OBSERVATIONS ON THE INITIATION OF SUSTAINED VENTRICULAR-TACHYCARDIA BY PROGRAMMED STIMULATION

We analyzed the initiation of sustained monomorphic ventricular tachycardia (VT) by programmed ventricular stimulation (PVS) in 50 consecutive patients who had clinical VT or aborted sudden cardiac death with remote myocardial infarction. In 25 of 50 patients, the first induced QRS complex of VT was morphologically identical to the succeeding QRS complexes of VT (type I). In 25 other patients, the first VT beat had a different morphology (type II). Type I had a significantly longer VT cycle length than type II (333 +/- 65 msec and 293 +/- 66 msec, P = 0.036). Type II VT initiation required more aggressive stimulation protocol than type I (type I: type II; number of extrastimulus required for induction 2.5 +/- 0.9:3.0 +/- 0.6, P = 0.026; shortest extrastimuli coupling interval 244 +/- 28 msec:220 +/- 23 msec, P = 0.002). The interval between the last extrastimulus and the onset of the first VT beat was 408 +/- 88 msec in type I and 336 +/- 75 msec in type II (P = 0.004). Furthermore, there was good correlation between the VT cycle length and the interval from last extrastimulus to the onset of nonpaced beat in type I but not in type II. We conclude: (1) in type I VT initiation, when compared to type II, the induced sustained VT had slower rates and required less extrastimuli and longer coupling interval of extrastimulus for induction; (2) in type I, initiation of VT suggests establishment of a reentrant circuit with the first nonpaced beat; (3) in type I, shortening of refractoriness is more important than the conduction delay in establishing sustained VT; and (4) in type II, the mechanisms of initiation of VT are heterogeneous.