Mahaim tachycardias.

Patients with Mahaim fibres form a distinct subgroup of the pre-excitation syndromes (less than 3%). They have episodes of a pre-excitation during sinus rhythm. Originally, Mahaim fibres have been classified into two main groups, nodoventricular and fasciculoventricular fibres. Recent evidence from both surgery and catheter ablation has shown that the substrate for tachycardia arises due to a slowly conducting fright atrioventricular (AV) accessory pathway (AP) with decremental properties. The pre-excited tachycardia (antidromic re-entrant tachycardia) is distinctive with a left bundle branch block (LBBB) pattern, long AV interval (due to the long conduction time over the AP) and short VA interval (over the AV node). The majority of these patients do not have episodes of narrow QRS complex, due to the absence of retrograde conduction of the AP. There are several ECG features that suggest Mahaim tachycardia as a cause of LBBB pattern tachycardia: QRS axis superior or between 0 degrees and 75 degrees, QRS duration of 0.15 s or less and precordial transition in lead V4 or after. Clinically, Ebstein's anomaly is relatively common and multiple APs are also observed with an increased frequency. Small studies and cases reports have demonstrated sensitivity to various classes of AA drugs. Class IA, IC and beta-blocker may be effective in preventing tachycardias. Small surgical series have reported excellent results in patients with accessory AV connections and Mahaim fibres tachycardia. However catheter ablation offers a definitive therapy in such patients with a high success rate and minimal morbidity.