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 reentr ant ventricular tachycardia, occurring gradually or abruptly, causing the t achycardia to be polymorphic, have been described in clinical cases. The pu rpose of this study was to determine the mechanism for such changes in a ca nine infarct model. Methods and Results: Reentrant circuits were mapped in the epicardial borde r zone during sustained ventricular tachycardia in the canine heart, 4 days after left anterior descending coronary occlusion, In 10 tachycardias, the re was either an abrupt change in QRS morphology or a gradual change that t ook up to 25 cycles. When the latter occurred, the ECG resembled torsades d e pointes. Maps show ed that the predominant mechanism for the change in QR S was a shift in the exit route by which the impulse left the reentrant cir cuit to activate the ventricles (9/10 tachycardias), Such shifts resulted f rom 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 th e change in QRS in only one of these experiments, in which the reentrant me chanism also changed from functional to anatomic. Conclusion: Subtle changes in conduction in reentrant circuits can alter QR S morphology, Changes in the exit route from a stable reentrant circuit can cause the ECG characteristics to resemble torsades de pointes.