Relaxation responses of the coronary microcirculation after cardiopulmonary bypass and ischemic arrest with cardioplegia: Implications for the treatment of postoperative coronary spasm

Abstract

Coronary arteriolar spasm may occur following cardiopulmonary bypass and ischemic arrest, resulting in impaired cardiac function. Because myocardial perfusion is principally regulated by the microcirculation, the in vitro effects of various clinically used vasodilating drugs on porcine coronary microvessels less than 200 μm in diameter were examined following cardioplegic arrest and reperfusion. After 1 hour of ischemic arrest using either crystalloid or blood cardioplegia solutions followed by 1 hour of reperfusion, microvessels were studied in a pressurized (40 mmHg), no-flow state, and imaged with a video tracking device. Vessels were preconstricted by 30% to 60% of their resting diameter using acetylcholine, and various vasodilatory agents were applied extraluminally. Responses to the β-adrenergic receptor agonist isoproterenol and the nitrovasodilator sodium nitroprusside were minimally altered by either cardioplegia solution as compared to control. In contrast, relaxation responses to both the calcium channel antagonist nifedipine and nitroglycerin were diminished after ischemic arrest and reperfusion. Relaxation reponses were similar with crystalloid or blood cardioplegia for all drugs tested. Despite its somewhat attenuated response, nifedipine remained the most potent vasodilator of those studied. It is concluded that (1) following ischemic arrest with either crystalloid or blood cardioplegia solutions, responses to sodium nitroprusside and isoproterenol were minimally altered, while responses to nifedipine and nitroglycerin were attenuated; (2) relaxation responses of coronary arterioles were not significantly different with either blood or crystalloid cardioplegia; and (3) despite a slightly decreased response after cardioplegia, nifedipine was the most potent vasodilator of coronary arterioles, and may be the best choice for treating postoperative coronary arteriolar spasm.