Changes in coronary vessel resistance during postischemic reperfusion and effectiveness of nitroglycerin

Objective: Microvascular incompetence after ischemia and reperfusion may co mpromise the normal postischemic coronary perfusion and additionally jeopar dize the recovery of the myocytes. We investigated whether such a form of a cute endothelial dysfunction occurs in the routine operative setting despit e the use of protective measures. For this purpose, we measured pressure-fl ow relations in the coronary vasculature during heart operations before and after ischemia and after reperfusion and their reaction to the nitric oxid e donor nitroglycerin. Methods: Forty-eight patients with a low risk profile scheduled for routine coronary artery bypass surgery were included. During normothermic extracor poreal circulation, the fibrillating heart was completely excluded from byp ass by clamping of the ascending aorta and snaring of the caval veins. It w as relieved of blood by opening the right atrium and venting the left atriu m and ventricle to avoid distention. The coronary vessels were perfused und er controlled flow, and the perfusion pressures were monitored. This protoc ol was performed in 24 patients before and immediately after ischemia and a fter a reperfusion period. Results: Compared with the preischemic control, vascular resistance was dec reased by 17% (P < .003) immediately after ischemia but increased again by 46% (P < .0001) during an average of 25 minutes of reperfusion and, even mo re important, by 23% (P < .028) in comparison with the preischemic values. In two groups of 12 patients, nitroglycerin was added to the perfusate eith er in a dosage of 3 mug . kg . min(-1) or as a bolus injection of 2 mg. Low -dose nitroglycerin did not reduce the elevated postreperfusion resistances significantly, but bolus injection did (P < .0002). Coronary vessel resist ance increased during reperfusion in particular in patients with a history of hypertension. Conclusion: Coronary vasoconstriction during postischemic reperfusion is re gularly present in the routine operative setting in cardiac surgery, despit e myocardial protection measures. The amount of va so constriction varies c onsiderably and is particularly increased in patients with hypertension. Th e nitric oxide donor nitroglycerin can normalize the elevated resistances, but only in high dosages. This demonstrates a preserved ability of vascular smooth muscle to relax. The phenomenon had no sequelae in our low-risk pat ients having elective operations. However, it may gain significance in the case of severe left heart hypertrophy and in patients at risk with both a p ostoperative low-output syndrome and reduced mean arterial pressures during reperfusion.