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Left Ventricular Oxygen Tensions in Dogs During Coronary Vasodilation by Enflurane, Isoflurane and Dipyridamole

*Institute of Surgical Research and the Institute of Anesthesiology, Ludwig-Maximilians-University, Munich, West Germany.

Abstract

The purpose of this study was to investigate the effects of the anesthetics enflurane and isoflurane and of the coronary vasodilator dipyridamole on myocardial oxygen balance and myocardial tissue oxygen tensions. The studies were performed in 24 open-chest dogs during basal anesthesia with a narcotic. Myocardial blood flow (MBF) was measured using radioactive microspheres, myocardial surface tissue PO₂ by means of a platinum multiwire surface electrode. One control group and three experimental groups were studied: enflurane (1.1 vol%), isoflurane (0.7 vol%, both end-tidal concentrations), and dipyridamole (0.4 mg/kg). Mean arterial pressure significantly decreased to an average of 70 mm Hg in all three experimental groups. Although MBF was unchanged during enflurane (–18%) and isoflurane (+20%), it increased during dipyridamole (+304% p < 0.05 vs baseline and control, enflurane, and isoflurane groups). Myocardial oxygen consumption decreased significantly during enflurane and isoflurane but remained unchanged during dipyridamole. Thus, the ratio between myocardial oxygen delivery and consumption increased 6% with enflurane (p < 0.05 vs baseline), 47% with isoflurane (p < 0.05 vs baseline and control group) and 280% with dipyridamole (p < 0.05 vs baseline and control, enflurane, and isoflurane groups). Coronary venous PO₂ remained unchanged during enflurane but increased significantly during isoflurane and dipyridamole. Left ventricular surface tissue PO₂ was unchanged in enflurane and isoflurane animals and decreased slightly, yet significantly, during dipyridamole. All variables remained unchanged in the control group. Thus, isoflurane and dipyridamole interfered with MBF autoregulation and increased myocardial oxygen delivery out of proportion to myocardial demands. The lack of simultaneous increase in tissue oxygen pressures may reflect occurance of functional shunting on a cardiac microvascular level. These results also suggest that coronary vasodilating agents may not necessarily be beneficial for oxygenation of the intact myocardium.

Key Words: HEART: blood flow, myocardial • HEART: myocardial oxygenation • ANESTHETICS, VOLATILE: isoflurane, enflurane