Patrick Hranitzky, MD, Luigi Di Biase, MD, PhD, Rodney Horton, MD, J. David Burkhardt, MD, Andrea Natale, MD
A 37-year-old woman presented with a long-standing history of intermittent palpitations that initially occurred 1 to 2 times per month but which have now progressed to 8 times per month. She was originally diagnosed with anxiety disorder after a 24-hour Holter monitor revealed sinus rhythm and sinus tachycardia, and she was placed on a β-blocker. She had minimal improvement in her symptoms and was intolerant of the β-blocker due to fatigue. One month prior to her initial clinic visit, she had an emergency department (ED) visit where her heart rate was documented at 180 bpm, but was interpreted as sinus tachycardia despite abrupt termination in the ED prior to obtaining a 12-lead ECG. She denied any history of syncope and stated that her palpitations would initially only last seconds, but now last anywhere from 10 to 45 minutes. She cannot identify any specific triggers other than when she assumes an upright posture after leaning over to pick something up. Her past medical history is otherwise unremarkable. An echocardiogram revealed normal left ventricular systolic function and no structural abnormalities. Laboratory values were all within normal limits. A 12-lead ECG was obtained at the initial clinic visit (Figure 8-1) and revealed sinus rhythm without preexcitation. She was scheduled for electrophysiologic testing where she was discovered to have both typical and atypical atrioventricular nodal reentrant tachycardia (AVNRT). A successful radiofrequency ablation was performed of the slow pathway. At follow-up there has been no recurrent arrhythmia or palpitations.
FIGURE 8-1 Initial ECG revealing sinus rhythm with a normal axis and intervals and no evidence of preexcitation.
The history and symptoms are quite consistent with AVNRT, which is typically characterized by an abrupt onset and termination. Episodes may last from seconds to hours. Many patients will report that they will often experience the episodes after they lean over to pick something up. The heart rate is usually rapid, ranging from 150 to 250 beats per minute (bpm). It is usually 180 to 200 bpm in adults; in children, the rate may exceed 250 bpm. In the absence of structural heart disease, it is usually well tolerated.
Common symptoms include the following:
• Chest discomfort/fullness
• Neck pulsations (often due to near simultaneous contraction of the atria and ventricles)
Syncope is rare, but may occur in patients with a rapid ventricular rate or prolonged tachycardia due to poor ventricular filling, decreased cardiac output, hypotension, and reduced cerebral perfusion.
The lack of ECG documentation is not unusual as these episodes are often quite brief. In the atypical form of AVNRT the ECG often reveals a long RP interval and can be misinterpreted as sinus tachycardia; however, the P waves (if discernable) are usually inverted in the inferior leads (II, III, and AVF).
Paroxysmal supraventricular tachycardia (PSVT) has a prevalence of 2.25 per 1000 population and an incidence of 35 per 100,000 person-years. AVNRT is the most common cause of PSVT and is the underlying arrhythmia in 60% of these patients.1-3 Of the patients presenting with AVNRT, 11% to 15% will present with the atypical form.4-7 There is a 3:1 predominance of AVNRT in women.8-10 AVNRT may occur in persons of any age. It is common in young adults, but some patients do not present until their seventh or eighth decade or later. AVNRT is considered less common in newborns and increases in prevalence throughout childhood. However, some reports suggest that AVNRT may be underrecognized in infancy.11
ETIOLOGY AND PATHOPHYSIOLOGY
AVNRT (typical and atypical) occurs due to reentry within two (or more) anatomically and functionally distinct pathways located within the region of the AV node; they are known as the fast and the slow pathways and have different electrophysiologic characteristics. The fast pathway is characterized by its more anterior and superior location within the right atrial septum and its relatively shorter conduction time and longer effective refractory period (ERP), whereas the slow pathway is typically located inferiorly and posteriorly and has a relatively longer conduction time and an ERP that typically is short when compared to fast pathway ERP (Figure 8-2). There may be multiple slow pathways in as many as 5% of patients with AVNRT.12 As previously mentioned, these separate pathways are usually anatomically discrete, but dual AV nodal physiology is a common finding during EP studies and is not synonymous with AVNRT. There are two forms of AVNRT that are usually described and include the typical form (ie, slow-fast) and the atypical form (ie, fast-slow or slow-slow), referring to the anterograde-retrograde conduction over the pathways during tachycardia. In the typical form the conduction moves in the anterograde direction through the slow pathway and in the retrograde direction through the fast pathway. In the atypical form, the conduction moves either anterograde in the fast pathway and retrograde in the slow pathway, or anterograde and retrograde through two slow pathways. Either of these scenarios can result in a long RP interval during tachycardia.6,7,13-18Typically, retrograde CS activation is concentric (Figures 8-3A and B and 8-4) as the inputs of the fast and slow pathways usually reside at the anterior/superior right atrial septum and the inferior/posterior right atrial septum, respectively. However, eccentric CS activation is seen in approximately 5% of all AVNRT and 32% to 80% of atypical AVNRT due to leftward inferior nodal extensions which are densely populated with transitional cells.7,15,18 In most patients with AVNRT, the tachycardia is initiated when a premature complex (atrial or ventricular) is blocked in the one pathway (usually the fast pathway with a longer refractory period) and conducts in the other pathway (usually the slow pathway with a shorter refractory period). While the impulse conducts in one pathway, the other pathway recovers so that the impulse can conduct in the opposite direction thus completing the loop of reentry.
FIGURE 8-2 Schematic representing an RAO projection of the right atrial septum. The fast and slow pathways are depicted in red and yellow, respectively. The dotted red line represents the borders of the triangle of Koch.
FIGURE 8-3 Programmed electrical stimulation is performed from the high right atrial catheter. Simultaneous recordings are taken from the proximal and distal high right atrium (HRA p, HRA d), the proximal, mid, and distal His bundle (HIS p, HIS m, HIS d), the proximal to distal coronary sinus (CS_7,8 to CS_d), and the proximal and distal right ventricular apex (RVA p, RVA d). (A): Decremental conduction has occurred down the fast pathway, and the AH interval measures 246 ms. (B) After a 10-ms decrement in the S2, there is a 64 ms “jump” in the AH interval consistent with dual AV nodal physiology.
FIGURE 8-4 Typical AVNRT in this patient as evidenced by a long AH interval and short VA interval during tachycardia.
The definitive diagnosis of AVNRT (typical or atypical) can only be made with electrophysiologic testing. The hallmark of AVNRT is the presence of dual AV node physiology which is defined as a >50 ms “jump” in the atrial-His interval (AH) between a 10 ms S1S2 decrement during atrial-programmed stimulation. This is evident in this patient (see Figures 8-3A and B and 8-4). An HA jump during ventricular pacing or programmed stimulation is frequently seen in patients with atypical AVNRT and was present in this patient (Figure 8-5).
FIGURE 8-5 During a ventricular pacing drive in this patient there is a 72-ms HA “jump” indicating a switch from conduction up the retrograde fast pathway to the retrograde slow pathway. Note that the earliest atrial activation switches from the proximal His (HIS p) to the proximal CS (CS_7,8). The asterisks on the first and last beat indicate the retrograde His deflection.
Approximately 85% of patients presenting with atypical AVNRT will also have inducible typical AVNRT.7 AVNRT can be initiated by ectopic atrial or ventricular beats. The typical form is usually initiated by atrial ectopic beats that block in the fast pathway and travel anterograde via the slow pathway. The resulting arrhythmia results in near simultaneous activation of the atria and ventricles (see Figure 8-4). The atypical form can be induced with atrial or ventricular extrastimuli or may occur during an atrial or ventricular pacing drive (Figure 8-6). Some of the distinguishing features of the various forms of AVNRT are as follows7,17:
FIGURE 8-6 Initiation of atypical AVNRT during an atrial pacing drive. Notice the long AH interval on the HIS catheter and the longer VA time when compared to the typical form in Figure 8-4.
Slow-fast → VA <60 ms and AH > HA during tachycardia, earliest retrograde atrial activation at the His bundle
Fast-slow → often no jump seen, VA >60 ms and HA > AH during tachycardia, earliest retrograde atrial activation near CS os
Slow-slow → AH jump often seen, VA >60 and AH > HA during tachycardia, earliest retrograde atrial activation near CS os
There is frequently quite a bit of cycle length variability during atypical AVNRT, which may be due to multiple slow pathways or presence of an upper or lower common pathway6,12,15,16 and is seen in this patient (Figures 8-7 and 8-8).
FIGURE 8-7 Beat-to-beat variations in the VA time during atypical AVNRT in this patient. Note the long VA time (>100 ms).
FIGURE 8-8 Example of a switch from the retrograde slow pathway to the retrograde fast pathway during tachycardia. Note the difference in VA times between the beats annotated with asterisks.
AVNRT must be differentiated from other forms of supraventricular tachycardia (SVT) including:
• Atrial tachycardia
• Orthodromic reciprocating tachycardia
• Sinus tachycardia
• Sinus node reentry
Differentiation of these forms of SVT is done during electrophysiologic testing. Sinus mechanisms as well as nonseptal atrial tachycardias demonstrate the earliest atrial activation away from the septum and thus can usually be excluded by looking at atrial activation.
A septal atrial tachycardia with 1:1 conduction to the ventricle can mimic AVNRT and can usually be excluded with ventricular entrainment. A VAV response to ventricular entrainment will typically exclude atrial tachycardia as the mechanism.
Orthodromic reciprocating tachycardia using a septal accessory pathway is a bit more challenging and can be differentiated using one or more of the following criteria19-22:
• SA-VA (if >110 ms → AVNRT, if <100 ms → ORT via septal AP)
Where SA is the interval between the stimulus artifact and the atrial signal on the HRA catheter during RV pacing and VA is the interval between the ventricular signal on the RV catheter and the atrial signal on the HRA catheter during tachycardia.
• PPI-TCL (if >115 ms → AVNRT)
Where PPI is the postpacing interval measured on the RV catheter between the last entrained beat and the first return beat and the TCL is the tachycardia cycle length.
• Introduction of a ventricular extrastimulus during His bundle refractoriness → if no atrial preexcitation this is consistent with AVNRT
• Preexcitation index (if >100 → AVNRT)
Preexcitation index (PI) is the V1V1 interval during tachycardia minus the longest V1V2 at which atrial preexcitation occurs
PI = V1V1 - V1V2
• Para-Hisian pacing (if nodal response → AVNRT)
Demonstrations of these maneuvers in this patient are seen in Figures 8-9 to 8-11.
FIGURE 8-9 Entrainment from the RV catheter reveals a V-A-H-V response excluding an atrial tachycardia. The postpacing interval minus the tachycardia cycle length is 146 ms, which is consistent with AVNRT.
FIGURE 8-10 This figure demonstrates a His-synchronous ventricular extrastimulus during tachycardia. There is no change in the atrial cycle length as annotated with the asterisks. In addition, the preexcitation index is calculated and is 110. Both findings are consistent with AVNRT.
FIGURE 8-11 Para-Hisian pacing is performed on the patient. There is a nodal response as the SA interval varies by greater than 60 ms between His capture (left asterisk) and RV capture (right asterisk).
Relaxation or vagal maneuvers alone may terminate an episode of AVNRT. The successful management of an acute episode, however, depends on the symptoms, the presence of underlying heart disease, and the natural history of previous episodes.
• Vagal maneuvers
Carotid sinus massage (avoid if known cerebrovascular disease)
Dive reflex → submerge face in cold water
■ These maneuvers can also be tried after each pharmacologic approach
• Pharmacologic intervention
Intravenous adenosine 6-12 mg
■ eg, metoprolol 5 mg (may be repeated if necessary)
Intravenous calcium channel blocker
■ eg, erapamil 5 mg
■ eg, diltiazem 10 mg
• Synchronized direct current cardioversion
Used when the patient has hemodynamic compromise or if drug conversion fails and the patient continues to be symptomatic
100 joules usually adequate
■ Rarely necessary for AVNRT
Prevention can be obtained in part by avoidance of exacerbating factors or triggers. Some of these include:
• Foods high in theobromine (eg, coffee, tea, chocolate)
• Pharmacologic stimulants (eg, pseudoephedrine, theophylline)
Long-term management is usually accomplished with pharmacologic or catheter-based therapy.
• Pharmacologic (given orally)
β-Blockers (eg, metoprolol, atenolol)
Calcium channel blockers (eg, verapamil, diltiazem)
Digitalis (rarely used)
(The previous three agents must be given with caution in patients with prolonged PR intervals to avoid high degree AV block. However, the fast pathway inserts into the central fibrous body distal to the AV node and is less subject to beta-blockers and calcium channel blockers)
Antiarrhythmic drugs (rarely used)
■ Sodium channel blockers (eg, flecainide, propafenone)
• Flecainide depresses conduction in the fast pathway
■ Potassium channel blockers (eg, sotalol)
• More drug intolerance
• Considered drug of choice for refractory AVNRT in pregnant women not undergoing catheter ablation
***The fast pathway inserts into the central fibrous body distal to the AV node, is less subject to β-blockers and calcium channel blockers, and has less decremental conduction.
• Catheter-based therapy
The cornerstone of therapy for patients with symptomatic AVNRT is catheter ablation. This procedure is highly effective (91%-99% acute success for all forms of AVNRT), safe (<2% serious complication risk), and provides a durable, long-term cure.
■ AV block in <1%.
■ Typically involves ablation of the slow pathway.
• Successful slow pathway site usually results in accelerated junctional rhythm when radiofrequency energy is applied (Figure 8-12).
FIGURE 8-12 Accelerated junctionals during radiofrequency application at the slow pathway.
■ Fast pathway ablation is done superior to the Tendon of Todaro.
• Less frequently performed due to higher recurrence rates and greater risk of AV block.
■ Single echos following catheter ablation are not associated with higher recurrence rate and occur in ~25% following catheter ablation.
■ Ablation may be required inside the CS at the site of earliest retrograde atrial activation.15,16,18
• More frequent in atypical form.
• Ablation at these sites may not produce junctional rhythm.
• In rare cases, ablation on the left side of the septum maybe required (Figure 8-13).
FIGURE 8-13 Example of a rare case where ablation on the left side of the septum was required to achieve slow pathway ablation and AVNRT noninducibility. Top left shows a 3-D map cloud of the His bundle (yellow dots) and the left-sided lesions (red dots) where the slow pathway was ablated. Top right image shows a fluoroscopy image of the ablation catheter in the left side of the septum at the successful site. The bottom image shows junctional rhythm during ablation.
Usually junctional rhythm is seen at this site.
■ Recurrence rate is higher for atypical versus typical AVNRT (3%-8.3% versus 0.5%),8-10,17,23-25
■ Safe and effective in patients with prolonged PR interval.26-29
■ Cryothermal ablation may be advantageous in patients with a smaller triangle of Koch.30
• Less destruction of tissue architecture, thus should AV block occur it is likely reversible.
• Cryoablation results in adherence of catheter to heart tissue providing greater stability.
• No junctional rhythm is seen during cryoablation.
• Similar efficacy as radiofrequency.
• Inappropriate sinus tachycardia is seen in ~10% following catheter ablation of the slow pathway.
Usually persists for 1 to 6 weeks, up to 6 months.
The prognosis for patients with AVNRT is usually good in the absence of structural heart disease. Most patients respond to medications to prevent recurrence or to catheter ablation, which is approximately 95% curative and has a low risk of complications. Catheter ablation is the preferred method of treatment for most patients.
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