Mechanisms for spontaneous changes in QRS morphology sometimes resembling torsades de pointes during reentrant ventricular tachycardia in a canine infarct model

Introduction: Spontaneous changes in QRS morphology during sustained reentrant ventricular tachycardia, occurring gradually or abruptly, causing the tachycardia to be polymorphic, have been described in clinical cases. The purpose of this study was to determine the mechanism for such changes in a canine infarct model. Methods and Results: Reentrant circuits were mapped in the epicardial border zone during sustained ventricular tachycardia in the canine heart, 4 days after left anterior descending coronary occlusion, In 10 tachycardias, there was either an abrupt change in QRS morphology or a gradual change that took up to 25 cycles. When the latter occurred, the ECG resembled torsades de pointes. Maps show ed that the predominant mechanism for the change in QRS was a shift in the exit route by which the impulse left the reentrant circuit to activate the ventricles (9/10 tachycardias), Such shifts resulted from small changes in conduction velocity in segments of the circuit, either speeding or slowing, which modified the length of the functional lines of block. Movement of the circuit to a different region was responsible for the change in QRS in only one of these experiments, in which the reentrant mechanism also changed from functional to anatomic. Conclusion: Subtle changes in conduction in reentrant circuits can alter QRS morphology, Changes in the exit route from a stable reentrant circuit can cause the ECG characteristics to resemble torsades de pointes.