Origin of Epicardial ST‐T Wave Potentials in the Intact Dog

Ventricular repolarization was analyzed by measuring epicardial potential distributions in intact dogs with single or multiple ectopic foci. During the S-T segment there was a maximum at each ectopic focus and a minimum at the terminal sile(s) of excitation. During the latter half of the T ware the distributions became more complex, and two maxima evolved from the initial one at each ectopic site. The measured epicardial potentials were simulated by means of a model of ST-T wave events that is suitable for study of single and multiple ectopic beats with fusion, a model we call the "SI model." lntracellular potentials around the ventricles during repolarization were calculated from measured excitation sequences and known action potential shapes. The extracellular potentials around the ventricles were computed from the intracellular ones by a simplified ventricular geometry. The satisfactory agreement between the theoretical and measured extracellular potential distributions shows that the complex changes which occur throughout the ST-T wate are predicted well on the basis of changes, in the intracellular potential distributions. In contrast to the well known lability of the T wave from beat to beat, the results show that for any single beat the events of repolarization proceed in an entirely repeatable and deterministic fashion. The results present a way to directly represent cardiac extracellular events during the ST-T wave, a method analogous to the use of isochrones during QRS, and they imply that in the future it will be possible to achieve a more precise quantitative understanding of the events of the ST-T wave than thus far has been possible for QRS.