Induction of ventricular fibrillation versus monomorphic ventricular tachycardia during programmed stimulation : role of premature beat conduction delay

Premature stimuli can cause ventricular fibrillation (VF) during electrophysiological testing. The electrophysiological correlations associated with the onset of VF were evaluated in 40 patients who had this rhythm induced during programmed ventricular stimulation. These parameters were compared with those observed in 51 patients who had inducible sustained monomorphic ventricular tachycardia (VT) and 45 patients who had no inducible sustained ventricular tachyarrhythmias. Shortest premature coupling intervals for S2, S3, and S4 at induction of tachycardia or before achieving refractoriness, corresponding conduction latencies (defined as the time from the premature stimulus to the upstroke of the depolarization wave front recorded 35 mm away from the stimulation site), and ventricular activation times (defined as the time from the premature stimulus to the end of the depolarization wave) were compared. The mean coupling intervals were longest in the inducible VT patients: 300 +/- 30, 254 +/- 57, and 228 +/- 32 msec for S2, S3, and S4, respectively. In the inducible VF group, the coupling intervals were 260 +/- 37, 208 +/- 20, and 213 +/- 30 msec. In the group with no inducible VT or VF, these coupling intervals were 251 +/- 24 (p less than 0.01 versus inducible VT group), 209 +/- 27 (p less than 0.001 versus inducible VT group), and 194 +/- 21 msec (p less than 0.05 versus inducible VT and VF groups). The coupling interval of the last premature extrastimulus was above 200 msec in 70% of the patients in whom VF was induced. The largest increases in latency and activation times were recorded in patients in whom VF was induced. The cumulative increase in latency, defined as increased conduction time from baseline, summed for all the premature stimuli was also the greatest at initiation of VF. In contrast, the smallest increases in these parameters were noted in the patients with no inducible VT or VF. Measurements of total activation time yielded similar results as those recorded for latencies. The most important parameters distinguishing the VT patient population from the other two groups were the low ejection fractions and the longer coupling intervals at which VT was induced, whereas in the VF group, the most important discriminating factor was cumulative activation time. Sixty-three percent of the inducible VF patients presented with abnormal hearts (myocardial infarction or cardiomyopathy), whereas 88% of the inducible VT patients had abnormal hearts. In