Porcine brain and myocardial perfusion during enflurane anesthesia without and with nitrous oxide

Brain and myocardial blood flow (MBF) were examined in 10 previously instrumented swine during isocapnic conditions using 15 micron in diameter radionuclide-labeled microspheres that were injected into the left atrium. Minimum alveolar concentration (MAC) of enflurane required to prevent 50% of the pigs from responding by gross purposeful movement to a noxious stimulus was 1.66%. In pigs, 50% nitrous oxide decreased enflurane requirement for 1.0 MAC anesthesia by 0.82%. Each animal was studied during the following conditions: (a) unanesthetized (control); (b) 1.0 MAC enflurane anesthesia (1.66% end-tidal concentration); (c) 1.5 MAC (2.49%) enflurane anesthesia; (d) the equivalent of 1.0 and 1.5 MAC anesthesia produced by enflurane (0.84 and 1.67%) plus 50% nitrous oxide. Cerebral and total brain blood flow values were the same as control values during both levels of enflurane anesthesia. However, blood flow in the brainstem and cerebellum exhibited a dose-related increase; the increment of 28% for each of these regions at 1.5 MAC achieved statistical significance. Vascular resistance in all regions of the brain decreased with enflurane anesthesia. Substitution of 50% nitrous oxide for enflurane to maintain the same level of anesthesia markedly increased cerebral blood flow. At 1.0 and 1.5 MAC anesthesia produced using enflurane plus 50% nitrous oxide, cerebral blood flow was 151 and 183% of the control value, respectively. During enflurane plus nitrous oxide anesthesia equivalent to 1.5 MAC, cerebellar and brain stem blood flow were 135 and 180% of respective control values. MBF in all regions decreased in a dose-related manner with enflurane anesthesia. At 1.5 MAC enflurane, perfusion values in the walls of the left and the right ventricles were 52 and 59% of respective control values. During both levels of enflurane plus 50% nitrous oxide anesthesia, transmural MBF in all regions remained close to awake values. Subendocardial/subepicardial perfusion ration in both ventricles exceeded 1.00 during all steps of the protocol, thereby suggesting that subendocardial O2 delivery kept pace with O2 demand. These experiments have demonstrated that usage of 50% N2O with enflurane to produce equipotent anesthesia resulted in a dramatic increase in cerebral blood flow while MBF remained near awake value.