Atriofascicular ("Mahaim") pathway tachycardia

INTRODUCTION — The term preexcitation was originally used in the first half of the 20^th century to describe premature activation of the ventricles in patients with the Wolff-Parkinson-White (WPW) syndrome [1]. This term has since been broadened to include other less common conditions in which antegrade ventricular activation occurs partially or totally via an abnormal conduction pathway. This topic will discuss the anatomy, clinical manifestations, diagnosis, and treatment of patients with atriofascicular ("Mahaim") pathway tachycardias. Additional details regarding WPW syndrome and other syndromes with accessory conductions pathways are presented separately. (See "General principles of asynchronous activation and preexcitation" and "Wolff-Parkinson-White syndrome: Anatomy, epidemiology, clinical manifestations, and diagnosis" and "Treatment of arrhythmias associated with the Wolff-Parkinson-White syndrome" and "Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction".)

NOMENCLATURE, ANATOMY, AND PHYSIOLOGY

Nomenclature and anatomy — Several types of abnormal conduction pathways have been identified (figure 1), although firm histopathologic correlation with clinical arrhythmias has not been established for all [2].

●Accessory atrioventricular (AV) pathways — Classic Wolff-Parkinson-White (WPW) syndrome is caused by a small band of myocytes running between atrial and ventricular muscle that bridges the normal fibrous insulation along the AV junction, resulting in a pattern of short PR interval and a delta wave on the electrocardiogram (ECG). These connections, formerly referred to as bundles of Kent, are now designated by the more accurate label of "accessory AV pathways" [3]. (See "Wolff-Parkinson-White syndrome: Anatomy, epidemiology, clinical manifestations, and diagnosis".)

●Accessory atrio-Hisian pathways — Accessory atrio-Hisian pathways (also referred to as James fibers) have been implicated as the potential cause of the Lown-Ganong-Levine syndrome. In the initial report (authored by James) of these pathways, a potential microscopic conduction fiber pathway was seen linking atrial myocardium directly to the bundle of His, thereby bypassing the AV node and resulting in an ECG appearance of a short PR interval, but with a normal QRS and no delta wave [4]. What remains uncertain, however, is whether this is a true clinical entity or simply conventional AV nodal reentry in a patient who happens to have enhanced conduction in their AV nodal fast pathway. (See "Lown-Ganong-Levine syndrome and enhanced atrioventricular nodal conduction".)

●Atriofascicular pathways — Atriofascicular tachycardia is now understood to involve a specialized conduction pathway arising from the lateral right atrium that extends far down into the body of the right ventricle (RV) and is quite distinct from the AV node and bundle of His. Although nodoventricular fibers might still be involved in some rare cases of atypical supraventricular tachycardia [5-7], atriofascicular tachycardia is now known to be caused by a reentry circuit that is critically dependent on an atriofascicular pathway or a long AV fiber with unique conduction properties. The term "Mahaim" fiber is still retained by some as a shorthand synonym for an atriofascicular pathway, although it is universally understood that the label is imprecise.

Clarification of the true mechanism for atriofascicular pathway tachycardia came about in the 1980s as a result of careful intracardiac electrophysiologic studies and fortuitous observations during arrhythmia surgery. Initial observations suggested the possibility of an unusual type of pathway along the right AV groove possessing decremental conduction properties [8]. These observations were further supported by the elimination of presumptive "Mahaim" fiber tachycardia; this was done by surgically interrupting atriofascicular pathways in two patients along the anterolateral tricuspid valve, proving that the culprit conduction pathway was truly remote from the AV node [9].

Electrophysiology — Atriofascicular pathways exhibit structural and functional features that can almost be likened to a secondary AV conduction system. The atrial end is comprised of cells with AV nodal-like properties situated along the right AV groove in the lateral or anterolateral position. Conduction at the atrial end exhibits clear decrement in response to premature beats, and a Wenckebach-type of periodicity in response to trains of rapid atrial pacing, very similar to behavior of the normal AV node. There is even histologic evidence of cells resembling AV nodal tissue at this site [10].

Traveling away from the atrial end is a long fiber that crosses the AV groove and descends into the body of the RV along the endocardium of the anterior free wall. The fiber is electrically insulated along its length similar to the His bundle or bundle branches, such that the site of earliest ventricular activation is not at the AV groove as seen with conventional accessory AV pathways in WPW syndrome, but much further towards the apex of the RV in the region where the moderator band attaches to the free wall. Because a major extension of the normal right bundle branch also runs along the moderator band towards the free wall, the terminal portion of an atriofascicular pathway is closely approximated or even attached to normal conduction tissues.

Conceptually, one can envision three potential models for the terminal portion of an atriofascicular fiber [11-13]:

●Termination far down on the RV free wall but not attached to the right bundle branch (which would most accurately be described as a "long AV fiber") [14].

●Termination within the muscle of the moderator band, close to, but not in direct contact with, the distal right bundle branch (intermediate between "AV" and "atriofascicular").

●Fused in direct electrical continuity with the distal right bundle branch (true "atriofascicular").

All three models can account for the clinical behavior of this particular pathway, but based on detailed analysis of conduction patterns during electrophysiologic testing [15], the vast majority of cases seem to involve the fusion model of direct continuity. Occasionally, cases are reported with subtle electrophysiologic findings that suggest a long fiber with decremental conduction that does not quite make contact with right bundle tissue [16], but the distinction is usually not critical from a treatment point of view, nor is it always easy to make the distinction. Hence, the label "atriofascicular" is applied unless there is very convincing evidence to the contrary.

CLINICAL FEATURES — Many patients with atriofascicular pathways have minimal ECG changes in sinus rhythm, though a subtle delta wave with a normal PR interval may be seen in some cases. Patients typically have a structurally and functionally normal heart and do not come to medical attention until they experience a tachycardia event. The lone exception is patients with Ebstein anomaly of the tricuspid valve, a situation which is strongly associated with atriofascicular pathways (as well as WPW syndrome) [17,18]. (See "Ebstein anomaly: Clinical manifestations and diagnosis" and "Wolff-Parkinson-White syndrome: Anatomy, epidemiology, clinical manifestations, and diagnosis", section on 'Anatomy'.)

Arrhythmias and symptoms — Antidromic reentry (antegrade conduction over the accessory pathway with retrograde conduction via the AV node) is the most common tachycardia seen with atriofascicular pathways (waveform 1). Less frequently, the retrograde limb involves a second conventional accessory AV pathway, or retrograde conduction may shift between the AV node and one of these secondary accessory AV pathways [13,17,19,20]. Other tachycardia substrates often coexist with an atriofascicular fiber. Approximately 40 percent of patients with a proven atriofascicular fiber have other accessory AV pathways and/or dual AV nodal physiology [17,19,21-23]. Often, a conventional Wolff-Parkinson-White pathway can mask the presence of an atriofascicular pathway, which only becomes apparent after surgical or catheter ablation of the primary accessory pathway [21]. Atriofascicular pathways can also function as a bystander and conduct anterograde impulses during AV nodal reentrant tachycardia, atrial flutter, or any other sort of atrial tachycardia (figure 2) [19,22].

The two most distinctive functional characteristics of an atriofascicular pathway include its decremental properties and the fact that it is only capable of antegrade conduction [24]. Therefore, when actively involved as part of a reentrant tachycardia, it will only support antidromic reentry. Passive antegrade conduction may also occur over an atriofascicular pathway during sinus rhythm or any other type of supraventricular tachycardia.

Some atriofascicular pathways are known to exhibit intrinsic automaticity, which can present as single premature beats or brief salvos that appear to be of ventricular origin [25,26].

ECG findings

Resting ECG — The resting ECG in patients with an atriofascicular fiber is often normal [17,19,21,24]. This is due to preferential ventricular activation via the AV node at normal resting heart rates. There are several conditions under which conduction via the atriofascicular pathway occurs:

●Enhanced vagal tone, rapid pacing, or a premature beat may slow AV nodal conduction and favor conduction partially or exclusively over the atriofascicular pathway [17].

●The relative refractoriness of the AV node and fiber may change with time, resulting in a variable degree of preexcitation during sinus rhythm.

●Premature atrial complexes (PAC; also referred to a premature atrial beat, premature supraventricular complex, or premature supraventricular beat) or an ectopic rhythm arising near the atrial insertion can preferentially engage the atriofascicular pathway.

Whenever some or all conduction proceeds over an atriofascicular pathway, the PR interval still falls into the normal range, but the QRS is variably distorted by early RV activation, resulting in a pattern of partial or complete left bundle branch block. The absence of a sharp septal Q wave due to relative delay in left bundle activation results in a somewhat slurred QRS upstroke, but whether the initial portion of the QRS should be referred to as a true delta wave is a matter of semantics. Regardless, conduction over an atriofascicular pathway does indeed satisfy the strict definition of preexcitation in that a ventricular site is activated ahead of the normal His-Purkinje system.

One study of sinus rhythm ECGs of patients with atriofascicular pathways identified a narrow QRS complex with an rS pattern in lead III in 20 of 33 cases (61 percent) [16]. In contrast, among 200 young adults with a history of palpitations but no structural heart disease, a narrow QRS with an rS in III was found in only 6 percent. More recently, terminal notching and slurring in the non-preexcited QRS has been described as a common finding in patients with atriofascicular pathways that resolves once the involved fibers are successfully eliminated with catheter ablation [27].

ECG with antidromic reentrant tachycardia — Antidromic reentry involving an atriofascicular pathway has a regular rate and abrupt onset. Several ECG features that have been suggested as characteristic of antidromic reentry involving an atriofascicular pathway include (waveform 1) [21,28]:

●QRS axis between 0 and minus 75°

●QRS duration of 0.15 seconds or less

●R-wave in lead 1

●rS complex in lead V1

●Precordial transition in lead V4 or later

●Cycle length between 220 and 450 ms (heart rates of 130 to 270)

Although these surface ECG markers can be a useful starting point in the acute setting to raise suspicion about an atriofascicular mechanism, they are not diagnostic, and invasive electrophysiology studies are usually required for an accurate diagnosis. (See 'Evaluation' below.)

EVALUATION — Patients with suspected atriofascicular ("Mahaim" fiber) tachycardia should all have had an ECG as part of the initial presentation. Additional testing for such patients also typically includes:

●Transthoracic echocardiography to assess for any structural cardiac abnormalities (especially Ebstein anomaly).

●Invasive electrophysiology studies (EPS) for confirmation of the diagnosis and, in many cases, therapeutic catheter ablation of the abnormal pathway.

When invasive EPS is performed, baseline sinus rhythm intervals are often normal or show only slightly shortened HV intervals (waveform 2). Atrial pacing maneuvers should uncover the antegrade conduction abnormalities [29]. The AH interval and the A-V time will prolong in the expected fashion as progressively premature atrial extrastimuli are delivered, but at some point there will be a shift (often subtle at first) in antegrade conduction away from the AV node and towards the atriofascicular pathway. This manifests itself as a progressively shortened HV interval and a more preexcited-appearing QRS with left bundle branch block features and earliest ventricular timing towards the RV apical region.

Coaxing conduction antegrade down the atriofascicular pathway can be highly dependent on the site of atrial stimulation. The closer the stimulation site is to the atrial end of the fiber, the more obvious and dramatic the preexcitation. Thus, atrial stimulation from the right atrial appendage or lateral right atrium along the AV groove is the preferred technique for uncovering these fibers. Stimulation from the coronary sinus or some other left atrial site will often fail completely to engage an atriofascicular pathway since the AV node has a chance to conduct long before the atrial activation wavefront ever reaches the right lateral region.

As atrial extrastimuli are delivered more prematurely, conduction begins to approach maximal preexcitation where the His signal becomes obscured by the preexcited QRS. Even at this point, the A-QRS interval may continue to prolong because of decremental conduction within the pathway. Eventually, a critical coupling interval can be reached where the normal AV node is refractory and antegrade conduction occurs exclusively over the atriofascicular pathway. This is the point at which antidromic reentry can develop, since conduction may now return to the atrium in a retrograde fashion via the His bundle and AV node to complete the circuit. Tachycardia can also be induced with rapid atrial pacing that achieves the same critical shift towards exclusive antegrade conduction in the atriofascicular pathway.

During induced antidromic tachycardia involving an atriofascicular pathway, the QRS will have a pattern of complete left bundle branch block with no preceding His bundle potential. Instead, the His potential will be observed after the QRS, and on close inspection of the His bundle electrogram, the His-Purkinje system can usually be confirmed to be activated in a retrograde manner from distal-to-proximal as opposed to the normal proximal-to-distal pattern seen in sinus rhythm [29,30]. When the RV is mapped carefully, the earliest ventricular activation will be localized to the anterior free wall near the insertion of the moderator band [31]. Unless another retrograde AV bypass tract or a retrograde AV nodal slow-pathway is present, earliest retrograde atrial timing during antidromic tachycardia will usually be recorded on the His bundle catheter. Premature atrial extrastimuli delivered during preexcited tachycardia not altering the atrial activation at the septal level, but resulting in advancement of the next ventricular and atrial complexes (ie, tachycardia reset) definitely indicate that the atriofascicular pathway is an integral part of the reentrant circuit (ie, the tachycardia is sustained by antidromic reentry) [30,32].

DIAGNOSIS — In some instances, the presence of an atriofascicular pathway may be suspected based on the surface ECG, on which the PR interval remains normal but the QRS is variably distorted by early RV activation, resulting in a pattern of partial or complete left bundle branch block. However, given that the ECG findings may be subtle or entirely absent at rest, intracardiac electrophysiologic study is usually required to make a firm diagnosis of an atriofascicular pathway. Baseline sinus rhythm intervals can be normal or may show a slightly short HV interval with RV activation occurring a bit earlier than expected. However, when atrial pacing maneuvers are performed, the antegrade conduction abnormalities become more apparent [29]. (See 'Evaluation' above.)

TREATMENT

Acute management — In the acute setting at initial presentation of sustained antidromic tachycardia involving an atriofascicular pathway, the wide-QRS pattern on ECG may be hard to distinguish from several other tachycardia mechanisms, including ventricular tachycardia. It is therefore best to approach acute treatment according to the standard algorithm for any wide-QRS complex tachycardia. (See "Wide QRS complex tachycardias: Approach to management".)

For patients with a known established diagnosis of atriofascicular pathway tachycardia (ie, a supraventricular tachycardia [SVT] rather than ventricular tachycardia or wide-QRS complex tachycardia of unknown origin), the approach to acute therapy is similar to other SVTs. Since antidromic atriofascicular reentry is regular and monomorphic, this means that a cautious trial of adenosine is a reasonable first step if the patient is hemodynamically stable. Adenosine is nearly always effective in this setting. (See "Overview of the acute management of tachyarrhythmias", section on 'Narrow QRS complex tachyarrhythmias'.)

Chronic management — For chronic treatment of atriofascicular pathway tachycardia, most patients are suitable candidates for catheter ablation, but the techniques used for mapping differ somewhat from those used for a conventional Wolff-Parkinson-White pathway. In rare circumstances, pharmacologic therapy can be used in an effort to suppress arrhythmias.

Catheter ablation — The atrial end of the atriofascicular pathway is the most productive and reliable site for ablation [29,33], but because atriofascicular pathways are capable only of antegrade conduction, the atrial insertion cannot be easily mapped with simple analysis of retrograde atrial activation patterns. Instead, electrophysiologists must rely on one or more of the following techniques:

●Stimulus to "delta-wave" mapping (aimed to disclose the atrial pacing site along the tricuspid annulus associated with the shortest "stimulus-to-delta" interval) [33].

●Premature right atrial stimulation during antidromic tachycardia (to identify the site from which the latest coupled atrial extrastimulus advances the next ventricular electrogram) [30].

●Direct recording of a discrete potential from the fiber near the tricuspid annulus [29,31,34].

Identification of a site with a discrete potential has become the most widely used and dependable technique for mapping atriofascicular fibers (waveform 2). Promising sites will register a sharp signal that is very similar in frequency and amplitude to a His bundle potential, with timing midway between the atrial and ventricular electrograms during antidromic tachycardia or preexcited atrial pacing [29,31,34-37]. These potentials can be recorded on the atrial side of the tricuspid valve at the AV groove, as well as on the ventricular side just below the valve. The combination of its long ventricular endocardial course, arborization of its distal segments, and potential fusion with the right bundle branch, all make identification of the true ventricular insertion of these fibers difficult [31]. Fortunately, precise location of the ventricular end is usually not critical since ablation is never performed at this site.

Once thorough mapping has been performed and a high-quality atriofascicular potential has been localized, radiofrequency (RF) ablation can be performed using standard techniques. A brief period of accelerated automaticity from the atriofascicular fiber often occurs as the tissue is heated (similar to ablation near the normal AV node and His bundle) and is usually a promising sign. This automaticity should resolve within the first few seconds of the RF application if the catheter tip is in good position. The reported permanent success rate for RF ablation in this condition is excellent, in the range of 87 to 100 percent [29,38].

Pharmacologic therapy — Pharmacologic therapy can be used as an alternative to catheter ablation if necessary. Prior to the development of ablation techniques, empiric drug therapy was the preferred treatment of "Mahaim" fiber tachycardia. Since this is a relatively rare condition, there have been no large-scale comparative trials of different classes of drugs. The published data are limited to case reports and small studies demonstrating sensitivity to various classes of antiarrhythmic agents. Because drug therapy is so uncommonly used (and studied) in this condition, and since for most patients catheter ablation is by far the preferred option, it is not possible to make a recommendation for any one agent.

Antegrade conduction in atriofascicular pathways appears to be acutely sensitive to adenosine but not necessarily to other agents that predominately affect the AV node, such as calcium blockers and beta blockers [39]. In some cases, however, the AV nodal blocking agents may be effective in preventing tachycardia by affecting AV nodal conduction in the retrograde limb of the tachycardia. Both class IA and IC agents, as well as class III agents, also may slow or prevent tachycardia in patients with antidromic tachycardia related to atriofascicular pathways [40].

SUMMARY AND RECOMMENDATIONS

●Cardiac preexcitation describes premature activation of the ventricles over an abnormal pathway distinct from the normal cardiac conduction system. (See 'Nomenclature, anatomy, and physiology' above.)

•The classic form of cardiac preexcitation occurs in Wolff-Parkinson-White (WPW) syndrome, involving a short connection along the atrioventricular (AV) groove, referred to as an "accessory AV pathway."

•Atriofascicular pathways, formerly referred to as "Mahaim" fibers, are now understood to involve a specialized conduction pathway arising from the lateral right atrium that extends far down into the body of the right ventricle and is quite distinct from the AV node and bundle of His.

●Atriofascicular pathways exhibit structural and functional features that can almost be likened to a secondary AV conduction system. (See 'Electrophysiology' above.)

●Many patients with atriofascicular pathways have minimal ECG changes, but in some cases a subtle delta wave with a normal PR interval can be seen. The most common tachycardia in this condition is antidromic reentry using the atriofascicular pathway as the antegrade limb and the AV node as the retrograde limb (See 'Clinical features' above.)

●Patients with suspected atriofascicular ("Mahaim" fiber) tachycardia should all have had an ECG as part of the initial presentation. Additional testing for such patients also includes transthoracic echocardiography and, frequently, invasive electrophysiology studies for confirmation of the diagnosis and, in many cases, therapeutic catheter ablation of the abnormal pathway. (See 'Evaluation' above and 'Diagnosis' above.)

●Most patients are suitable candidates for transcatheter ablation, but the techniques used for mapping differ somewhat from those used for a conventional WPW pathway. In rare circumstances, pharmacologic therapy can be used in an effort to suppress arrhythmias. (See 'Treatment' above.)