Mahaim’s bundle

ECG 8A: Tachycardia of 180 bpm, left delay, left axis; atrial activity difficult to pinpoint (possible 1:1 ratio with atrial activity in the T wave);

ECG 8B:

ECG 8B: The tachycardia is interrupted following vagal maneuvers; relatively short PR pattern with slightly pre-excited QRS;

Comments: The pattern of the tachycardia and of the sinus rhythm electrocardiogram is evocative of a tachycardia due to an accessory pathway called Mahaim-type. An electrophysiological study procedure allowed revealing a right atriofascicular accessory pathway with decremental conduction which was subsequently ablated, suppressing the tachycardias and correcting the pre-excitation pattern on the sinus electrocardiogram.

In 1937, Mahaim initially described fibers providing a connection between the bundle of His and the right ventricular myocardium (fasciculo-ventricular); the term Mahaim fibers was subsequently extended to the connections between the atrioventricular node and the myocardium (nodo-ventricular). Fasciculo-ventricular fibers are rare although their actual incidence is minimized by the absence of clinical translation. The PR interval is normal and the QRS complex can be broadened with an initial slurring. The nodo-ventricular fibers are even much rarer and expose to reciprocating tachycardia episodes (broad QRS, left delay pattern). The atria are not included in the reentry circuit. In sinus rhythm, the PR interval is normal or somewhat short, the QRS complex is normal or weakly altered.

For many years, the following association suggested the presence of Mahaim fibers: 1) 1:1 tachycardia with broad QRS, left delay, 2) little or no pre-excited sinus rhythm pattern, 3) characteristic electrophysiological study: normal AH interval, short HV interval; under rapid pacing at increasing rate: the PR and AH intervals are prolonged, the HV interval becomes shorter and the QRS exhibits a left delay pattern, with depolarization dependent on the accessory bundle.

It would appear, however, that a large number of accessory pathways corresponding to this electrocardiographic clinical picture do not correspond to the topology described by Mahaim, but are of the atriofascicular or atrioventricular type as in this patient. They are hence termed “pseudo-Mahaim”. Tachycardias with left delay are linked to the anterograde use of particular atrioventricular connections located on the lateral wall of the tricuspid annulus, remotely from the atrioventricular node, with a septal ventricular insertion toward the emergence of the right branch and exhibiting decremental conduction properties. The anterograde conduction is via the accessory pathway and the retrograde conduction via the atrioventricular node or a second accessory bundle. They constitute right accessory paths for which some of their properties (slow and decremental conduction) can be assimilated to an accessory atrioventricular node located on the lateral wall of the tricuspid annulus. These accessory pathways have an electrophysiological behavior similar to that of the fibers initially described by Mahaim with prolongation of the AH interval and shortening of the HV interval occurring during the appearance of the left delay during atrial pacing at increasing rate. The tachycardias are exclusively antidromic, this type of fibers only conducting in anterograde manner (no retrograde conduction). Only atrioventricular dissociation is impossible whereas possible with the classical Mahaim fibers since the atrium is an integral component of the reentry circuit.

The typical pattern of tachycardia on this type of accessory pathway includes a wide QRS, a left delay pattern (these are right ventricular pathways), a left axial deviation, a most often late precordial transition of the QRS complex (R/S ratio > 1).

The sinus rhythm electrocardiogram shows an often normal or moderately shortened PR interval (possible association with an improved conduction of the nodo-Hisian pathway), moderate pre-excitation (incomplete left bundle branch block pattern) or absent from the QRS complex (no delta wave), one of the only visible signs being the potential disappearance of the septal q wave in V5-V6.

The electrophysiological study procedure restores the characteristics of this type of pathway: exclusive anterograde conduction with decremental conduction; highlighting of a particular potential (M potential) generated by the accessory pathway at the level of the tricuspid annulus in its lateral portion. This potential represents the target of the ablation procedure.

Take-home message: Initially, a bundle of Mahaim corresponded to fasciculo-ventricular or nodo-ventricular fibers; nowadays, the term “pseudo-Mahaim” is used in the context of atriofascicular or atrioventricular fibers with decremental conduction and the possibility of antidromic wide QRS tachycardias with a left delay pattern and left axis deviation;