EEG changes during carotid occlusion and hypotension in the rat: the effects of nitrous oxide

It has been suggested that N2O may alter the sensitivity of the brain to ischemia. To test this hypothesis. we examined the effects of N2O on the development of left-right hemispheric asymmetry in the electroencephalogram (EEG) during hemorrhagic hypotension in rats subjected to unilateral carotid occlusion. Rats were anesthetized with halothane/O2/air, and ventilated to normocarbia (PaCO2 approximately 40 mm Hg). Catheters were placed in the femoral artery and vein, and both common carotid arteries (CCA) were exposed. Bilateral fronto-occipital screws were then placed to record left and right hemispheric EEGs, which were processed by computer and stored on disc. Animals were then randomly assigned to one of three treatment groups (n = 8 each): group 1, 0.5 MAC (0.5%) halothane + 0.5 MAC (70%) N2O; group 2, 1 MAC (1.0%) halothane + 70% nitrogen; and group 3, 1 MAC halothane + 70% N2O. After stabilization, the left CCA were occluded. Animals with EEG changes at this point were discarded. Beginning 5 min later, venous blood withdrawal was started at a rate of 0.25 ml/min, while mean arterial blood pressure (MAP) and EEG were continuously recorded. After exsanguination was complete, EEG data (raw and processed) were re-examined by an individual who was unaware of the anesthetics administered to determine the MAP at which any evidence of EEG asymmetry appeared. There were no intergroup differences in weight, PaO2, PaCO2, pHa, blood glucose. hematocrit, or starting (prebleed) MAP. The earliest change in the EEG was typically a decrease in total amplitude over the hemisphere ipsilateral to the carotid occlusion. Adding 70% N2O to a 1 MAC halothane background (group 2 vs. group 3) had no effect on the MAP at which this EEG asymmetry appeared (54 +/- 13 vs. 53 +/- 10 mm Hg). However, this MAP was significantly higher in animals breathing 0.5 MAC halothane + 0.5 MAC N2O (group 1, 78 +/- 17 mm Hg, p = 0.0019 by ANOVA). We conclude that 70% N2O had no direct effects on the MAP at which EEG abnormalities appear (group 2 vs. 3), and that the observed differences are more closely related to the concentration of volatile agent. Whether these differences are related to anesthetic-induced differences in the brain's tolerance to reduced CBF or whether there are differences in cerebral blood flow (CBF) autoregulation are unknown.