His bundle electrograms were recorded in 2 patients with ectopic beats arising in accessory atrioventricular tracts. Case 1 had Wolff-Parkinson-White (WPW) type A and a left-sided Kent tract with a short effective refractory period. Though ectopic impulse formation most probably occurred within the Kent tract itself, a vulnerability-related origin in the ventricular muscle close to the distal end of the Kent tract could not be excluded. In Case 2, with a Mahaim tract extending from His bundle to ventricles, there were three types of QRS morphologies resulting from : (a) atrioventricular conduction exclusively through the normal pathways; (b) atrioventricular conduction through both, normal pathway, and Mahaim tract; and (c) ectopic impulse formation in the Mahim tract. Specialized electrophysiological studies were essential to diagnose these unusual arrhythmias.
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