Mahaim fibers were first described as accessory connections arising from the atrioventricular node (AVN), from the His-bundle, or from fascicles to the right ventricle (RV) by Ivan Mahaim in 1938,¹ and have since been recognized as atriofascicular, nodofascicular, nodoventricular, and fasciculoventricular bypass tracts with antegrade decremental properties.^2,3 Here, we present a case of wide QRS complex tachycardia (WQRST) involving the atriofascicular bypass tract in the right atrium (RA) that was initially misdiagnosed as atypical atrioventricular nodal reentrant tachycardia (AVNRT) at another hospital and that recurred following the initial procedure.

A 45-year-old female presented with suspected recurrent WQRST after initial catheter ablation. She had undergone an electrophysiologic study (EPS) for intermittent palpitations with documented tachycardia two years prior (Figure 1A). On initial EPS, dual AVN physiology with an AH jump was demonstrated. The concentric retrograde atrial activation sequence was not advanced by His-refractory premature ventricular extrastimulus during tachycardia. The VA interval during tachycardia was 130 ms. However, tachycardia was still induced despite several attempts of slow pathway ablation including within the coronary sinus. Tachycardia frequently recurred starting three days after the procedure.

A baseline 12-lead electrocardiogram (ECG) revealed normal sinus rhythm with a PR interval of 118 ms and a QRS duration of 90 ms (Figure 1B). EPS was performed using a duodecapolar catheter (Livewire™; St. Jude Medical, Minneapolis, MN, USA) in the RA, a decapolar catheter in the coronary sinus, and two quadripolar catheters in the right ventricle (RV) and the His-bundle region. Atrial pacing augmented the degree of preexcitation with His potential followed by the potential of the RV apex. The degree of preexcitation over an accessory pathway was changed with different atrial pacing cycle lengths. Preexcitation was augmented at faster pacing, which led to prolongation of both the AH interval and stimulus-to-RV interval with reversal of activation sequences of the His and RV potentials (Figure 2). Adenosine blocked antegrade conduction through the AVN but did not affect the bypass tact. WQRST was induced by ventricular programmed extra stimulation with a drive cycle length of 400 ms and a single extrastimulus interval of 250 ms. Tachycardia was terminated by a single ventricular extrastimulus. The earliest ventricular activation in the tricuspid annulus was mapped by atrial pacing and was ablated with Mahaim potential (Figure 3). Preexcitation disappeared in three seconds, along with the transition of the ventricular activation sequence in the tricuspid annulus.

On post-ablation EPS, tachycardia was not induced. There was no antegrade conduction through the bypass tract on atrial pacing or adenosine injection. The patient remains free from symptoms 12 months after the procedure.

Mahaim fibers were found to affect approximately 6% of accessory pathways in a previous study.⁴ Although the decremental antegrade conduction of these fibers is a unique electrophysiologic characteristic distinguishing them from other accessory pathways, there are several limitations to diagnosis. First, overt preexcitation is not usually seen in 12-lead ECGs of patients with an atriofascicular accessory pathway during sinus rhythm. Minimal preexcitation is supported by an rS pattern in lead III, absence of septal Q waves in leads I and V6, and terminal QRS slurring of notching. Second, although antidromic atrioventricular reentrant tachycardia (AVRT) over a Mahaim fiber typically has a left bundle branch block (LBBB) morphology, various QRS morphologies and axes may be observed, which can be difficult to differentiate from supraventricular tachycardia with LBBB aberrant conduction.⁵ Third, other heart conductions are associated with Mahaim fibers. Nearly 50% of patients with Mahaim fibers exhibit a dual AVN pathway or other accessory pathways.^6–9 In cases such as these, the presence of dual AVN pathways may complicate the diagnosis of clinical tachycardia.

Progressive atrial pacing is helpful in identifying the presence of preexcitation in patients with WQRST. Rapid atrial pacing rate is expected to increase the degree of preexcitation via delayed conduction through the AVN, thereby promoting the contribution of accessory pathway conduction to ventricular excitation.

In conclusion, we report a right free wall accessory pathway with decremental antegrade conduction participating in antidromic AVRT, which was initially thought to be an instance of AVNRT. The diagnostic utility of atrial pacing for detecting inapparent preexcitation is highlighted.