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Acute occlusions of the left circumflex coronary artery were performed in open-chest dogs. A control group (n = 19) was compared with three groups (total n = 17) pretreated once daily with different doses of the cardioselective beta-blocking drug atenolol (ICI 66 082) given by mouth for 5 days. Only animals without coronary collateral vessels were examined, having a mortality rate of 100% in the control group. Arrhythmias and ventricular fibrillation during the first 30 min after coronary occlusion showed a biphasic distribution in time (phase 1a and 1b). A lower degree of beta-adrenoceptor blockade reduced the incidence of arrhythmias and ventricular fibrillation in phase 1a, but fibrillation occurred in all animals during phase 1b. A higher dose of the beta-blocking drug protected the animals from ventricular fibrillation, and arrhythmias in phase 1a were greatly reduced. At all times the ventricular fibrillation threshold in the group pretreated with atenolol was significantly higher than in the control group. In both groups a significant decrease in ventricular fibrillation threshold was found only during phase 1a. The greater sensitivity of phase 1a arrhythmias to beta-blockade and the lack of a decrease in ventricular fibrillation threshold during phase 1b might indicate differences in the genesis of arrhythmias and fibrillation in phases 1a and 1b.
Menken et al. (Mon,) studied this question.