MECHANISMS CAUSING SUSTAINED VENTRICULAR-TACHYCARDIA WITH MULTIPLE QRS MORPHOLOGIES - RESULTS OF MAPPING STUDIES IN THE INFARCTED CANINE HEART

Background Sustained reentrant ventricular tachycardias (VTs) with dif ferent QRS morphologies have been observed to occur spontaneously and during programmed stimulation in human hearts. We determined mechanism s that can cause tachycardias with multiple morphologies in a canine m odel of myocardial infarction by mapping reentrant circuits. Methods a nd Results Reentrant VT with multiple QRS morphologies was induced in 11 canine hearts with 4-day-old infarcts. Comparison of activation map s of the reentrant circuits in the epicardial border zone associated w ith each morphology indicated two basic mechanisms. Less frequently, V Ts of different morphologies in the same heart were caused by reentran t circuits in different regions of the infarct. Most commonly, the ree ntrant circuits associated with different morphologies were in the sam e region. Three different factors caused different exit routes from ci rcuits in the same region, leading to the multiple morphologies. (1) T he reentrant wave front for each morphology rotated around the same li ne of block but in different directions. (2) Reentrant circuits associ ated with each morphology were similar, but there were small changes i n the extent of the central line of block. (3) Reentrant circuits with completely different sizes and shapes caused different morphologies. Conclusions In this canine model, tachycardias with multiple morpholog ies most commonly arise from reentrant circuits in the same region of the infarct, suggesting that most often only one area has electrophysi ological properties necessary to sustain reentry.