Conductive property of the zone of slow conduction of reentrant ventricular tachycardia and its relation to pacing induced terminability.

In order to assess the functional characteristics of the zone of slow conduction of reentrant VT, rapid pacing was performed to entrain VT. The orthodromic conduction time was measured as the interval between the stimulus and the orthodromically captured electrogram recorded distal to the zone of slow conduction, but not precisely at the exit point, and its response to rapid pacing was evaluated. In 32 of 33 consecutive patients, rapid pacing was performed to entrain VT. Of these, rapid pacing was repeated in 28 patients at 3-10 cycle lengths in steps of 10 msec before VT was terminated, or rapid pacing produced an acceleration of the rate. A pacing induced prolongation of the orthodromic conduction time (slowed conduction) was observed in 16 (57.1%) patients and in another 12 (42.9%) patients, the conduction time was constant. The pacing induced termination was observed in 93.8% of VT with slowed conduction and in 50% of VT with constant conduction, and the difference was significant (P < 0.05). There was no difference in the cycle length of VT or the shortest paced cycle length between VT with and without slowed conduction. The zone of slow conduction in human VT showed different conductive properties and VT with slowed conduction was associated with an easier and safer terminability with rapid pacing. The fact might be useful in selecting patients for antitachycardia pacing.