[The phenomenon of accelerated A-V conduction. Electrophysiological study with a "selective" calcium antagonist...

In order to discover slow response calcium dependent fibers within the structure with enhanced A-V conduction, an acute i.v. test with Verapamil (a highly selective calcium antagonist) was done in 13 patients with enhanced A-V conduction syndrome (4 had A-V by-pass, Kent type, associated). The action of Verapamil on the following parameters was studied: 1) A-H interval in sinus rhythms; 2) delta A-H interval (i.e. the maximum increment of A-H interval after incremental atrial pacing); 3) antegrade and retrograde function curves of the structure with enhanced A-V conduction; 4) R-R cycle of the reciprocating tachycardia, antegrade and retrograde conduction time (after identification of the reentry circuit) and pattern of induction and interruption of the tachycardia. Only in 1 patient a total atrio-hisian by-pass was identified. In all the other patients the electrophysiologic response to i.v. Verapamil suggested the presence of slow response calcium dependent fibers within the structure with enhanced A-V conduction which was always depressed by the drug even if to a lesser degree than in a normal A-V node. From these data and from the analysis of the epidemiology of enhanced A-V conduction, the Authors hypothesize that an infantile type A-V node could be in most cases the anatomical basis of enhanced A-V conduction while only rarely a total or partial A-V node by-pass.