Alternation in the fusion complex during constant pacing of sustained ventricular tachycardia

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 420 msec and overdrive pacing initially performed at a paced cycle length of 400 msec and repeated at decrements of 10 msec until the VT was interrupted at a paced cycle length of 320 msec. 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 350 msec in each paced cycle length. However, only at a pacing cycle length of 360 msec two conduction times were alternatively observed, one of 350 msec and the other of 365 msec. When the conduction time changed from 350 msec to 365 msec, 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.