1993 Volume 34 Issue 2 Pages 227-234
In a patient with sustained ventricular tachycardia (VT), we observed two different conduction times through the reentry circuit at the critical paced cycle length. The cycle length of the VT was 420msec and overdrive pacing initially performed at a paced cycle length of 400msec and repeated at decrements of 10msec until the VT was interrupted at a paced cycle length of 320msec. During rapid pacing, constant fusion and progressive fusion were confirmed. The first post-pacing return cycle was identical to each paced cycle length. The conduction time between the stimulus artifact and the orthodromically captured electrogram at the left ventricle was constant at 350msec in each paced cycle length. However, only at a pacing cycle length of 360msec two conduction times were alternatively observed, one of 350msec and the other of 365msec. When the conduction time changed from 350msec to 365msec, morphological alternation both in the surface QRS complex and in the orthodromically captured electrogram was evident. Dual slow pathways or a single slow pathway with plural exits from the reentry circuit is a likely mechanism of the alternation.
