Spontaneous changes in ventricular tachycardia cycle length and their relation to earliest sites of epicardial activation in a canine model.

Background: The purpose of this study was to examine the spontaneous changes in cycle length during episodes of sustained monomorphic (MVT) and polymorphic (PVT) ventricular tachycardias and to relate these changes with the earliest epicardial activation site of the beat.

Methods: Isochronal activation maps were obtained from 127 unipolar electrograms recorded from the surface of both ventricles with a sock electrode array in 24 open chest anesthetized dogs. After atrioventricular block, the left anterior descending coronary artery was occluded for 60 min under ventricular pacing (140/min), followed by reperfusion. In 7 dogs the left stellate ganglion was stimulated 5 min after reperfusion.

Results: In 7 MVTs (reperfusion) and 4 PVTs (sympathetic stimulation), cycle length changes showed an initial acceleration, reaching a minimum cycle length and then decelerating before termination. Isochronal maps showed radial spread from earliest activation, without conduction block. Cycle length (481 +/- 80 msec) in MVT had beat to beat variations of 15 +/- 17 msec corresponding to small shifts in sites of the earliest activation, clustered along the border of the ischemic myocardium. In PVTs the cycle length (352 +/- 90 msec, p < 0.01) had a variability of 62 +/- 23 msec, corresponding to wide changes in the sites of earliest activation in right and left ventricles. Linear regression analysis showed a strong and significant correlation between cycle length variability and the number of electrodes with the earliest activation (r = 0.77, p < 0.0001).

Conclusion: In these models of monomorphic and polymorphic ventricular tachycardias, cycle length variability showed a significant correlation with the number of electrodes with the earliest activation. MVTs showed concentrated origins with regular cycle length, whereas PVTs showed dispersed origins with irregular cycle length. These results suggest that the earliest epicardial activation site of the beat could be a factor in determining the dynamics in the cycle length.