Regional blood flow and tissue oxygen pressures of the collateral-dependent myocardium during isoflurane anesthesia in dogs

The authors investigated the effects of isoflurane on blood flow and tissue oxygen pressures of a collateral-dependent myocardium. Seventeen dogs divided into two groups were studied 3-4 weeks after implantation of ameroid coronary artery constrictors to completely occlude the proximal part of the left anterior descending artery. Experiments were performed during anesthesia with an opiate that was infused intravenously throughout the experiments. In Group 1 (n = 9), measurements were obtained during control and during isoflurane- (1.6-2.2 vol%) induced hypotension (mean arterial pressure, 60 mmHg). In Group 2 (n = 8), the identical protocol was applied, but norepinephrine was infused to maintain normotension. Dipyridamole effects were studied in five animals of Group 2 after a second control period at least 1 h after discontinuation of isoflurane. Isoflurane-induced hypotension caused reductions of blood flow and surface tissue oxygen pressures in the collateral flow-dependent area. Vasodilation in the normal left ventricular areas was demonstrated by an unchanged blood flow despite a reduced oxygen consumption and by a significantly increased coronary sinus hemoglobin oxygen saturation. When arterial pressure was maintained at its control level by norepinephrine, tissue oxygen pressures remained constant and collateral as well as normal area flow increased significantly during isoflurane. Coronary vascular resistance was lower during administration of isoflurane and norepinephrine compared with that during isoflurane induced hypotension, suggesting a significant contribution of tissue oxygen demand in regulation of coronary vascular resistance. At comparable levels of arterial pressure and left ventricular oxygen consumption, normal zone blood flow was significantly higher during dipyridamole than during isoflurane and norepinephrine. Thus, isoflurane-induced hypotension decreased blood flow and tissue oxygen pressures of collateral flow-dependent myocardial areas. However, neither isoflurane nor dipyridamole caused such alterations when arterial pressure was normal.