Mechanisms Causing Sustained Ventricular Tachycardia With Multiple QRS Morphologies

BACKGROUND: Sustained reentrant ventricular tachycardias (VTs) with different QRS morphologies have been observed to occur spontaneously and during programmed stimulation in human hearts. We determined mechanisms that can cause tachycardias with multiple morphologies in a canine model of myocardial infarction by mapping reentrant circuits. METHODS AND RESULTS: Reentrant VT with multiple QRS morphologies was induced in 11 canine hearts with 4-day-old infarcts. Comparison of activation maps of the reentrant circuits in the epicardial border zone associated with each morphology indicated two basic mechanisms. Less frequently, VTs of different morphologies in the same heart were caused by reentrant circuits in different regions of the infarct. Most commonly, the reentrant circuits associated with different morphologies were in the same region. Three different factors caused different exit routes from circuits in the same region, leading to the multiple morphologies. (1) The reentrant wave front for each morphology rotated around the same line of block but in different directions. (2) Reentrant circuits associated with each morphology were similar, but there were small changes in 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 morphologies most commonly arise from reentrant circuits in the same region of the infarct, suggesting that most often only one area has electrophysiological properties necessary to sustain reentry.