Effects of the ventricular premature beat on the alternation of the repolarization phase in ischemic myocardium during acute coronary occlusion in dogs.

Effects of ventricular premature beats (VPB) on the alternans of the ST segment (ST alternans, STA) in the epicardial ECG and of the monophasic action potential (MAP) were examined during acute coronary occlusion in dogs. When STA was recorded simultaneously from four different points it was discordant in most cases. The discordant STA was accompanied by discordant alternation of the repolarization phase of MAP. A VPB transformed the discordant alternans into a concordant one and potentiated the degree of alternans. Transient prolongation or shortening of the cycle length showed effects similar to those of the VPB. The concordant alternans which was transformed from a discordant one by a VPB was not remarkably potentiated by a VPB, or by prolongation or shortening of the cycle length. It is possible that the transformation of the discordant alternans into a concordant one may contribute to the potentiation of STA by a VPB, and that the effects of VPB are due to the effects of the compensatory pause as well as the short cycle length associated with the VPB.