CARDIAC ARRHYTHMIAS INDUCED BY INJECTION OF ISOPROTERENOL INTO THE CORONARY ARTERIES

The threshold doses of isoproterenol for induction of atrioventricular nodal rhythm by closearterial injection into the left circumflex coronary artery in cyclopropane-anesthetized dogs did not differ from the threshold doses determined in pentobarbital- or ether-anesthetized animals. Injections of isoproterenol or epinephrine into the anterior descending coronary artery caused arrhythmia only in cyclopropane-anesthetized dogs. The threshold dose of isoproterenol for nodal rhythm in pentobarbital-anesthetized vagotomized animals was not affected by the level of systemic blood pressure. An effect of pressure was seen occasionally in cyclopropane-anesthetized animals. In the latter, isoproterenol thresholds after injection into the anterior descending coronary artery were usually decreased by mechanically induced increases in systemic blood pressure. These thresholds were increased by decreases in pressure. Quinidine may sensitize the atrioventricular node to induction of pacemaker activity following isoproterenol injection into the left circumflex artery. On the other hand quinidine caused an increase in threshold for arrhythmias produced by isoproterenol injection into the anterior descending artery. The level of systemic blood pressure affects both these effects of quinidine.