Cerebral effects of nitrous oxide in the dog

The cerebral effects of nitrous oxide, 60 per cent, were examined in 27 dogs. During administration of halothane, 0.2 per cent, nitrous oxide increased cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMR02) to a maximum of 203 and 121 per cent of control, respectively. Cerebrospinal fluid pressure paralleled the change in CBF. The electroencephalogram (EEG) showed low-voltage slow-wave activity. With halothane, 0.8 per cent, nitrous oxide increased CBF and CMR02 to maximum values of 164 and 108 per cent of control, respectively. After administration of thiamylal, 8 mg/kg, intravenously, nitrous oxide did not increase CBF or CMR02 for the first 30-min period, but thereafter, CMR02 increased to 11 per cent above control. Pretreatment with reserpine, 0.5 mg/kg, intramuscularly, for two days did not modify the cerebral circulator and metabolic responses to nitrous oxide. These results indicate that nitrous oxide causes cerebral metabolic stimulation accompanied by an increase in CBF and slowing of the EEG. Sympathoadrenal stimulation would appear not to be the mechanism for the increases in CBF and CMR02. The cerebral effects of nitrous oxide are modified by the background anesthesia.