathy (dilated or hypertrophic), and cardiac tumors are possible causes.
4. Arrhythmogenic right ventricular dysplasia (right ventricular [RV] cardiomyopathy) may be the cause in children with symptomatic tachycardia. (See the section on primary myocardial disease in Chapter 18.)
5. Long QT syndrome (LQTS; see later section)
6. Congenital or acquired heart disease, preoperative or postoperative
7. Drugs such as catecholamines, theophylline, caffeine, amphetamines, digitalis toxicity, and some anesthetic agents are possible causes.
8. Mitral valve prolapse (MVP) is a possible cause.
1. Occasional PVCs are benign in children, particularly if they are uniform and disappear or become less frequent with exercise.
2. PVCs are more significant if the following are true:
a. They are associated with underlying heart disease (preoperative or postoperative status, MVP, cardiomyopathy).
b. There is a history of syncope or a family history of sudden death.
c. They are precipitated by or become more frequent with activity.
d. They are multiform, particularly couplets.
e. There are runs of PVCs with symptoms.
f. There are incessant or frequent episodes of paroxysmal VT (more likely myocardial tumors).
3. Ventricular parasystole does not appear to have any consequences in children.
1. In children with otherwise normal hearts, occasional isolated uniform PVCs that are suppressed by exercise do not require extensive investigation or treatment. ECG, echocardiography studies, and 24-hour Holter monitoring suffice.
a. ECGs are used to detect QTc prolongation, ST-T changes, and other abnormalities.
b. Echocardiography studies detect structural heart disease or functional abnormalities.
c. 24-Hour ambulatory ECG (Holter monitoring) or event recorder detects the frequency and severity of the arrhythmia.
2. Children with uniform PVCs, including ventricular bigeminy and trigeminy, do not need to be treated if the echocardiography findings and exercise stress test results are normal. Arrhythmias that are potentially related to exercise are significant and require documentation of the relationship. The induction or exacerbation of arrhythmia with exercise may be an indication of underlying heart disease. In children, PVCs characteristically are reduced or eliminated by exercise.
3. Asymptomatic children with multiform PVCs and ventricular couplets should have 24-hour Hotter monitoring, even if they have structurally normal hearts, to detect the severity and extent of ventricular arrhythmias.
4. All children with symptomatic ventricular arrhythmias and those with complex PVCs (multiform PVCs, ventricular couplets, unsustained VT) should be treated. Tables A-4 and A-5 in Appendix A provide a quick review of antiarrhythmic drugs.
a. Beta-blockers (e.g., atenolol, 1–2 mg/kg orally in a single daily dose) are effective for cardiomyopathy and occasionally for RV dysplasia.
b. Other antiarrhythmic drugs, such as phenytoin sodium (Dilantin) and mexiletine, may be effective. Antiarrhythmic agents that prolong the QT interval, such as those of class IA (quinidine, procainamide), class IC (encainide, flecainide), and class III (amiodarone, bretylium), should be avoided (Box 24-1).
c. Frequent PVCs occasionally require treatment with an IV bolus of lidocaine (1 mg/kg per dose) followed by an IV drip of lidocaine (20–50 μg/kg/min).
5. For patients with symptomatic ventricular arrhythmias or sustained VT and seemingly normal hearts, magnetic resonance imaging (MRI) may be indicated to investigate for RV dysplasia.
6. Children with multiform PVCs and runs of PVCs (VT) with or without symptoms need to be evaluated by an electrophysiologist. Invasive electrophysiologic studies with RV endomyocardial biopsy may be indicated.
Accelerated Ventricular Rhythm
1. Accelerated ventricular rhythm (AVR) is also known by many other names, such as slow VT, idioventricular tachycardia, slow ventricular rhythm, and nonparoxysmal VT.
2. A wide QRS complex rhythm of short duration is present (usually several beats but can be longer than 120 beats).
3. The ventricular rate approximates the patient’s sinus rate, within ±10% to 15% of the sinus rate (isochronicity). The isochronicity with sinus rhythm is more important than the rate per minute.
4. The ventricular rate is usually 120 beats/min or less in children (and 140–180 beats/min in newborns). The rate helps differentiate AVR from VT (with a rate >120 bpm).
5. The QRS morphology is LBBB pattern in the great majority.
1. In the majority of the patients, AVR is usually an isolated finding. Rarely, it may be associated with underlying heart disease, such as CHD, myocarditis, digitalis toxicity, hypertension, cardiomyopathy, metabolic abnormalities, postoperative state, or myocardial infarction (in adults).
2. The mechanism of AVR is unknown; an ectopic ventricular focus may accelerate its rate enough to overcome sinus rate.
BOX 24-1 Acquired Causes of QT Prolongation∗
Antibiotics: erythromycin, clarithromycin, telithromycin, azithromycin, trimethoprim-sulfamethoxazole
Antifungal agents: fluconazole, itraconazole, ketoconazole
Antiprotozoal agents: pentamidine isethionate
Antihistamines: astemizole, terfenadine (Seldane) (Seldane has been removed from the market for this reason)
Antidepressants: tricyclics such as imipramine (Tofranil), amitriptyline (Elavil), desipramine (Norpramin), and doxepin (Sinequan)
Antipsychotics: haloperidol, risperidone, phenothiazines such as thioridazine (Mellaril) and chlorpromazine (Thorazine)
Class IA (sodium channel blockers): quinidine, procainamide, disopyramide
Class III (prolong depolarization): amiodarone (rare), bretylium, dofetilide, N-acetyl-procainamide, sotalol
Lipid-lowering agent: probucol
Antianginal agent: bepridil
Diuretics (through K loss): furosemide (Lasix), ethacrynic acid (Edecrin)
Oral hypoglycemic agents: glibenclamide, glyburide
Promotility agents: cisapride
Hypokalemia: diuretics, hyperventilation
Underlying Medical Conditions
Bradycardia: complete atrioventricular block, severe bradycardia, sick sinus syndrome
Myocardial dysfunction: anthracycline cardiotoxicity, congestive heart failure, myocarditis, cardiac tumors
Endocrinopathy: hyperparathyroidism, hypothyroidism, pheochromocytoma
Neurologic: encephalitis, head trauma, stroke, subarachnoid hemorrhage
Nutritional: alcoholism, anorexia nervosa, starvation
∗ A more exhaustive updated list of medications that can prolong QTc interval is available at the University of Arizona Center for Education and Research on Therapeutics’ website (www.torsades.org or www.qtdrugs.org).
1. It is hemodynamically insignificant, usually asymptomatic, and benign. It is rarely seen in patients with syncope, presyncope, or palpitation.
2. Sometimes it is found by routine ECG or Holter monitoring in asymptomatic patients.
3. Exertional sinus tachycardia usually converts it to sinus rhythm.
1. In children, AVR is generally considered benign.
2. AVR is notably resistant to antiarrhythmic agents (no treatment is required).
3. Follow-up Holter monitoring is useful and it usually shows resolution of AVR.
1. VT is a series of three or more PVCs with a heart rate of 120 to 200 beats/min. QRS complexes are wide and bizarre, with T waves pointing in opposite directions (see Fig. 24-6).
2. VT may be classified in various ways by its onset, duration, and morphology.
a. The onset may be paroxysmal (sudden) or nonparoxysmal.
b. By duration, VT may be (1) a salvo of VT (a few beats in a row), (2) nonsustained VT (a duration of <30 seconds), (3) sustained VT (longer than 30 seconds), and (4) incessant VT (lengthy, sustained VT that dominates the cardiac rhythm).
c. By morphology, it may be (1) monomorphic (one dominant QRS form), (2) polymorphic (a beat-to-beat change in the QRS shape), or (3) “bidirectional VT” (a specific form of polymorphic VT in which the QRS axis shifts across the baseline).
3. Torsades de pointes (meaning “twisting of the points”) is characterized by a paroxysm of VT during which there are progressive changes in the amplitude and polarity of QRS complexes separated by a narrow transition QRS complex. It is a distinct form of polymorphic VT, occurring in patients with marked QT prolongation.
4. Differentiating VT from SVT with aberrant conduction (see later discussion) is sometimes difficult. However, in children, almost all wide QRS tachycardias are VT. They should be treated as such until proven otherwise.
1. VT may occur in patients with structural heart diseases such as tetralogy of Fallot (TOF), aortic stenosis, hypertrophic or dilated cardiomyopathy, or MVP.
2. Postoperative CHDs (e.g., TOF, dextrotransposition of the great arteries, or double-outlet RV).
3. Myocarditis, cardiomyopathies, Chaga’s disease (trypanosomiasis in South America), myocardial tumors, myocardial ischemia, and infarction
4. Pulmonary hypertension
5. Arrhythmogenic RV dysplasia (in patients of southern European ancestry), Brugada syndrome (young men from Southeast Asia), and LQTS
6. Metabolic causes include hypoxia, acidosis, hyperkalemia, hypokalemia, and hypomagnesemia.
7. Mechanical irritation from an intraventricular catheter
8. Pharmacologic and chemical causes include catecholamine infusion, digitalis toxicity, cocaine, and organophosphate insecticides. Most antiarrhythmic drugs (especially classes IA, IC, and III) are also proarrhythmic.
9. Certain drugs that prolong the QT interval may cause VT, including antiarrhythmic drugs (especially class IA [quinidine, procainamide], class IC [encainide, flecainide], class III [amiodarone, bretylium]), antipsychotic agents (phenothiazines [chlorpromazine, thioridazine], tricyclic antidepressants (imipramine, desipramine), and certain antibiotics (erythromycin, trimethoprim-sulfamethoxazole) (see Box 24-1). Class II and IV antiarrhythmic agents do not prolong the QT interval. For a quick review of antiarrhythmic drugs, readers should refer to Tables A-4 and A-5 in Appendix A.
10. VT may occur in healthy children who have structurally and functionally normal hearts. This group is discussed under a separate heading (see later discussion).
1. VT may signify a serious myocardial pathology or dysfunction and can be a cause of sudden cardiac death. These events are, however, unusual in patients with structurally normal hearts.
2. Presenting symptoms may be dizziness, syncope, palpitation, or chest pain. The family history may be positive for ventricular arrhythmia or sudden death.
3. With a fast heart rate, cardiac output may decrease notably, and the rhythm may deteriorate to ventricular fibrillation (VF), in which effective cardiac output does not occur.
4. Chronic low-rate VT may lead to a tachycardia-mediated cardiomyopathy.
5. A condition called accelerated ventricular rhythm is similar to VT, but the heart rate is slower (<120 beats/min) and occurs in patients with structurally normal hearts. (This is discussed later in this chapter under a separate heading.)
6. Polymorphic VTs are more significant than monomorphic ones.
7. Those associated with abnormal cardiac structure (pre- and postoperative) or function are more significant than those seen in patients with structurally and functionally normal hearts.
8. Some VTs are provoked by exercise, but others are suppressed by exercise. The former is usually more significant than the latter.
9. VTs associated with certain forms of cardiomyopathy (arrhythmogenic RV dysplasia, hypertrophic or dilated cardiomyopathy) and genetic electrical heart diseases (LQTS; Brugada syndrome) can be a cause of sudden death.
1. The following investigations may be indicated.
a. A history of congenital or acquired heart disease or substance abuse and a family history of syncope, seizure, sudden cardiac death, or familial arrhythmia are important.
b. Echocardiography and Doppler studies identify most conditions that may cause sudden cardiac death, such as cardiomyopathies (hypertrophic, dilated, noncompaction), myocarditis, coronary artery anomalies (congenital, Kawasaki’s disease), primary pulmonary hypertension, and various CHDs (pre- and postoperative).
A structurally and functionally normal heart on echocardiography studies may include a group of benign conditions as well as some potentially lethal conditions, including the following:
(1) Right ventricular outflow tract (RVOT) VT, right bundle branch block (RBBB) VT (Belhassen’s type), and accelerated ventricular rhythm (AVR) are generally benign conditions. These conditions are discussed further under separate headings.
(2) Electrical myopathies: LQTS, Brugada syndrome, and RV dysplasias are potentially fatal conditions. The ECG usually suggests these conditions.
c. Holter monitoring may be useful for assessing the frequency of PVCs or VTs.
d. Exercise stress test is useful for detecting exercise-induced VT and for assessing the effectiveness of therapy (either medical or ablation).
e. MRI is useful to rule out arrhythmogenic RV dysplasia.
f. Electrophysiologic investigation may be indicated for:
(1) Those with high-density PVCs and symptoms suspicious for tachyarrhythmia
(2) Those with underlying heart disease, especially those in postoperative status, with potentially life-threatening inducible sustained VT
(3) To target the VT focus or reentry circuit for ablation
(4) To check for the effectiveness of orally administered antiarrhythmic therapy
2. Acute therapy
a. Symptomatic VT must be treated promptly with synchronized DC cardioversion (0.5–1 joule/kg) if the patient is unconscious or has cardiovascular instability with clinical evidence of low cardiac output.
b. Rarely, if the patient is conscious, an IV bolus of lidocaine (1 mg/kg per dose over 1–2 minutes) followed by an IV drip of lidocaine (20–50 μg/kg per minute) may be effective. Lidocaine or procainamide is often initiated after cardioversion in an attempt to suppress reinitiation of the tachycardia.
c. IV amiodarone is used in patients with drug-refractory VT, particularly as seen in postoperative patients. The mechanism of action of amiodarone appears to be by reducing transmural heterogeneity of repolarization in the ventricular muscle.
d. IV injection of magnesium sulfate is reportedly an effective and safe treatment for torsades de pointes in adults (2 g in an IV bolus).
e. A trial of adenosine may be helpful in some patients with structurally normal hearts. VT with RBBB and superior axis (LV septal origin) may be calcium channel dependant and respond to a slow IV push of verapamil.
3. The physician should research for reversible conditions contributing to the initiation of VT (e.g., hypokalemia, hypoxemia, postoperative TOF with severe pulmonary regurgitation) and correct the conditions if possible.
4. Chronic therapy
a. Conservative management may be safe for those asymptomatic patients with repetitive nonsustained VT in the absence of any evidence of ventricular dysfunction.
b. Antiarrhythmic drug treatment: Complete pharmacologic suppression may not be achieved without serious complications. Therefore, controlling the rate to an asymptomatic level may be adequate. A combination of 24-hour Holter monitoring and treadmill exercise testing is the best noninvasive means of evaluating drug effectiveness.
(1) Virtually all classes of antiarrhythmic agents have been used for various types of VTs, including all class I and III drugs, with varying levels of success. Tables A-4 and A-5 in Appendix A provide a quick review of antiarrhythmic drugs.
FIGURE 24-9 Tracing from a 4-year-old girl who is asymptomatic even while having ventricular tachycardia (VT). The child is on atenolol. In this figure, P waves are not identifiable in front of each QRS complex. The QRS duration is wide (0.10 second), and ventricular tachycardia is present with the ventricular rate of 160 beats/min. The QRS complexes have left bundle branch block morphology, indicating the right ventricle (RV) as the ectopic focus, and the axis of VT is directed inferiorly. Spontaneous temporary interruption of VT occurred while recording the V4, V5, and V6 leads.
(2) Beta-blockers may be very effective for patients who have no underlying heart disease and those who have exercise-provokable monomorphic VT from the RVOT or LV outflow tract. Beta-blockers have fewer side effects than most other antiarrhythmic agents.
(3) In patients with reduced LV function, digitalis and an afterload reducer may be beneficial (to improve LV function).
c. Patients with LQTS are treated with beta-blockers, which alleviate symptoms in 75% to 80% of patients. An implantable cardioverter-defibrillator (ICD) is sometimes recommended as initial therapy.
d. Catheter ablation is most successful in patients with structurally normal hearts with a focal origin of the tachycardia. Patients with underlying heart disease are more likely to have reentry circuits and have lower effectiveness than those with focal origin of VT.
e. ICD has become the established standard for treating many, if not most, forms of VT, which are potentially lethal. ICDs that can be used without thoracotomy are gaining experience in the young.
Ventricular Arrhythmias in Children with Normal Hearts
Although recurrent sustained VT usually signals an organic cause of the arrhythmia, some VTs are seen in healthy adolescents and young adults with structurally and functionally normal hearts. The prognosis is good. So-called RVOT VT and RBBB VT are examples of this group of VT.
1. RVOT VT: This special form of VT seen in children with structurally and functionally normal hearts originates from the RV conal septum and thus has inferior QRS axis and LBBB morphology (Fig. 24-9). This is usually benign tachycardia. It may manifest as PVCs or short runs or salvos of VT, but many children are asymptomatic or minimally symptomatic. Exercise stress may not completely abolish the tachycardia. Beta-blockers are sufficient for treatment. Verapamil and other agents may also prove to be effective. Radiofrequency ablation can be curative.
2. RBBB VT (Belhassen’s tachycardia): It appears to arise from the septal surface of the LV and is less common than RVOT VT. It is characterized by RBBB morphology and superior QRS axis. They may be calcium channel dependent and respond to a slow IV push of verapamil or adenosine. Long-term treatment with verapamil can prevent recurrences. When refractory to medical therapy, radiofrequency ablation or surgery is effective. The long-term outcome is excellent.
When a supraventricular impulse prematurely reaches the AV node or bundle of His, it may find one bundle branch excitable and the other still refractory. Therefore, the resulting QRS complex resembles a bundle branch block pattern. The right bundle branch usually has a longer refractory period than the left bundle branch, producing QRS complexes similar to those of RBBB. The following features help in differentiating aberrant ventricular conduction from ectopic ventricular impulses:
1. An rsR′ pattern in V1 that resembles QRS complexes of RBBB suggests aberration. In ventricular ectopic beats, the QRS morphology is bizarre and does not resemble the classic form of RBBB or LBBB.
2. Occasional wide QRS complexes after P waves with regular PR intervals suggest an aberration.
3. The presence of a ventricular “fusion” complex (see previous discussion) is a reliable sign of ventricular ectopic rhythm.
Description. Ventricular fibrillation is rare in the pediatric population. It is characterized by bizarre QRS complexes of varying sizes and configurations. The rate is rapid and irregular (see Fig. 24-6). The arrhythmia is maintained by multiple reentrant circuits because portions of the myocardium are depolarizing constantly.
Causes. All of the causes listed for VT can cause VF. Predisposing factors include electrolyte abnormalities, proarrhythmic medications, increased sympathetic activities or catecholamine infusion, hypoxia, or ischemia. Certain CHD (pre- and postoperative) and hereditary conditions are possible causes.
Abrupt VF after blunt chest wall trauma that typically occurs in young participants in sports (notably, ice hockey, lacrosse, baseball, and softball) can cause the arrhythmia (called commotio cordis).
Significance. VF usually is a degeneration of VT and is terminal arrhythmia because it results in ineffective circulation. Immediate resuscitation must be provided.
Management. Resuscitation from VF is successful if performed in a timely fashion; the longer the myocardium is allowed to fibrillate, the more difficult conversion to a sinus rhythm. Electric shocks (delivered in an asynchronous fashion) are aimed at depolarizing the myocardium to terminate fibrillating rhythm and allow an intrinsic cardiac pacemaker to resume control.
1. Acute care
a. Immediate cardiopulmonary resuscitation (CPR), providing the ABCs (airway, breathing, and circulation), airway management with 100% oxygen, and rhythm monitoring, is essential.
b. Defibrillation with 2, 4, and 6 joule/kg if needed
c. Administration of epinephrine via IV or intraosseous (IO) route, 0,01 mg/kg (1:10,000 solution, 0.1 mL/kg) or via an endotracheal (ET) tube 0.l mg/kg (1:1000 solution, 0.1 mL/kg)
d. One should identify and treat causes, including the metabolic environment (hypoxia, acidosis).
e. One of the following antiarrhythmic agents may be used:
(1) Amiodarone 5 mg/kg bolus, IV or IO
(2) Lidocaine 1 mg/kg IV, or IO, or ET
(3) Magnesium sulfate 25 to 50 mg/kg IV or IO (not to exceed 2 g) for torsades de pointes or hypomagnesemia
2. A child with susceptibility to VF and those resuscitated from the arrhythmia should have a comprehensive pediatric electro-physiology evaluation.
3. An ICD is often indicated in patients who survive VF. This device can be deployed through epicardial patch or transvenous lead placement.
Long QT Syndrome
Long QT syndrome is a genetic disorder of myocardial repolarization characterized by a prolonged QT interval on the ECG and ventricular arrhythmias, usually torsades de pointes, that may result in sudden death. There are four groups of patients with congenital types of LQTS: those with Jervell and Lange-Nielsen syndrome, those with Romano-Ward syndrome, those with a sporadic form of Romano-Ward syndrome, and those with two new rare forms of the condition.
1. Jervell and Lange-Nielsen (1957) first described families in Norway in whom a long QT interval on the ECG was associated with congenital deafness, syncopal spells, and a family history of sudden death. This syndrome is transmitted in an autosomal recessive manner.
2. Roman-Ward syndrome, reported independently by Romano and colleagues in Italy (1963) and Ward in Ireland (1964), has all of the features of Jervell and Lange-Nielsen syndrome but without deafness. This syndrome transmits in an autosomal dominant mode and is much more common than Jervell and Lange-Nielsen syndrome.
3. A significant number of individuals with Romano-Ward syndrome (with normal hearing) appear to represent sporadic cases, with a negative family history of the syndrome.
4. Two additional syndromes, Anderson-Tawil syndrome and Timothy syndrome, have been added recently (Table 24-2). Anderson-Tawil syndrome is sometimes designated as LQT7, in which the QU interval, rather than the QT interval, is prolonged, along with muscle weakness (periodic paralysis), ventricular arrhythmias, and developmental abnormalities. Timothy syndrome is associated with webbed fingers and toes and long QT measurement.
The QT prolongation may be congenital or acquired.
1. Congenital LQTS is a heterogeneous disorder. LQTS is caused by mutations of cardiac ion channel genes. Recent advances in molecular biology have revealed that ion channels that govern the electrical activity of the heart are defective in congenital LQTS. Based on genetic background, six types of Romano-Ward syndromes and two types of Jervell-Lange Nielsen syndromes are identified. Two additional syndromes (Anderson-Tawil syndrome and Timothy syndrome) are considered different subgroups. Table 24-2 lists molecular genotypes, frequency, chromosomes involved, mutant genes, and defective ionic channels according to subtypes of congenital LQTS. This discussion focuses on congenital LQTS.
2. Acquired prolongation of the QT interval can be caused by a number of drugs, electrolyte disturbances, and other underlying medical conditions (see Box 24-1). In the acquired type of LQTS, a similar ionic mechanism may be involved as is observed in congenital LQTS. Individuals who manifest acquired LQTS are believed to be genetically predisposed for the condition.
CONGENITAL CAUSES OF LONG QT SYNDROME
Adapted from Collins KK, van Hare GF. Advances in congenital long QT syndrome. Curr Opin Pediatr18:497-502, 2006.
Prolonged QT interval on the ECG represents prolonged recovery from electrical excitation that contributes to an increased likelihood of dispersion of refractoriness with some parts of the myocardium remaining refractory to subsequent depolarization. Consequently, the wave of excitation may pursue a distinctive pathway around a focal point in myocardium (circus reentry rhythm), leading to VT.
1. The family history is positive in about 60% of patients, and deafness is present in 5% of patients with the syndrome.
2. Presenting symptoms may be syncope (26%), seizure (10%), cardiac arrest (9%), presyncope, or palpitation (6%). The majority of these symptoms occurs during exercise or with emotion. Symptoms of LQTS are related to ventricular arrhythmias. The first symptom of the syndrome appears usually in the first decade but by the end of the second decade of life. The first manifestation may be cardiac arrest.
3. Syncope occurs in the setting of intense adrenergic arousal, intense emotion, and during or after rigorous exercise. Swimming appears to be a particular trigger among exercises. Abrupt auditory signals such as a loud doorbell, alarm clock, telephone, or security alarm can trigger symptoms.
4. The ECG shows the following:
a. A prolonged QT interval with a corrected QT interval (QTc) usually longer than 0.46 second (Fig. 24-10); the upper limit of normal is 0.44 second.
b. Abnormal T-wave morphology (bifid, diphasic, or notched) is frequent.
c. Bradycardia (20%), second-degree AV block, multiform PVCs, and monomorphic or polymorphic VT (10%–20%) may be present. All of these ECG findings are considered risk factors for sudden death.
5. Echocardiographic studies usually show a structurally and functionally normal heart.
FIGURE 24-10 Electrocardiogram from a patient with Romano-Ward syndrome at age 6 years demonstrates the longest QTc interval (0.56 second). The precordial leads are not shown. Two negative P waves in aVF suggest a junctional mechanism. This child received 10 mg of propranolol four times a day until the age of 13 years, with complete cessation of syncopal attacks. There were seven cases of sudden death associated with syncopal attacks on the maternal side of the family. His mother (age 27 years) and sister (age 5 years) had moderate prolongation of QTc interval but experienced no syncopal attacks.
Correct diagnosis and treatment are important to prevent sudden death. However, the diagnosis of this disease, which has a poor prognosis, should not be made lightly because it implies a lifelong commitment to treatment.
Any child who has a prolonged QTc of 0.46 sec or longer or a compelling borderline QTc interval with symptoms, family history, or unusual T waves (T wave alternans or notched T waves) should be carefully evaluated.
1. Accurate measurement of the QTc interval is necessary for the diagnosis of LQTS. A 12-lead ECG is the current screening tool for the identification of LQTS.
a. Lead II is the preferred lead to measure the QT interval because a q wave is usually present in this lead, but precordial leads (V1, V3, or V5) may also be used because they provide better definition of T waves.
b. The QTc interval is calculated by using Bazett’s formula (see Chapter 3). The QTc interval represents the QT interval normalized for a heart rate of 60 beats/min.
c. The presence of sinus arrhythmia creates problems in measuring an accurate and reliable QTc interval because the QTc interval varies with the R-R interval. It has been recommended that the longest QTc interval after the shortest R-R interval be used. By this method, it is rare to have LQTS with a QTc of 0.46 msec or less (Martin et al, 1995). This recommendation is still controversial. The Bazett’s formula is reliable only for the steady state, but sinus arrhythmia is not steady state. In addition, there is a phenomenon called QT hysteresis in which the QT interval adapts slowly to changing heart rate and certainly not in one beat. Therefore, it may be worthwhile repeating the measurement of the QTc interval in a tracing that does not have marked sinus arrhythmia.
d. In patients with prolonged QRS duration (as seen in bundle branch block), the QT interval may be prolonged secondary to the lengthening of the QRS duration even in the absence of repolarization abnormalities; the QT interval includes both depolarization and repolarization. In such cases, the JT interval may be a more sensitive predictor of repolarization abnormalities than the QTc interval. The JT interval is measured from the J point (the junction of the S wave and the ST segment) to the end of the T wave. Rate correction is accomplished by the use of Bazett’s formula (Berul et al, 1994). A normal JTc interval (mean ± SD) is 0.32 ± 0.02 second with the upper limit of normal of 0.34 second in children and adolescents.
2. The diagnosis of LQTS is clear cut when there is a marked prolongation of the QTc interval with positive family history of the syndrome. However, many cases are at borderline, making it difficult to make or reject the diagnosis. Schwartz et al refined the diagnostic criteria in 1993. A point system is used to distinguish the likelihood of a patient having disease. The criteria take into account the ECG findings, clinical history, and family history and rank the findings by points based on the “importance” of the findings (Table 24-3). According to these criteria, the scoring of the probability of LQTS is done as follows.
≤1 point = Low probability of LQTS
2 to 3 points = Intermediate probability of LQTS
≥4 points = High probability LQTS
3. Initial diagnostic strategy: Initially, the following five steps are considered in making the diagnosis of LQTS.
a. History of presyncope, syncope, seizure, or palpitation and family history are carefully examined.
b. Causes of acquired LQTS are excluded (see Box 24-1).
c. ECG is examined for the QTc interval and morphology of the T waves. ECGs are also obtained from immediate family members
d. The LQTS score is calculated (see Table 24-3), and the diagnostic possibility is graded as described earlier
SCHWARTZ DIAGNOSTIC CRITERIA FOR LONG QT SYNDROME
LQTS, long QT syndrome.
Adapted from Schwartz PJ, Moss AJ, Vincent GM, Crampton RS. Diagnostic criteria for the long QT syndrome: an update. Circulation 88:782-784, 1993.
e. Patients with an LQTS score of 4 or above or abnormal exercise test results are considered to have LQTS; an LQTS score of 1 or below is excluded from the diagnosis. Patients with an LQTS score of 2 or 3 are followed up for possible LQTS.
4. For borderline cases, some centers carry out additional testing, such as Holter monitoring, exercise testing, pharmacologic testing, or electrophysiology study. However, the interpretation and significance of these tests are controversial.
a. Holter monitoring: This may detect intermittent QT prolongation with changing heart rate (with QTc prolongation at a faster rate), bradyarrhythmia, macrovolt T-wave alternans, and T-wave notching. It may also detect VT. However, the results of Holter monitoring should be interpreted with caution because the standards for QTc on ambulatory monitoring are not established. The QTc interval is longer during sleep; therefore, Holter monitoring may show the QTc interval to be 0.05 second longer than the interval on a standard ECG. Tape speed of the monitor may vary according to the device used. Accordingly, comparison of the Holter with a standard ECG for the QTc interval may be inaccurate.
b. Exercise testing: Some centers routinely perform exercise testing. In most children and young adults, the QT intervals shortens with exercise with an increasing heart rate. However, in patients with LQTS, the QT interval may fail to shorten or may lengthen at higher heart rates and with exertion. A lack of appropriate shortening of the QTc duration and abnormal T-wave morphology are most often seen in LQTS patients. Ventricular arrhythmias may develop during the test in up to 30% of patients.
c. Epinephrine test: Continuous ECG monitoring during an infusion of IV epinephrine at escalating doses (0.025–0.3 mcg/kg per min) usually shortens the QT interval in normal subjects. The QTc prolongs in all patients with LQT subgroups. The prolongation is greatest in patients with LQT1 (141 msec in LQT1 compared with 69 and 3 msec for LQT2 and LQT3, respectively).
5. Genetic testing may identify genotypes of the LQTS. Commercially available genetic testing can identify the five most common gene mutations, all in Romano Ward syndrome, including KCNQ1, KCNE1, KCNH2, KCNE2, and SCN5A. The genetic testing has limitations, though. It is important to realize that the testing can identify a particular mutation, but it cannot rule out LQTS; a negative genetic test result does not rule out LQTS.
1. The known risk factors should be considered when making a treatment plan for the congenital type of LQTS.
a. Known risk factors for sudden death include:
(1) Bradycardia for age (from sinus bradycardia, junctional escape rhythm, or second-degree AV block)
(2) An extremely long QTc interval (>0.55 second)
(3) Symptoms at presentation (syncope, seizure, cardiac arrest)
(4) Young age at presentation (<1 month)
(5) Documented torsades de pointes or VF
b. T-wave alternation (major changes in T-wave morphology) is a relative risk factor.
c. Noncompliance with medication is an important risk factor for sudden death.
2. General measures
a. Physicians should be aware of the conditions and medications that prolong the QT interval (see Box 24-1). Physicians should avoid prescribing QT-prolonging medications (and discontinue those medications if possible).
b. A list of drugs that are known to prolong the QT interval (see Box 24-1) along with the websites for it (www.qtdrugs.org) should be given to the parents.
c. Catecholamines and sympathomimetic drugs should also be avoided if possible because they can potentially trigger torsades de pointes in patients with LQTS.
d. A no competitive sports policy applies. This is particularly important in patients with LQT1. Physicians should also advise against swimming.
e. Alarm clocks and bedside telephones should be removed because these are known triggers of VT in patients with LQTS.
f. The patients and the parents should be educated about the importance of being compliant with their medication because noncompliance can result in sudden death.
3. Treatment of congenital LQTS: For congenital LQTS, the initial treatment is aimed at interrupting sympathetic input to the myocardium with beta-blockers. An ICD may be needed in some patients who continue to have symptoms while taking beta-blockers. Left cardiac sympathetic denervation is no longer popular because of the availability of other options, such as pacing and ICD.
a. Beta-blockers: The present therapy of choice is treatment with beta-blockers. The protective effect of beta-blockers is related to their ability to reduce both syncope and sudden cardiac death. There is a consensus that all symptomatic children with LQTS should be treated with propranolol or other beta-blockers (e.g., atenolol, metoprolol). Moderate doses of beta-blockers may be better than larger doses because moderate doses lessen the trend toward bradycardia, a known risk factor for sudden death, especially in patients with sinus or AV node disorders.
Beta-blockers are effective in preventing cardiac events in approximately 70% of patients; cardiac events continue to occur despite beta-blocker therapy in the remaining 30% of patients. Even with treatment with beta-blockers, sudden death can occur. More than 80% of cases of sudden death occur while patients are on medications; some of these cases of sudden death are caused by noncompliance.
Whether to start beta-blockers on asymptomatic children with QTc prolongation has been controversial. Any patients who scores 4 or greater on the Schwartz diagnostic criteria should be treated regardless of symptoms. However, it may be prudent to follow asymptomatic children whose QTc intervals are at borderline (0.46–0.47). Symptoms are more likely to occur in patients with QTc intervals longer than 0.48 second. In addition, beta-blocker treatment may be dangerous to some patients with the syndrome because treatment tends to produce bradycardia, a known risk factor for sudden death. Definitive treatment of asymptomatic patients with congenital LQTS has been suggested by Schwartz (1997) in the following circumstances:(1) newborns and infants, (2) patients with sensorineuronal hearing loss, (3) affected siblings with LQTS and sudden cardiac death, (4) extremely long QTc (>0.60 sec) or T-wave alternans, and (5) to prevent family or patient anxiety.
b. Cardiac pacemakers: Implantation of cardiac pacemakers (with continuous ventricular or dual chamber pacing) has been considered helpful because pacing eliminates arrhythmogenic bradycardia which may result from high doses of beta-blockers. In this situation, a maximally tolerated dose of beta-blockers (e.g., atenolol 50–200 mg/day, metoprolol to 50 to 200 mg/day, propranolol 60–120 mg/day) may be used because the pacemaker is expected to prevent bradycardia from occurring. However, pacemakers did not provide complete protection from sudden death (occurring in 16% of patients) because the pacemaker lacks defibrillation capability. In view of the availability of an ICD that can defibrillate as well as pace when bradycardia develops, cardiac pacemakers are no longer popular in this condition.
c. Left cardiac sympathetic denervation: Left cardiac sympathetic denervation is another method to reduce cardiac events in patients who continue to have symptoms by reducing sympathetic discharge. After a high thoracic sympathectomy, there was a dramatic reduction in the incidence of cardiac events, although sudden death still occurred (8%). Beta-blockers are usually continued after the surgical procedure. Because of the availability of other options, such as pacing and ICD, this procedure is rarely performed.
d. ICD: Implantation of an ICD appears to be the most effective therapy for high-risk patients, defined as those with aborted cardiac arrests or recurrent cardiac events despite conventional therapy (with beta-blockers), and those with extremely prolonged QTc intervals (e.g., >0.60 second). An ICD is expected not only to prevent bradycardia (a risk factor, which may result from a high dose of beta-blockers) but also to convert ventricular arrhythmias. ICD has been shown to be safe and effective in preventing sudden death in limited clinical experience with children. These patients should be kept on beta-blockers.
Complications to the ICD placement may include infection, lead fracture and dislodgement, inappropriate discharge, psychiatric sequelae, and electrical storm.
e. Targeted pharmacologic therapy has reported some success. The sodium channel blocker mexiletine was used in patients with mutations in the sodium channel gene SCN5A (LQT3) with significant shortening of the QTc. In patients with LQT2 (with potassium channel abnormalities), potassium supplementation in combination with spironolactone was associated with a significant reduction in QTc and some improvement in T-wave morphology.
f. “Gene-specific” approach: Gene or gene-specific therapy has not gained wide clinical application at this time.
Treatment of Acquired Long QT Syndrome
The management of acquired LQTS involves acute treatment of arrhythmias (with IV magnesium), discontinuation of any precipitating drug, and correction of any metabolic abnormalities (e.g., hypokalemia or hypomagnesemia).
Long QT syndrome is a serious disease, and treatment is at best only partially effective. The prognosis is very poor in untreated patients, with an annual mortality rate as high as 20% and 10-year mortality rate of 50%. Beta-blockers may reduce mortality to some extent, but they do not completely protect patients from sudden death. An ICD appears promising in improving prognosis.
Short QT Syndrome
Recently, a familial short QT interval has been reported to be a cause of sudden death. Short QT syndrome is characterized by a very short QTc (≤300 msec), symptoms of palpitation, dizziness or syncope, and family history of sudden death. The cause of death is believed to be VF. Symptoms including syncope and cardiac arrest occur most often during periods of rest or sleep. Although it usually occurs in the adult (median age 30 years), a sudden cardiac death was observed in infancy. This syndrome is transmitted in an autosomal dominant manner and a few affected families have been identified. Recently, the use of an antiarrhythmic agent, particularly quinidine (which prolongs the QT interval) has been suggested. An ICD may become standard practice.
This inherited arrhythmogenic disorder with a high risk of sudden cardiac death occurring during sleep, resulting from ventricular tachyarrhythmias, appears to be inherited as an autosomal dominant pattern. It is primarily a disease of men seen most commonly in Southeast Asian men (with a reported mean age at sudden death of 40 years). However, this syndrome has also been demonstrated in children and infants. No male preponderance is observed in children, raising a possibility of high level of androgen in the occurrence of the fatal event. Mutations in the sodium channel (SCN5A) appear to be the cause of the condition, at least in 20% of sufferers.
The ECG is abnormal but without demonstrable structural abnormalities of the heart. The ECG typically shows concave ST-segment elevation (>2 mm) with J-point elevation followed by a negative T wave in the right precordial leads (V–V3) and RBBB appearance. This so-called type 1 ECG pattern may be present either spontaneously or after provocation with ajmaline or flecainide. The PR interval is frequently prolonged. Fever is an important precipitating factor of syncope, including one that develops after vaccination. SVT, including atrial flutter, is also frequent. The patient may present with complaints of syncope or palpitations. Most syncope takes place at rest (90%). There may be a family history of sudden death. Cardiac examination findings are usually normal.
The diagnosis is suspected based on the ECG appearance, which may not always be present. The condition can carry a poor prognosis, particularly in those who are symptomatic (i.e., at least a 10% death rate per year). Beta-blockers do not appear to reduce the risk of death in these patients. In many centers, an ICD is standard practice to prevent sudden cardiac death. Hydroquinidine has been shown to be a good alternative to ICD implantation in adult patients, and it appears to be effective in preventing syncope in children also (Probst et al, 2007).