MAHAIM TACHYCARDIAS

Patients with Mahaim fibres form a distinct subgroup of the pre-excita tion syndromes (less than 3%). They have episodes of a pre-excited tac hycardia but usually do not exhibit ventricular pre-excitation during sinus rhythm. Originally, Mahaim fibres have been classified into two main groups, nodoventricular and fasciculoventricular fibres. Recent e vidence from both surgery and catheter ablation has shown that the sub strate for tachycardia arises due to a slowly conducting right atriove ntricular (AV) accessory pathway (AP) with decremental properties. The pre-excited tachycardia (antidromic re-entrant tachycardia) is distin ctive with a left bundle branch block (LBBB) pattern, long AV interval (due to the long conduction time over the AP) and short VA interval l 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 tachycardi a 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 precordi al transition in lead V-4 or after. Clinically, Ebstein's anomaly is r elatively common and multiple APs are also observed with an increased frequency. Small studies and case reports have demonstrated sensitivit y to various classes of AA drugs. Class IA, IC and beta-blockers may b e effective in preventing tachycardias. Small surgical series have rep orted excellent results in patients with accessory AV connections and Mahaim fibres tachycardia. However catheter ablation offers a definiti ve therapy in such patients with a high success rate and minimal morbi dity.