In 1941, Mahaim and Winston described the histology of anomalous connections that arise from the AV node and insert into the right ventricle.1 This was the first description of nodoventricular or so-called Mahaim accessory pathways. Accessory pathways with decremental conduction properties that connected the atrium to the right bundle branch (RBB) were subsequently mapped mainly at the lateral aspect of the tricuspid annulus, and thus the term atriofascicular was also adopted. [2][3][4] We know now that decrementally conducting connections can be between the right atrium or the AV node and the right ventricle in or close to the RBB. 5-7 Thus, although they are anatomically distinct from the initially described nodoventricular pathway, they present with similar electrocardiographic and electrophysiological characteristics and the term 'Mahaim' has been adopted to describe pathways with the following features:
Electrocardiographic Features
ECG During Sinus RhythmDuring sinus rhythm overt pre-excitation is usually absent (see Figure 1). Only subtle ECG abnormalities, such as an rS pattern in lead III, absence of septal Q waves in leads I and V6, and terminal QRS slurring or notching, suggest the presence of Mahaim conduction.
9,10However, patients with short, rapidly conducting fibres may have typical pre-excitation.
10
ECG during tachycardiaAlthough, typically, antidromic atrioventricular reentrant tachycardia over a Mahaim fibre has a left bundle branch (LBBB) morphology (see Figure 2), various QRS patterns and axis may occur (see Figure 3). 6,7 It seems that these pathways insert into or near the RBB, and variations in the frontal plane axis can be explained by the location of the exit of the RBB and a variable degree of fusion of ventricular activation between anterograde conduction over the pathway and, following retrograde invasion into the RBB, partial anterograde left ventricular activation over the left-sided conduction system, especially the anterior fascicle (see Figure 4). The various QRS patterns and rate changes seen during the change from short to long V-A tachycardia can be explained by the mode of retrograde conduction over the bundle branch system.
6,7
Electrophysiological Properties
Nodoventricular or Atriofascicular?Although the first case of this arrhythmia was studied electrophysiologically by Wellens and published in 1971, and considered to be based on a nodo-ventricular pathway, 11 with the advent of surgical and then catheter ablation in the 1980s, it was discovered that most fibres with Mahaim conduction characteristics originated at the lateral aspect of the tricuspid annulus, and the term atriofascicular pathway was adopted. [2][3][4][5][12][13][14][15][16][17] However, posteroseptal locations could also be found, 7,14 and true nodoventricular fibres have been identified (see Figure 5). 1,14,16 In addition, some of these pathways, so-called 'short'as opposed to 'long' Mahaims, may insert at the ventricle near rather than in the RBB. [5][6][7]18 Thus, pathways with Mahaim characteristic...