Effects of verapamil on regional myocardial blood flow and ST segment. Role of the induced bradycardia

The effects of verapamil on regional myocardial blood flow and on the $\text{ST}$ segment were studied in both normal and ischemic regions in dogs with and without cardiac pacing.In the absence of cardiac pacing, verapamil (0.05 mg/kg/min/10 min) induced marked bradycardia, a drop in blood pressure and an increase in epicardial and endocardial flows both in normal and ischemic regions of the heart. However, in ischemic regions, redistribution was favorable since the endo/epi ratio increased from 0.46 to 0.61 (p<0.01) whereas this ratio did not vary in normal regions (0.94 vs. 0.92). This increase is accompanied by less marked $\text{ST}$ segment elevation during coronary occlusion in dogs treated with verapamil.In dogs with cardiac pacing, administration of verapamil under the same conditions again induced a drop in blood pressure and, in normal regions only, a homogeneous but less marked increase in epicardial and endocardial flows, the endo/epi ratio varying from 0.94 to 0.98. In contrast, in ischemic regions, blood flow was not modified by verapamil, but the endo/epi ratio dropped from 0.46 to 0.31 (p<0.01), while the $\text{ST}$ segment elevation observed did not differ from that recorded during the control occlusion.These results demonstrate the major role of verapamil-induced bradycardia in the anti-anginal effects of this drug.