Abstract
Atriofascicular accessory pathways are rare decremental bypass tracts with unidirectional antegrade conduction. They most commonly connect the right atrium to the right bundle. In this chapter, we will discuss the anatomy, pathophysiology, diagnostic features, as well as the differential diagnosis of atriofascicular pathways, which include supraventricular tachycardia with left bundle branch block (LBBB) aberration, WPW, ventricular tachycardia (VT), and other atypical pathways such as decremental atrioventricular (AV) pathways, nodoventricular/nodofascicular, and fasciculoventricular pathways. The chapter also discusses the most effective approaches and the challenges that might be faced during mapping and ablating atriofascicular pathways and their variants. Diagnosis of atriofascicular pathways can be challenging in sinus because of absent or subtle preexcitation at baseline. They participate in antidromic tachycardia with a LBBB morphology, antegrade conduction over the accessory pathway, and retrograde conduction over the atrioventricular node or another accessory pathway. Active participation in AV reentry tachycardia can be confirmed with a late premature atrial extrastimulus during AV node refractoriness that advances the V and the subsequent H and A with the exact activation sequence and timing. Entrainment of the tachycardia can be associated with prolonged stimulus to QRS, despite pacing from within the reentry circuit because of decrement in the accessory pathway with more rapid pacing; however, the PPI will still be equal to the tachycardia cycle length. Mapping and ablating the discrete M potential along the tricuspid annulus is the recommended strategy. Prolonged transient accessory pathway block may be observed during mapping because of mechanical trauma. Catheter instability can be overcome by using long steerable sheaths and 3-dimensional mapping systems.
Keywords
atriofascicular, atrioventricular, atypical, decremental, fasciculoventricular, Mahaim, nodoventricular
Key Points
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Atriofascicular accessory pathways are decrementally conducting accessory pathways that are most commonly located along the right free wall, connecting the atrium to the right bundle branch.
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They participate in antidromic tachycardia with a left bundle branch block morphology, antegrade conduction over the accessory pathway, and retrograde conduction over the atrioventricular node or another accessory pathway.
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The most effective method for ablating the atriofascicular pathway involves mapping and ablating the discrete M potential along the tricuspid annulus.
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Sources of difficulty include catheter instability and prolonged transient loss of accessory pathway conduction because of mechanical trauma during mapping. Locating the atrial insertion may be time consuming, and ablating the ventricular insertion is more challenging because of extensive arborization.
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Long and steerable sheaths may help stabilize the catheter along the tricuspid annulus, and 3-dimensional mapping systems may guide ablation.
Atriofascicular Accessory Pathways
In 1938 Dr. Ivan Mahaim first described nodoventricular fibers that connected the atrioventricular (AV) nodal region to the ventricular myocardium. The description was histologic, and no physiologic properties were reported. More recently, in the early 1980s, slowly conducting AV accessory pathways (AP) with AV node-like decremental properties participating in antidromic tachycardia were postulated to be nodoventricular fibers. However, others also identified right lateral APs having similar properties that were independent of the AV node. Nevertheless, the eponym Mahaim fibers became widely adopted to describe all atypical APs with decremental conduction properties instead of the initially defined anatomic fibers. In the early 1980s, the most prevalent thought was that nodoventricular pathways, possibly those described by Mahaim, were the predominant cause of these types of antidromic reentry arrhythmia. They had the potential to be involved in reentrant pathways or in “bystander” activation of the ventricles during supraventricular tachycardia such as AV nodal reentry. In the late 1980s, independent publications from two different groups questioned this concept, with further case reports demonstrating that these types of antidromic tachycardia were using atrioventricular as opposed to nodoventricular pathways in their reentrant circuits. With additional publications from other groups, it became apparent that the majority (>90%) of reentry tachycardias involving slowly conducting atypical pathways were in fact attributed to atriofascicular, or the less frequent atrioventricular pathways with decremental conduction and a separate atrial origin from the atrioventricular node. , , , , Thus the term Mahaim pathway fell out of favor, and instead the more precise description of atypical decrementally conducting pathways that also includes the correct anatomy is now preferred. Interestingly a lateral AV connection with an AV node-like structure, which most likely represented an atriofascicular pathway, was described by Dr. Stanley Kent in 1914; however, he misinterpreted the structure as parts of the then-postulated normal conduction system.
Anatomy
The atriofascicular pathway consists of a long, insulated fiber with decremental conduction properties that connect atrial tissue to the distal His-Purkinje network. This differentiates it from other slowly conducting atrioventricular pathways that insert directly into the myocardium rather than into the normal His-Purkinje system (HPS). It acts as a parallel conductive system that is structurally and physiologically similar to the AV node and the His-bundle. It almost always originates from the right atrial free wall along the tricuspid valve annulus, where slow potentials similar to those at the AV node can be occasionally recorded ( Fig. 26.1 ). A single left-sided atriofascicular accessory pathway has been reported. However, the majority of decremental pathways that originate outside the right atrium are atrioventricular pathways. They arise from the left atrium and can insert into the left ventricle along the mitral annulus, the mitral annulus–aorta junction, or into the myocardium abutting the left coronary cusp.
When mapping an atriofascicular pathway, a His-like structure can be frequently identified along the tricuspid annulus and is characteristic of such pathways. Decremental conduction typically occurs through a structure between the atrial tissue and the His-like structure. The distal insertion of the pathway is usually located several centimeters from the tricuspid valve, at the distal right bundle (RB) branch or its branching Purkinje fiber, toward the apical third of the right ventricular free wall (see Fig. 26.1 ). Extensive arborization via the Purkinje network is common and suggested by the presence of a QS pattern on unipolar electrograms or sharp potentials fusing with the earliest bipolar ventricular potentials over a wide area of ventricular tissue. However, simultaneous activation via local His-Purkinje network may be an alternative explanation.
Pathophysiology
Atriofascicular pathways are associated with unique tachyarrhythmias given their characteristic anatomy and electrophysiological properties. The AP provides the substrate for AV reentry tachycardia by acting as the antegrade limb, whereas the AV node-HPS or another AP acts as the retrograde limb. Typically they are not capable of retrograde conduction, and to date there is only one report of an atriofascicular pathway participating in both antidromic and orthodromic reentry tachycardia. Retrograde conduction block of atriofascicular pathways occurs near the atrial insertion. This facilitates initiation of atrioventricular reentry tachycardias with premature ventricular beats as they block retrogradely in the atriofascicular AP and conduct up the AV node-HPS allowing for reentry to occur. As with other types of APs, atriofascicular accessory pathways may preexcite the ventricle during an atrial tachycardia or participate in a figure-of-8 manner with a coexisting orthodromic reentry or AV nodal reentry.
Atriofascicular pathways are mostly right sided, with a few exceptions, inserting in the right bundle branch (RBB) or its distal Purkinje fibers, giving rise to a QRS with a left bundle branch block (LBBB) morphology during antidromic AV reentrant tachycardia. However, identifying atriofascicular pathways during sinus rhythm may be challenging as their slow antegrade decremental conduction leads to minimal and sometimes no preexcitation in sinus rhythm. Similar to the native AV node-HPS, they are sensitive to autonomic changes, adenosine, and occasionally to verapamil, leading to variations in the amount of preexcitation depending on the degree of their contribution to ventricular activation. In addition, similar to the AV node, there have been reports of atriofascicular pathways with dual conduction properties leading to tachycardia because of a 1:2 response or beat-to-beat variation in the antidromic AV reentry tachycardia cycle length.
Diagnosis
12-Lead Electrocardiogram
The diagnosis of atriofascicular pathways can be challenging in sinus rhythm, as preexcitation can be absent at baseline because of faster conduction along the AV node compared with the AP. However, subtle preexcitation can be detected in approximately 60% of patients in the form of an absence or a decrease of the septal q in leads I and V5–V6 and the presence of an rS pattern in lead III ( Fig. 26.2 ). Day-to-day variation in the degree of preexcitation is common and does not necessarily reflect an AP with a long refractory period. Sinus tachycardia or rapid atrial pacing, especially pacing the atrium near the pathway versus pacing from the coronary sinus, can manifest the preexcited QRS morphology, as conduction decrements more significantly down the AV node and preferential AP conduction occurs to the ventricles. The conduction scenarios showing varying degrees of preexcitation are schematically illustrated in Fig. 26.3 .
Classically, the maximally preexcited QRS, either achieved with rapid atrial pacing or during antidromic tachycardia, is similar to a typical LBBB: R wave in V1 is minimal or absent (rS, or QS), precordial R wave transition is beyond lead V4, and an R in limb lead I. The QRS is usually less than 150 ms in duration as the atriofascicular pathway allows for more rapid activation of the ventricles through a close connection to the HPS. For the same reason, the initial QRS forces are typically rapid, especially in the anterior chest leads, with absence of a delta wave that is classically seen with pathways that insert into the myocardium near the annuli. The QRS typically shows left axis deviation. However, the frontal axis can vary widely between –75 and +60 degrees. This variation is dictated by the location of ventricular insertion as well as the retrograde conduction properties of the RBB (see later discussion).
Electrocardiogram (ECG) criteria can suggest a decrementally conducting atrioventricular pathway and help differentiate it from ventricular tachycardia (VT), which allows for a more thorough intracardiac electrophysiological evaluation. However, it cannot distinguish between atriofascicular mediated tachycardia, and supraventricular tachycardia (SVT) with LBBB aberration, as both can have a typical LBBB pattern. In addition, in one study the maximally preexcited QRS width or axis during antidromic tachycardia could not predict the location of the atriofascicular AP.
Intracardiac Electrograms
During Sinus Rhythm
A three component electrogram: an atrial component A , a His-Purkinje like potential, sometimes referred to as an M potential, and a ventricular component V can frequently be identified near the proximal (atrial) insertion along the tricuspid annulus. The M potential is a His-like potential, which represents depolarization of the bundle forming the distal portion of the atriofascicular pathway that inserts into the distal RBB or its Purkinje branches. In sinus rhythm, ventricular activation is predominantly over the AV node and His bundle, minimizing any ventricular preexcitation. Atrial pacing close to the origin of the AP can manifest the preexcitation as fusion occurs between ventricular activation over the AP and the His bundle ( Fig. 26.4 ).
The distal (ventricular) insertion of atriofascicular pathways occurs at the distal RB or its branches located in the anterior right ventricular free wall. During more rapid atrial pacing, ventricular activation may occur preferentially over the atriofascicular pathway because of greater conduction slowing in the AV node. Similar to the AV node, decremental conduction in the atriofascicular pathway occurs in the proximal segment of the AP between the atrial insertion and the M potential.
Induction of Antidromic Atrioventricular Reentrant Tachycardia
This tachycardia can be induced via atrial stimulation, ventricular stimulation, or both. Induction with atrial stimulation can be achieved with burst atrial pacing or with the administration of atrial extrastimuli. It requires anterograde block in the AV node, and conduction over the AP with subsequent retrograde conduction over the AV node. This occurs when the refractory period of the AP is shorter than the refractory period of the AV node anterogradely. It also requires retrograde AV node conduction to recover sufficiently for the reentry to occur. Thus isoproterenol may be needed for induction. Initiation of tachycardia may be more readily achieved when atrial stimulation is closer to the AV node than the AP, such as from the coronary sinus, which allows more time for the AV node to recover and facilitate retrograde conduction.
In many cases, the AV node has more decremental properties than the atriofascicular pathway as revealed with rapid atrial pacing, atrial premature stimuli, or after the administration of intravenous adenosine. The A-His interval prolongs more than the A-M interval. The HV interval gradually shortens and the QRS widens and assumes a LBBB pattern as it becomes more preexcited because of increasing activation of the ventricle via the AP. Maximal preexcitation occurs when there is retrograde activation of the His bundle with reversal of distal RB, proximal RB, and His activation sequences. Antidromic AV reentry tachycardia is typically initiated after such reversal of RB and His activation ( Fig. 26.5 ).
Because initiation of tachycardia with atrial stimulation requires the AV node to block during atrial pacing while the AP still conducts, it is not always a reliable means of initiating the tachycardia, and in some cases it is not feasible to initiate the tachycardia. However, because these APs typically have conduction block in the retrograde direction, ventricular stimulation can almost always initiate the reentry circuit. Thus initiation of tachycardia is frequently easier to accomplish with ventricular pacing. With ventricular pacing one only needs to achieve sufficient retrograde activation delay via the normal HPS-AV node axis that would allow reentry into the ventricle to occur via the AP. This delay can be achieved either via slowing in the AV node when sufficiently short retrograde H-H intervals can be achieved, or by delay in retrograde HPS activation because of retrograde RBB block when a sufficiently short premature ventricular beat impinges on the HPS refractoriness ( Figs. 26.6 and 26.7 ). In some cases tachycardia can only be induced via ventricular stimulation. Davidson et al. reported on two patients with no evidence of preexcitation during sinus rhythm at baseline or even with atrial pacing maneuvers. However, an antidromic AV reentry tachycardia utilizing an atriofascicular pathway was readily induced with ventricular stimulation and ablated along the lateral tricuspid annulus in both patients. This was believed to be because of fast conduction over the AV node and slower conduction over the AP. In addition, the refractory periods of the pathways were longer than the respective refractory periods of the AV nodes, thus preventing initiation of tachycardia with atrial premature stimulation.
Electrophysiologic Evaluation During Tachycardia
Both the atrium and ventricle are essential parts of the macroreentrant circuit. The earliest retrograde atrial activation is typically found along the mid atrial septum at the region of the fast AV node pathway unless another AP is used as the retrograde limb. The AV interval is often more than 150 ms during antidromic tachycardia caused by the decremental conduction properties of the AP. The earliest ventricular activation is characteristically located along the right ventricular anterior-lateral free wall near the distal third toward the apex. Direct insertion of the atriofascicular pathway into the distal RBB or its fascicles leads to rapid simultaneous activation of both His and ventricle and a very short V–H interval (16 ± 5 ms). The V–H interval is shorter during antidromic tachycardia than the HV interval in sinus rhythm. This is because of the fact that His and the ventricle are simultaneously (instead of sequentially) activated via the RB. The V–H (and V–A) interval may prolong in the setting of transient retrograde right bundle branch block (RBBB), which may occur as a result of mechanical trauma during catheter manipulation or with ventricular premature beats. In this situation the tachycardia cycle length increases as conduction has to pass across the interventricular septum via the myocardium, up the left bundle, to reach the His bundle then the AV node and atrium (see Figs. 26.6 and 26.7 ). The His bundle–atrial (HA) interval remains constant as the final segment of the retrograde limb from the His-bundle to the atrium remains unchanged. Antidromic AV reentry tachycardia with retrograde RBBB is usually initiated with ventricular premature beats from the RV that block retrogradely in the RBB but conducts transeptally up the LBB to the His bundle and up the AV node ( Figs. 26.6 and 26.7 ). Because of anterograde penetration of the RBB, subsequent reentrant beats then continue to maintain the retrograde RBBB by a concealed conduction phenomenon, whereby serial anterograde and retrograde conduction in the RBBB are blocked by a mechanism of interference. Resolution of the retrograde RBBB is typically associated with an acceleration of the tachycardia cycle length accompanied by a shorter ventriculoatrial (VA) interval (see Figs. 26.6, 26.7 ). This resolution of retrograde RBBB has been demonstrated to occur through the migration of the site of retrograde block toward the His bundle. Minor changes in QRS morphology, axis, and duration may occur as a result of the retrograde RBBB (see Fig. 26.7 B, note change in QRS durations). In the absence of RBBB, rapid retrograde conduction over the RB to the His bundle may allow for anterograde left ventricular activation over the left anterior fascicle and fusion with transseptal activation leading to a narrower QRS sometimes with a more inferior axis. Ventricular activation during tachycardia with or without retrograde RBBB is schematically illustrated in Fig. 26.8 .
A late atrial premature stimulus delivered in the lateral right atrium during preexcited tachycardia that advances or delays the following ventricular activation without affecting the septal atrial activation around the AV node confirms the presence of an atriofascicular AP. Active involvement in the tachycardia can be confirmed by maintenance of the ventricular, HPS, and atrial activation sequence and timing after resetting the tachycardia. Cycle length oscillation following the extrastimulus is common and characteristic of decrementally-conducting APs ( Fig. 26.9 ). His refractory premature atrial beats typically advance the next ventricular activation and reset the tachycardia; however, in 15% of cases they delay the next ventricular activation. By contrast, failure to influence the tachycardia with a late atrial premature beat does not constitute sufficient evidence to rule out the presence of an atriofascicular AP as the pacing location may be far away from the origin of the AP and would not allow the premature beat to preexcite the reentrant circuit. Alternatively, the premature atrial beat may decrement in the pathway in such a manner as to obscure any changes in the cycle length of ventricular activation.
1:2 Atrial to Ventricular Conduction Over an Atriofascicular Pathway
In addition to participating in antidromic AV reentry tachycardia, atriofascicular pathways may rarely present with tachycardia secondary to 1:2 atrial to ventricular conduction over the accessory pathway similar to that reported with dual AV nodal physiology ( Fig. 26.10 ). Different explanations were postulated including longitudinal dissociation in the atriofascicular pathway similar to dual AV nodal pathways, distal division into two fibers with different conduction times, or less likely two closely located accessory pathways with markedly differing conduction times.
Differential Diagnosis
Antidromic AV reentrant tachycardia involving an atriofascicular pathway must be differentiated from other wide QRS complex tachycardias with LBBB type of QRS morphology.
Supraventricular Tachycardia with Left Bundle Branch Block Aberrancy
The tachycardia is associated with normal or prolonged HV interval with anterograde His-bundle activation that precedes RB activation.
Ventricular Tachycardia
Ventricular tachycardia can be easily differentiated from antidromic AV reentry tachycardia involving an atriofascicular pathway when VA dissociation occurs, which may occur spontaneously or be demonstrated by delivering a ventricular premature stimulus that blocks retrogradely to the atrium without terminating the VT. In addition, fusion beats with atrial pacing and atrial extrastimuli are possible without terminating VT, whereas during antidromic AV reentry tachycardia, the same premature atrial beat will advance (or delay) the next tachycardia beat with the same QRS morphology or terminate it. Although fusion is theoretically possible with an extremely slow tachycardia having markedly prolonged conduction in the atriofascicular pathway, such has not been seen by the authors nor reported in the literature.
Bundle branch reentrant VT (BBRVT) typically has a LBBB morphology with an HV interval that is comparable to that seen during sinus rhythm or somewhat prolonged. The H and RB activations show an anterograde sequence. Because these tachycardia tend to have a rapid rate, there is usually VA dissociation. However, when BBRVT has 1:1 VA conduction, late atrial premature beats will not be able to advance the next ventricular beat. Early atrial extrastimuli can potentially advance a slow tachycardia, with similar QRS morphology and His, RB, and ventricular activation sequence. When the cycle length of the LBBB QRS tachycardia prolongs in association with retrograde RBBB, one can exclude focal or reentrant VT as this phenomenon is characteristic of antidromic reentry related to a right-sided AP.
WPW
Rapidly conducting atrioventricular pathways do not exhibit notable decremental anterograde conduction, and therefore patients commonly demonstrate preexcitation during sinus rhythm especially with right-sided pathways. They are frequently capable of retrograde conduction and predominantly presents with orthodromic AV reentry tachycardia. Antidromic tachycardia occurs in only 10% of patients and is characterized by a short AV interval, ventricular activation that is earliest along the tricuspid or mitral annulus, and a wider QRS complex. The QRS tends to have a more slurred onset in the anterior chest leads as a result of slower conduction through the periannular myocardium as opposed to the rapid deflection seen in a typical LBBB QRS pattern.
Decremental Atrioventricular Pathways
The majority of decremental AV pathways insert several centimeters from the tricuspid annulus via a long, insulated bundle of fibers into the right ventricle free wall. However, shorter decremental accessory pathways that insert into the myocardium immediately contiguous to the tricuspid annulus have been reported. Less commonly we have even encountered left lateral decremental pathways that insert along the mitral annulus, and most recently a left-sided decremental pathway inserting at the outflow region by the left coronary cusp. The pathophysiology is also very similar to atriofascicular pathways; however, the maximally preexcited QRS of right-sided AV pathways typically shows a LBBB morphology with a wider QRS duration (165 ± 26 vs. 133 ± 10 ms) and broad r waves (>40 ms) in V2–V4 ( Fig. 26.11 ). The further the ventricular insertion is from the RB the longer the VH interval, as activation has to go through the more slowly conducting myocardial tissue. Short AV pathways are characterized by a longer VH interval during antidromic AV reentry tachycardia (46 ± 22 milliseconds), compared with long atrioventricular (37 ± 9 ms) and atriofascicular pathways (16 ± 5 ms).
