Propranolol in experimental myocardial ischemia: dissociation of effects on contraction and epicardial ST segments.

Recent studies have suggested that propranolol decreases the extent of myocardial injury in acute ischemia. Although other studies have shown that global myocardial performance is depressed, zonal effects of propranolol in this setting are unknown. Therefore, the effect of propranolol (1.0 mg. per kilogram) was investigated in nine dogs with the use of Walton-Brodie strain gauge arches and local epicardial electrograms (10 to 12 sites). The heart rate effects of propranolol were controlled by atrial pacing. After coronary occlusion, heart rate increased slightly without a significant change in blood pressure. Following the infusion of propranolol, heart rate decreased significantly from 165.0 +/- 3.5 to 126.2 +/- 4.7 beats per minute (p less than 0.001) while both the systolic and diastolic blood pressures showed insignificant changes. After coronary occlusion, nonischemic zone tension showed no significant changes; however, propranolol decreased total tension from 105.2 +/- 2.5 per cent to 66.8 +/- 4.7 (p less than 0.001). Similarly, propranolol further decreased total tension in the border zone from 84.4 +/- 6.7 per cent (p less than .02) to 50.6 +/- 5.1 (p less than 0.01). Ischemic zone tension also fell further (p less than 0.025) after propranolol. Restoration of prepropranolol heart rate had no significant effect on tension development. Following coronary occlusion, sigmaST increased from 5.7 +/- 2.2 to 72.9 +/- 20.1 mv. (p less than 0.001). Coincident with the decrease in heart rate and tension development induced by propranolol, sigmaST decreased to 60.8 +/- 18.8 mv. (p less than 0.05). When the heart rate was restored to prepropranolol level, sigmaST again rose to 73.2 +/- 16.9 mv. (p less than .005). Thus, propranolol does effect an improvement in ischemic injury which is related, at least in part, to the induced decrease in heart rate. A concomitant substantial decrease in local tension development also occurs, however. The latter observations may limit the potential usefulness of propranolol in this setting.