Atrial fibrillation in the Mahaim syndrome: atrial pacing and adenosine-induced AV block to unmask a Mahaim physiology

Abstract

There is now evidence that patients with the so-called Mahaim physiology have an accessory AV node located anywhere round the tricuspid ring from which atriofascicular fibres exit to insert distally in the distal component of the right bundle [1]. Antegrade decremental conduction and the absence of retrograde conduction are the most characteristic properties of these fibres [2,3]. The resting ECG in these patients is normal, as ventricular activation proceeds via the AV node His–Purkinje network; during antidromic tachycardia, the most commonly reported manifestation of this syndrome, the surface ECG, shows features that fit in a substrate as described above: [1] a left bundle branch block morphology (LBBBM), as the left bundle does not participate in the ventricular activation; and [2] a superior electrical axis, as the activation front, iniciated in the apex of the right ventricle, is leftward and superiorly directed. Instead of a pre-excited tachycardia, should atrial fibrillation (AF) take place, these ECG features might easily be overlooked, as right-sided AV pre-excitation (PX) or acceleration-dependent aberrancy could account for beats with LBBBM.