Color Atlas and Synopsis of Electrophysiology, 1st Ed.


Aman Chugh, MD


A 55-year-old man was referred to for electrophysiologic evaluation for Wolff-Parkinson-White (WPW) syndrome. Although he had noted palpitations for many years, they were initially brief and responded to vagal maneuvers. He had previously undergone an attempt at catheter ablation of a right-sided accessory pathway (AP) at another institution. The procedure was unsuccessful and was terminated secondary to transient atrioventricular (AV) nodal conduction block. An event monitor after the ablation procedure revealed supraventricular tachycardia (SVT) at a heart rate of 190 bpm with a right bundle branch block pattern. There was no history of syncope or a family history of unexplained sudden death. There is a remote history of cardiomyopathy, which was attributed to viral etiology, and has since resolved.

The 12-lead electrocardiogram (ECG) showed sinus rhythm with ventricular preexcitation. The pattern of preexcitation was a bit unusual but was compatible with a right anterior free wall accessory pathway (AP), likely inserting superiorly on the tricuspid annulus (Figure 9-1). The electrophysiology procedure was performed under conscious sedation. In the baseline state, no supraventricular tachycardia could be induced. With isoproterenol infusion, SVT at a cycle length of 340 ms could be easily induced (Figure 9-2). Diagnostic maneuvers confirmed the presence of orthodromic atrioventricular reciprocating tachycardia (AVRT) (see Figures 9-2 to 9-4). The earliest atrial activation during AVRT was at the anterior tricuspid annulus (Figure 9-5). Mapping during sinus rhythm also revealed that the earliest ventricular activation occurred at the anterior tricuspid annulus (approximately 12 o’clock in the left anterior oblique view) (Figure 9-6). Catheter stability at the target site was suboptimal, as the catheter would frequently dislodge into the right atrial appendage or into the right ventricle. Radiofrequency energy was applied during sinus rhythm with an irrigated-tip ablation catheter (Surround Flow, Biosense-Webster, Diamond Bar, CA) at 35 watts, which only had a transient effect on the AP. Increasing the power to 45 watts finally eliminated preexcitation (Figures 9-7 to 9-9), and also rendered the tachycardia noninducible despite isoproterenol infusion. The patient has been free of palpitations for 2 years in the absence of rate- or rhythm-controlling medications.


FIGURE 9-1 A 12-lead electrocardiogram showing sinus rhythm with evidence of ventricular preexcitation. The delta wave is negative in lead V1 (arrow) and the transition (asterisk) in the precordial leads occurs between leads V3and V4, compatible with a right free wall accessory pathway.


FIGURE 9-2 Induction of orthodromic atrioventricular reciprocating tachycardia (ORT) with programmed atrial stimulation. After a drive train at 400 ms (S1), a single atrial extrastimulus (S2) is introduced at 210 ms. The extrastimulus blocks in the accessory pathway and conducts over the specialized conduction system, yielding supraventricular tachycardia. The mechanism of a tachycardia whose induction depends upon conduction block in the accessory pathway is very likely to be ORT. Abl = ablation; CS = coronary sinus; H His; RV = right ventricle.


FIGURE 9-3 Response to a His-synchronous premature ventricular complex (“S”) during supraventricular tachycardia. The premature ventricular complex (PVC) results in advancement of the subsequent atrial electrogram (from 340 to 270 ms), which confirms the presence of an accessory pathway.


FIGURE 9-4 The ventriculoatrial (VA) time during supraventricular tachycardia with a right bundle branch block pattern is 185 ms, as compared with 130 ms during the narrow QRS complex (asterisk). This observation proves that the mechanism of the tachycardia is orthodromic reciprocating tachycardia (ORT) utilizing a right free wall accessory pathway. (Although the QRS morphology during tachycardia was that of a right bundle branch block pattern, there were several instances in which a narrow QRS complex appeared spontaneously. The narrow QRS complex was always followed by a shorter VA time.)


FIGURE 9-5 Prior to radiofrequency ablation, ORT was induced to ensure that a His potential was not present at target site, which had afforded the earliest ventricular activation during sinus rhythm. Note the possible accessory pathway potential (APP) between the ventricular and atrial electrograms.


FIGURE 9-6 Bipolar electrogram during sinus rhythm, recorded by the ablation (Abl) catheter at the site where radiofrequency energy eliminated conduction over the accessory pathway. The activation time (V-QRS) is relatively modest, but there may also be an accessory pathway potential (APP) present.


FIGURE 9-7 Fluoroscopic view showing the position of the ablation catheter (arrow) at the site on the tricuspid annulus where radiofrequency ablation eliminated preexcitation. AP = posterior; LAO = left anterior oblique.


FIGURE 9-8 Activation map of the right atrium (RA) showing that the earliest ventricular activation (arrow) was found at the anterior tricuspid annulus (12 o’clock). A His potential could be recorded relatively diffusely over the septal region. IVC inferior vena cava; RAA = right atrial appendage.


FIGURE 9-9 Initiation of radiofrequency (RF) energy at the anterior tricuspid annulus readily eliminates preexcitation (asterisk). There was no longer evidence of retrograde activation over the pathway or tachycardia.


AVRT is the most common sustained arrhythmia in patients with WPW syndrome. It is the second most common arrhythmia mechanism in patients presenting for catheter ablation of paroxysmal SVT. In patients undergoing catheter ablation of AVRT/WPW syndrome, right free wall APs account for about 15% of cases. Left free wall and septal accessory pathways are found in about 60% and 25% of patients, respectively. The prevalence of the WPW pattern, that is, evidence of ventricular preexcitation without arrhythmic symptoms, is estimated to be about 2 to 4 per 1000 individuals.1


Ventricular preexcitation is mediated by anomalous muscular tracts that connect the atrium and ventricle along the mitral or tricuspid annulus. Patients with WPW syndrome are usually otherwise healthy, and no significant abnormalities are found on cardiovascular testing. Occasionally, AVRT may occur in the context of overt heart disease, such as Ebstein anomaly or hypertrophic cardiomyopathy.

Orthodromic AVRT (or ORT) is due to a macroreentrant circuit that involves anterograde activation of the specialized conduction system and retrograde activation over the AP. During ORT, typically a narrow complex tachycardia is inscribed on the 12-lead ECG, with retrograde P waves obscured by the ST segment. In cases of a slowly conducting AP, retrograde conduction may be delayed such that the P wave is found after the T wave, resulting in a long RP tachycardia. Classically, such pathways insert near the posteroseptal area, resulting in negative P waves in the inferior leads. These slowly conducting pathways may result in frequent or nearly incessant bouts of tachycardia, ie, permanent junctional reciprocating tachycardia (PJRT). Frequent tachycardia may be associated with left ventricular dysfunction, which resolves after elimination of the AP.

Much less commonly, one may encounter antidromic AVRT or ART during which the circus movement involves anterograde conduction along the AP and retrograde conduction over the normal conduction system or another AP. The ECG in this case shows a wide QRS-complex tachycardia, the morphology of which does not resemble a typical bundle branch block pattern. The QRS complex during ART is maximally preexcited and may be mistaken for ventricular tachycardia.

One may encounter less common forms of AVRT mediated by APs that do not insert along the AV ring. These include atriofascicular, nodofascicular, and nodoventricular pathways. Conduction along these pathways is decremental and may not be apparent on the resting ECG. Atriofascicular pathways connect the atrium to the right bundle branch and are incapable of retrograde conduction. Nodofascicular or nodoventricular pathways connect the AV node and proximal right bundle or the right ventricular myocardium, respectively. Fasciculoventricular APs connect the distal right bundle branch to the ventricular myocardium. As such, the ECG shows only subtle preexcitation in patients with a fasciculoventricular pathway. These pathways are thought not to participate in AVRT but may act as bystanders during supraventricular tachycardia.


In a patient with a history of paroxysmal supraventricular tachycardia, the presence of preexcitation on the 12-lead ECG during sinus rhythm increases the odds that the mechanism of the tachycardia is ORT as opposed to atrioventricular nodal reentrant tachycardia (AVNRT) or atrial tachycardia (AT). Often, the 12-lead ECG during SVT does not contain clues that may be diagnostic of ORT. If a narrow complex tachycardia is initiated after anterograde conduction block over the AP, the mechanism is very likely to be ORT. Prolongation of the RP interval during bundle branch block is diagnostic of ORT utilizing an AP located ipsilateral to the blocked bundle branch.

Diagnosis of ORT is usually made in the electrophysiology laboratory. Development of a bundle branch block during ORT provides important diagnostic information. Prolongation of the ventriculoatrial (VA) interval by >30 ms during SVT with a bundle branch block is diagnostic of ORT utilizing an AP that is located ipsilateral to the blocked bundle branch (see Figure 9-4). There is less of an increment in the VA interval with development of a bundle branch block during ORT mediated by a septal accessory pathway. Insertion of a His-synchronous premature ventricular complex (PVC) during supraventricular tachycardia may provide mechanistic insights. Advancement or delay of the subsequent atrial electrogram confirms the presence of an AP. Termination of the tachycardia without affecting the atrial electrogram proves that the mechanism is ORT (Figure 9-10). Differentiation between typical AVNRT and ORT is straightforward. However, differentiation between atypical AVNRT and ORT using a septal AP usually requires a few diagnostic maneuvers. After ruling out the possibility of atrial tachycardia (by observing an A-V response after cessation of ventricular overdrive pacing that entrains the tachycardia), the difference between the postpacing interval and the tachycardia cycle length helps discern between the two possibilities. If the difference is less than 115 ms, the diagnosis is very likely to be ORT.2 Even if the tachycardia terminates during the entrainment attempt, the maneuver can still be helpful. The number of fully paced ventricular complexes (as compared to fused complexes) required to advance the atrial electrogram to the paced rate can be helpful in differentiating between AVNRT and ORT. A prior study showed that ORT could be reset with one fully paced complex as compared with a mean of 3.7 beats for AVNRT.3 The delta-AH (ie, the difference between the atrio-His (AH) interval during pacing from the high right atrium at the tachycardia cycle length and that during tachycardia) can be quite helpful in differentiating between atypical AVNRT and ORT using a septal AP. During ORT (and atrial tachycardia), the delta-AH is minimal. However, the AH interval during pacing is longer than that during AVNRT owing to sequentialactivation of the atrium and specialized conduction system during atrial pacing. During atypical AVNRT, after retrograde activation over the slow pathway, there is nearly simultaneous activation of the atrium and His bundle, leading to a pseudo-AH interval. A prior study showed that the delta-AH interval was >40 ms in 80% of patients with atypical AVNRT whereas it was never >10 to 20 ms in patients with ORT (or AT).4


FIGURE 9-10 In another patient with multiple right free wall accessory pathways, a His-synchronous PVC (“S”) terminates supraventricular tachycardia, proving that the mechanism of the tachycardia is ORT.

Para-Hisian pacing is a useful maneuver to determine whether retrograde activation is occurring over the AV node or septal accessory pathway. High-output pacing is performed from the His bundle catheter, and the output is then gradually decreased. With high output pacing, both the ventricular myocardium and the His bundle (V+H) are captured resulting in a relatively narrow QRS. As the output is lowered, only the ventricular myocardium (V) is captured resulting a wider QRS. Then the stimulus-to-atrial electrogram interval (SA) during the narrow and wide QRS is compared. If the SA interval is the same, then retrograde conduction is occurring over a septal AP since pathway activation depends only upon ventricular activation. If the SA interval with a narrow QRS paced complex is shorter, then retrograde activation is occurring over the AV node. The reason that the SA is shorter with the narrow QRS paced complex (ie, during V + H capture) is that retrograde conduction needs only to commence from the proximal aspect of the specialized conduction system to the atrium. With the wide QRS paced complex (ie, during ventricular capture only), the SA interval is longer because atrial activation depends upon a series of steps: ventricular activation, followed by engagement of the Purkinje network, His bundle, and then the AV node. Para-Hisian pacing is also helpful in determining whether a septal AP has been successfully ablated.


Asymptomatic Individuals with Preexcitation

An Italian study from a few years ago revealed that the prognosis for most asymptomatic persons with incidentally discovered preexcitation is good.5 Specifically, 293 patients with a WPW pattern underwent a diagnostic electrophysiologic evaluation. Over a median follow-up of 67 months, 262 individuals (89%) remained asymptomatic. AVRT was noted in 14 persons (5%) during follow-up. Potentially life-threatening arrhythmias were noted in 17 patients (6%). Although one of the latter individuals suffered an aborted cardiac arrest, there were no deaths. The investigators identified young age, inducibility, and a short AP anterograde effective refractory period (≤250 ms) as predictors of potentially life threatening events. Importantly, spontaneous disappearance of the delta wave on the ECG was noted in 30% of the study population. Since the event rate of individuals with incidental preexcitation is low, a conservative approach is reasonable in most situations. The care of pilots, bus drivers, athletes, or others in whom ventricular preexcitation is discovered on routine testing (ie, in the absence of symptoms) should be individualized.

Medical Management

The acute management of a patient with AVRT is similar to that of a patient with paroxysmal supraventricular tachycardia in general. In the hemodynamically stable patient, intravenous adenosine is indicated to terminate the tachycardia. In the acute setting (eg, a patient presenting to the emergency department with SVT) it may not be known whether the patient has manifest preexcitation during sinus rhythm. Since adenosine administration may result in atrial fibrillation (AF) in about 10% of patients, in the patient with underlying preexcitation, AF may result in rapid conduction over the accessory pathway. The ECG may show an irregular, wide complex tachycardia (owing to varying fusion between activation over the AP and the AV node) with heart rates >200 bpm. This may be met with hemodynamic collapse and/or ventricular fibrillation requiring immediate transthoracic cardioversion/defibrillation. Although the emergency department is well prepared to deal with such a situation, other medical facilities (eg, student health services) may be less prepared should adenosine administration result in preexcited AF.

Patients with hemodynamically stable antidromic AVRT may be treated with intravenous ibutilide or procainamide. Patients presenting with tachycardia and hemodynamic instability or serious symptoms such as angina or heart failure should undergo prompt cardioversion.


Patients with recurrent SVT that is thought to be due to AVRT or AF in the setting of ventricular preexcitation should undergo catheter ablation given the favorable risk/benefit ratio. The pathway responsible for AVRT can be eliminated in >95% of patients with a low risk of complications, such as serious bleeding, thromboembolism, and stroke (all <1%). Patients with a septal AP need to be counseled that the risk of AV block requiring a pacemaker is obviously higher than with free wall APs, but the risk is still small (about 1%). In a case of a para-Hisian accessory pathway (Figure 9-11), AV block may be avoided by targeting the noncoronary cusp of the aortic valve (Figure 9-12). In patients with a midseptal accessory pathway, cryomapping may be helpful in determining whether the pathway can be safely ablated. In some cases where the AP cannot be ablated without injury to the conduction system, patients are best treated with a combination of rhythm- and rate-controlling medications. Posteroseptal APs may be ablated outside the ostium of the coronary sinus, from the inferoseptal mitral annulus or from within the coronary sinus or one of its ventricular branches.6 Varying VA times may also reveal the presence of multiple accessory pathways responsible for ORT (Figure 9-13).


FIGURE 9-11 Catheter ablation of a para-Hisian accessory pathway. Shortly after initiation of RF current, there is a loss of preexcitation (*). A small His electrogram is apparent on the electrogram recorded by the distal bipole of the ablation catheter. Note that there is no RF-induced junctional ectopy, which would prompt the immediate discontinuation of RF energy.


FIGURE 9-12 An aortogram showing the location of the noncoronary cusp (NCC), where radiofrequency ablation was applied to eliminate a para-Hisian accessory pathway. “L” and “R” refer to left and right coronary cusp.


FIGURE 9-13 Example of multiple pathways responsible for ORT. Programmed atrial stimulation induced a tachycardia with varying ventriculoatrial (VA) times. The varying VA intervals were due to two different tachycardias, and the mechanisms of both were due to ORT. The longer VA interval corresponded to retrograde activation over a right lateral accessory pathway, and the shorter over a right anterior accessory pathway.


Some studies have suggested that the recurrence rate following catheter ablation of right free wall APs may be greater than that of left free wall accessory pathways. In most cases, this is probably related to suboptimal contact/stability during ablation of APs located at the lateral, anterolateral, and anterior aspects of the tricuspid annulus. Utilization of a long sheath is helpful in order to improve contact at these locations. Deflectable sheaths (eg, Agilis, St. Jude Medical) may also be useful in difficult cases. To approach the free wall aspect of the tricuspid annulus, the operator carefully advances the catheter and sheath assembly from the 6 o’clock position superiorly in the left anterior oblique view. The tension on the handle is released, and the assembly is rotated toward the target site on the annulus, while seeking an atrial and ventricular electrogram on the distal bipole of the ablation catheter. Some have also advocated a superior mapping approach from the right internal jugular vein in order to improve stability in this region. Tachycardia termination during radiofrequency ablation leads to a sudden change in the heart rate, and may lead to catheter dislodgement and suboptimal lesion delivery. Ventricular pacing at a rate approximating that of the tachycardia may help prevent the sudden change in rate and abrupt catheter dislodgement from the target site. It should also be noted that right free wall APs may insert at some distance away from the annulus, even as high as the base of the right atrial appendage. Three-dimensional mapping may be helpful in mapping challenging right free wall APs.7


Patients who are diagnosed with paroxysmal SVT and whose ECG during sinus rhythm reveals evidence of ventricular preexcitation (ie, WPW syndrome) should be advised about vagal maneuvers that may help abort long episodes of tachycardia. They should also be told about the risk of syncope and sudden death, although the risk is small. Patients with paroxysmal SVT that is thought to be mediated by an accessory pathway should be referred to an electrophysiologist to discuss the risks and benefits of catheter ablation. Patients should be told that catheter ablation successfully eliminates that AP responsible for ORT with a low risk of complication. The risk of AV conduction block should be discussed with patients with septal APs. Patients in whom ventricular preexcitation is discovered incidentally should be advised that a conservative approach is reasonable since of risk of syncope and sudden death is low.


1. Klein GJ, Gula LJ, Krahn AD, Skanes AC, Yee R. WPW pattern in the asymptomatic individual: has anything changed? Circ Arrhythm Electrophysiol. 2009;2(2):97-99.

2. Michaud GF, Tada H, Chough S, et al. Differentiation of atypical atrioventricular node re-entrant tachycardia from orthodromic reciprocating tachycardia using a septal accessory pathway by the response to ventricular pacing. J Am Coll Cardiol. 2001;38:1163-1167.

3. Dandamudi G, Mokabberi R, Assal C, et al. A novel approach to differentiating orthodromic reciprocating tachycardia from atrioventricular nodal reentrant tachycardia. Heart Rhythm. 2010;7:1326-1329.

4. Man KC, Niebauer M, Daoud E, et al. Comparison of atrial-His intervals during tachycardia and atrial pacing in patients with long RP tachycardia. J Cardiovasc Electrophysiol. 1995;6:700-710.

5. Santinelli V, Radinovic A, Manguso F, et al. Asymptomatic ventricular preexcitation: a long-term prospective follow-up study of 293 adult patients. Circ Arrhythm Electrophysiol. 2009;2:102-107.

6. Stavrakis S, Jackman WM, Nakagawa H, et al. Risk of coronary artery injury with radiofrequency ablation and cryoablation of epicardial posteroseptal accessory pathways within the coronary venous system. Circ Arrhythm Electrophysiol. 2014;7:113-119.

7. Long DY, Dong JZ, Liu XP, et al. Ablation of right-sided accessory pathways with atrial insertion far from the tricuspid annulus using an electroanatomical mapping system. J Cardiovasc Electrophysiol. 2011;22:499-505.