The duration of coronary occlusion influences adrenergic contributions to reperfusion ventricular arrhythmias

Effects of coronary artery occlusion and reperfusion on the idioventricular rate in anesthetized dogs

OBJECTIVES

We evaluated the effects of alpha 1- and beta-adrenergic receptor blockade on the changes in idioventricular rate induced by occlusion and reperfusion of the left anterior descending coronary artery in anesthetized dogs.

BACKGROUND

Because sinus rate usually exceeds idioventricular rate, the experimental induction of complete atrioventricular (AV) block may improve the definition of the effects of coronary occlusion and reperfusion on idioventricular pacemaker mechanisms.

METHODS

We injected formaldehyde into the AV node to induce complete AV block in 35 chloralose-anesthetized dogs. Animals were assigned to untreated (n = 22), prazosin-treated (n = 6) and propranolol-treated (n = 7) groups. We paced the hearts at 120 beats/min during a 20- to 60-min period of complete coronary occlusion, but we did not pace the heart during reperfusion.

RESULTS

In the untreated group, the idioventricular rate did not change significantly during coronary artery occlusions of 20 or 40 min, but it did increase significantly during the last 20 min of a 60-min occlusion. Prazosin and propranolol each attenuated but did not abolish this rate increase. The idioventricular rate increased markedly but transiently soon after reperfusion in all untreated animals. This tachycardia was virtually abolished by either prazosin or propranolol.

CONCLUSIONS

The increase in idioventricular rate that develops during the last 20 min of a 60-min coronary occlusion is modulated but probably not mediated by adrenergic mechanisms. The pronounced ventricular tachycardia after reperfusion is virtually abolished by either alpha 1- or beta-adrenergic antagonists. Hence, this tachycardia requires the simultaneous activation of alpha 1- and beta-adrenergic mechanisms in dogs.

Effects of beta-adrenoceptor antagonism upon delayed reperfusion arrhythmias in conscious dogs

Whereas isolated heart preparations or anesthetized animals have been used to assess the actions of beta-adrenoceptor antagonists upon reperfusion-induced arrhythmias, this study evaluated the possible antiarrhythmic effects of beta-adrenoceptor antagonism in conscious animals. Conscious, chronically instrumented dogs were subjected to a temporary occlusion of the left anterior descending coronary artery for 4 h. After the first hour of reperfusion, flestolol (N-(1,1-dimethyl-2- ureidoethyl)-2-hydroxy-3-(O-fluorobenzoyloxy)-propylamine sulfate), an ultra short-acting beta-adrenoceptor antagonist, was administered i.v. at infusion rates of 1 and 2.6 micrograms/kg per min. A control group received the equivalent volume of saline during this period. Delayed reperfusion-induced arrhythmias were evaluated by using a computerized analysis system. Treatment with flestolol did not affect the total number of beats, the number of normal and ectopic beats, and the arrhythmic ratio, i.e. the ratio of ectopic beats to the total number of beats. Hence, beta-adrenoceptor antagonism does not appear to suppress delayed ectopic activity during coronary reperfusion in conscious dogs.