Steven J. Kalbfleisch, MD
A 29-year-old man presented to the emergency department with complaints of progressive shortness of breath and occasional palpitations. The presenting ECG is shown in Figure 12-1. The patient was admitted to the hospital for further cardiac workup. An echocardiogram demonstrated a reduced left ventricular ejection fraction of 35% with no other significant abnormalities. While on telemetry, the patient was noted to have intermittent termination of his arrhythmia with brief periods of normal sinus rhythm and prompt recurrence of tachycardia. During the periods of sinus rhythm, a 12-lead ECG was obtained and was normal without evidence of preexcitation. The patient reported that his symptoms of palpitations dated back a few years, but they had become more persistent over the past 6 months.
FIGURE 12-1 Twelve-lead ECG demonstrating a long RP supraventricular tachycardia at a rate of 130 bpm. Note the deeply inverted P waves in the inferior leads (II, III, aVF), which clearly distinguish this from sinus tachycardia.
He was referred to the electrophysiology service for evaluation of possible catheter ablation of his arrhythmia. An electrophysiology study was performed which demonstrated an incessant atrioventricular (AV) reentrant tachycardia utilizing a decremental posteroseptal accessory pathway (AP) with a long conduction time. Mapping during tachycardia revealed a broad atrial insertion of the pathway spanning across the posteroseptal space from the mouth of the coronary sinus ostium to the left posteroseptal region of the mitral annulus. Ablation at the coronary sinus ostium and posteroseptal mitral annulus via a transeptal approach terminated the tachycardia and eliminated pathway conduction (Figure 12-2). The patient was seen at follow-up 2 months after the ablation procedure. His symptoms had abated, no further tachycardia was noted on Holter monitoring, and there was normalization of the ejection fraction by echocardiography.
FIGURE 12-2 Shown in this figure are the electroanatomic map and fluoroscopic view of the ablation procedure. The blue and red arrows point to the ablation sites at the coronary sinus ostium (CS os) and left posteroseptal region, respectively. The pathway had a broad atrial insertion requiring ablation on both sides of the posteroseptal space for elimination.
This patient had a classic case of permanent junctional reciprocating tachycardia (PJRT). PJRT is a long RP tachycardia that is associated with deeply negative P waves in the inferior leads and is caused by an “atypical” AP.
A “typical” AP is one that has a rapid and fixed conduction time and is located along the AV groove anywhere except at the anteromedial mitral annulus where the aortic valve sits.1,2 Both the atrial and ventricular insertions of a typical AP are close to the mitral or tricuspid valve annulus.
A simple definition of an “atypical” accessory pathway is any AP that has either unusual conduction characteristics (long conduction time or decremental conduction) or an unexpected location.
Pathways with unusual locations such as those coursing through the fibrous region of the aortomitral continuity, traveling around the aortic valve through the left anteroseptal region or connecting directly between the right or left atrial appendage and the ventricles, are very rare and have been reported only in small series or isolated case reports.3-6
During electrophysiologic testing in patients with an AP, the probability of finding decremental pathway conduction is not rare and in one series was reported to be on the order of 10%.7 In the majority of cases from this series, the pathways had relatively short conduction times with decremental conduction in the retrograde direction. The most common location for a decremental pathway was in the posteroseptal region (47%).7
Two specific variants of atypical APs occur frequently enough and have consistent enough characteristics to form distinct clinical subgroups. These are pathways associated with PJRT and Mahaim fibers. Both of these pathways types are fairly rare, and each type accounts for less than 2.5% of pathways in large ablation series of adult patients.7-9
The locations of the most common sites for atypical APs are shown in Figure 12-3.
FIGURE 12-3 The most common locations for atypical accessory pathways are shown in the areas outlined with a dashed line. The most common variant of Mahaim fibers, atriofascicular pathways, are located along the anterolateral tricuspid annulus. The posteroseptal region is the usual location for pathways responsible for PJRT and is the most common location for any pathway with decremental conduction properties. Pathways located in the aortomitral continuity are atypical by virtue of their location; very rarely pathways can traverse this area, which is a region of dense fibrous tissue that separates the left atrium and ventricle.
ETIOLOGY AND PATHOPHYSIOLOGY
APs of the PJRT type have very long conduction times with decremental conduction properties. These pathways essentially only conduct in the retrograde direction and are therefore concealed on the baseline ECG. They are usually located in the posteroseptal region close to the coronary sinus ostium (75% of patients), but pathways with these conduction characteristics can also be found in other locations.10
The characteristic tachycardia associated with this type of pathway is a long RP tachycardia with deeply inverted P waves in the inferior leads. The P wave timing is due to the long VA conduction time of the pathway and the morphology of the P wave is the result of earliest activation of the atrium being near the coronary sinus ostium. These tachycardias can be paroxysmal in nature, but in a high percentage of patients they can be nearly incessant. In one large ablation series of 49 patients with PJRT type pathways, the tachycardia was incessant in 47% and led to a tachycardia-induced cardiomyopathy in 16% of cases.10
Mahaim fibers, on the other hand, are APs with long conduction times and decremental conduction properties that conduct in the antegrade direction only. These pathways are typically very long in their course, usually originating in the anterolateral right atrium and inserting in the distal right ventricle near the right bundle branch.11 They behave like a duplicate AV node and His-Purkinje system and are referred to as atriofascicular pathways. The tachycardias associated with these pathways are wide complex preexcited tachycardias with a left bundle branch block configuration (Figure 12-4).
FIGURE 12-4 This 12-lead ECG was recorded during an episode of antidromic tachycardia in a patient with a Mahaim fiber. Because these pathways insert distally into the right ventricle close to the location of the right bundle branch, they have a left bundle superior axis morphology that can closely mimic left bundle branch aberration. The recording was performed at 50 mm/sec, twice the normal sweep speed.
Diagnosing an AP-mediated tachycardia usually relies on invasive electrophysiologic testing. Determining that the AP has atypical decremental conduction properties is dependent on demonstrating an increase in conduction time across the AP of at least 30 msec in the antegrade or retrograde direction with pacing maneuvers.7 Unlike typical APs, pathways with decremental properties are often sensitive to adenosine, and pathway conduction can be blocked or slowed after its administration.7
The diagnosis of a PJRT-type tachycardia should be considered whenever a long RP (RP interval > PR interval) tachycardia is demonstrated on the ECG. Many variants of the initially described tachycardia have been reported, and the term PJRT is now often loosely used to describe any orthodromic reciprocating tachycardia caused by an AP with a very long conduction time.
The pacing maneuver most commonly performed during tachycardia to prove the existence of an extranodal pathway and determine if the pathway participates in the tachycardia is delivery of a PVC when the His bundle is refractory (Figure 12-5). If a PVC is delivered when the His bundle is refractory and the atrial timing is advanced, then an extranodal AP has to be present. If the tachycardia is reset by advancing the atrial activation, then the pathway participates in the tachycardia and is not just a bystander. Other possible responses to a His refractory PVC include delaying the next atrial activation or terminating the tachycardia without affecting atrial activation. Either of these responses also proves both the existence and participation of an AP in the tachycardia.
FIGURE 12-5 These are the intracardiac recordings during an episode of PJRT at the time of delivery of a His refractory PVC. Shown are surface leads I, II, and V1 and intracardiac recordings from the high right atrium (HRA) and His bundle proximal, mid, and distal locations (HB p, HB m, and HB d, respectively). The tachycardia cycle length was 470 ms. A ventricular extrastimulus (S) was delivered and captured in the RV apex. The His bundle timing was not affected by the extrastimulus, but the subsequent atrial electrogram was advanced and occurred earlier than expected at an interval of 440 ms, proving the existence of an extranodal accessory pathway. After advancement of the atrial electrogram, the tachycardia was reset (ie, the next His and QRS complex were also advanced), this indicates that the accessory pathway was not just a bystander pathway but also participated in the tachycardia.
A Mahaim tachycardia should be considered in otherwise healthy patients who have a wide complex tachycardia of left bundle morphology. During a Mahaim tachycardia the left bundle morphology is usually relatively narrow (≤0.15 seconds), has a late precordial transition (>V4), and left axis deviation.12 These patients often do not manifest preexcitation on their baseline ECG; however, antegrade preexcitation can easily be demonstrated by pacing along the lateral right atrium in close proximity to the atrial insertion of the AP.
For any supraventricular arrhythmia with a 1:1 AV relationship, an AP-mediated tachycardia needs to be considered. The only ECG feature that effectively rules out the participation of an AP is the presence of AV block during the tachycardia. This is true because the ventricle is an obligatory part of any tachycardia, which is dependent on AP conduction.
For patients who have a long RP tachycardia with deeply inverted P waves in the inferior leads, the differential diagnosis includes PJRT, atypical AV node reentrant tachycardia, and a septal atrial tachycardia. Differentiating these can usually be accomplished by performing ventricular pacing maneuvers during the tachycardia. For patients with PJRT, the tachycardia should be able to be entrained with ventricular pacing with a postpacing atrial-ventricular response and a tachycardia cycle length postpacing interval difference of less than 115 ms (Figure 12-6).
FIGURE 12-6 This tracing is the same as Figure 12-5 with the edition of the right ventricular (RV) recording. The patient was in tachycardia with a tachycardia cycle length (TCL) of 460 ms. Ventricular overdrive pacing at a cycle length of 430 ms was performed. The tachycardia was entrained with acceleration of the atrium to the pacing cycle length. Upon termination of pacing there was an atrial-ventricular response and a postpacing interval (PPI) of 520 ms. The atrial-ventricular response rules out an atrial tachycardia, and the PPI-TCL difference of 60 ms rules out atypical AV node reentrant tachycardia, making AV reentry utilizing long conducting accessory pathway the correct diagnosis.
The differential diagnosis for patients presenting with a left bundle morphology tachycardia includes ventricular tachycardia, supraventricular tachycardia with aberration, and a preexcited tachycardia using an AP for antegrade activation of the ventricle. A short HV interval during tachycardia excludes supraventricular tachycardia with aberration from the differential. For patients with antegrade conduction over a Mahaim fiber during tachycardia, atrial stimulation during the tachycardia when the atrial septum is refractory will often preexcite the ventricle without changing the morphology of the ventricular complex thus proving the presence of an AP.
For patients with symptomatic or recurrent AP-mediated tachycardia, ablation of the AP has a class I indication based on current AHA and ACC guidelines.14
Ablation of atypical APs can be somewhat more challenging than typical APs since the conduction time across the pathway can be variable, the pathway course can be long and tortuous across the AV groove, and the insertion into the atrium or ventricle can be distant from the AV groove.
Patients with PJRT can be quite difficult to control with antiarrhythmic therapy, especially when the arrhythmia is incessant, and are usually best treated with ablation therapy.10 For patients with a tachycardia-mediated cardiomyopathy, the majority of improvement in cardiac function can be expected within the first 2 to 3 months after arrhythmia control.15 In these patients, close follow-up is required to ensure arrhythmia control. If the arrhythmia is recurrent, further decline in cardiac function can occur quickly and in some cases may be irreversible.
The most common variety of Mahaim fibers, atriofascicular pathways, by their very definition insert distally into the right ventricle near the right bundle branch and almost never conduct in the retrograde direction. The approach to ablating these pathways is to find and target the AP potential along the anterolateral tricuspid annulus (Figure 12-7).16 These pathways can be associated with Ebstein’s anomaly, the presence of multiple APs, and other supraventricular arrhythmias such as AV node reentrant tachycardia, so it is important to do a complete evaluation rather than to assume that the Mahaim fiber is the sole tachyarrhythmia.17
FIGURE 12-7 This tracing shows surface ECG recordings from leads I, III, V1 and V6, intracardiac recordings from the high right atrium (HRA), ablation proximal and distal electrode pairs (Abl prox and dist), coronary sinus (CS) proximal (prox) to distal (dist) electrode pairs, and right ventricular (RV) apex. The ablation catheter was placed on the lateral aspect of the tricuspid annulus. Notice the sharp deflection consistent with an accessory pathway (AP) potential between the atrial (A) and ventricular (V) electrograms on the ablation distal recordings. It should also be noted that the RV apical electrogram was early relative to the onset of the QRS complex (dotted line) and ventricular electrogram on the mapping catheter. This indicates that the ventricular insertion of the accessory pathway was distal in the right ventricle near the location of the RV apical catheter.
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