Cerebral autoregulation in awake versus isoflurane-anesthetized rats

We evaluated regional cerebral and spinal cord blood flow in rats during isoflurane anesthesia. Tissue blood flow was measured in cerebral cortex, subcortex, midbrain, and spinal cord using radioactive microspheres. Blood flow autoregulation was measured within the following arterial blood pressure ranges (mm Hg): 1 = <50, 2 = 50-90, 3 = 90-130, 4 = 130-170, 5 = >170. Arterial blood pressure was increased using phenylephrine infusion and decreased with ganglionic blockade and hemorrhage. Three treatment groups were studied: 1 = awake control, 2 = 1.0 minimum alveolar anesthetic concentration (MAG) isoflurane, 3 = 2.0 MAG isoflurane. Autoregulation was seen in awake rats from 50 to 170 mm Hg in all tissues. The autoregulatory coefficient (change in blood flow/change in blood pressure) was increased in midbrain and spinal cord during 1.0 MAC isoflurane and in all tissues during 2.0 MAG isoflurane (P < 0.05). Within the arterial blood pressure range of 90-130 mm Hg, isoflurane produced the following changes in tissue blood flow (percent of awake control): 1.0 MAG isoflurane: cortex = 87% ± 8% (P > 0.30), subcortex = 124% ± 11% (P > 0.05), midbrain = 263% ± 20% (P < 0.001), spinal cord = 278% ± 19% (P < 0.001); 2.0 MAC isoflurane: cortex = 137% ± 13% (P < 0.05), subcortex = 272% ± 24% (P < 0.001), midbrain = 510% ± 53% (P < 0.001), spinal cord = 535% ± 50% (P < 0.001). These results show that isoflurane produces dose-dependent cerebral vasodilation and loss of autoregulation which is regionally specific.