Cycle length dynamics at the onset of postinfarction ventricular tachycardias induced in canines: Dependence on interval-dependent excitation properties of the reentrant substrate

Tachycardia Onset Dynamics. Introduction: Postinfarction monomorphic ventri cular tachycardias induced by programmed stimulation may display initial cy cle length (CL) variations before stabilizing. Methods and Results: To show that tachycardia onset dynamics depend on rate -dependent electrical properties of the reentrant substrate, we extracted a ctivation times and maximum negative slopes of local activation complexes ( -dV/dt(max)) from 191 unipolar electrograms recorded in the anterior left v entricular wall of anesthetized, 3-day-old infarct canine preparations. Mea surements were made of the responses to programmed stimulation, as well as in early and later beats of tachycardias, which displayed either a constant trend in CL (group A, n = 5 preparations) or one in which CL prolongation occurred according to an exponential course before stabilizing (group B, n = 9), Stimulation protocols inducing the tachycardias were more aggressive and their CL was significantly shorter (CL = 159 +/- 24 msec) in group A th an in group B (stabilized CL = 206 +/- 34 msec), Reentrant activity occurre d in subepicardial areas in which the absolute value of -dV/dt(max) \-dV/dt (max) \) was heterogeneously depressed (<2 mV/msec), \-dV/dt(max)\ was redu ced and activation delay increased in the successive responses to extrastim uli. Further reductions in I-dV/dt(max)\ (10% to 23%) were shown to occur b etween early and later beats in 5 of the 9 tachycardias in group B (no chan ge in the 4 others), and they were associated with localized prolongation o f conduction times in reentrant pathways. In contrast, \-dV/dt(max)\ improv ed in all group A tachycardias (7% to 25%), Conclusion: This study provides evidence that the onset dynamics of postinf arction ventricular tachycardias are determined by interval-dependent elect rical changes occurring in the reentrant substrate.