Nitrous oxide does not alter infarct volume in rats undergoing reversible middle cerebral artery occlusion

This experiment was designed to determine if nitrous oxide alters neurologic and pathologic outcome from temporary focal cerebral ischemia in spontaneously hypertensive rats deeply anesthetized with a barbiturate. Two groups of rats were given intravenous methohexital such that a stable EEG pattern of burst suppression was achieved. In one group of rats (n = 11), the lungs were mechanically ventilated with 70% N₂O/30% O₂, and in the other group (n = 10), ventilation was done with 70% nitrogen/30% O₂. The middle cerebral artery was then occluded for 2 h, during which time mean arterial pressure, blood gases, hematocrit, plasma glucose, and head temperature were held constant between groups. The total doses of methohexital administered were similar in both groups as were the plasma methohexital concentrations immediately prior to onset of ischemia. After reperfusion of the middle cerebral artery, the animals were allowed to awaken. Neurologic evaluations were performed prior to ischemia and at 24 and 96 h postischemia. Cerebral infarct volume was measured at 96 h postischemia using triphenyl tetrazolium chloride staining and computer imaging techniques. There were no neurologic differences between the N₂O and nitrogen groups at any experimental interval although both groups exhibited deficits at both 24 and 96 h postischemia relative to preischemia values. The two groups also had nearly identical cerebral infarct volumes (N₂O = 231 ± 97 mm³; nitrogen = 226 ± 75 mm³; mean ± SD). In a subset of identically anesthetized rats not undergoing ischemia (70% N₂O; n = 5, 70% N₂: n = 5), the cerebral metabolic rate for glucose (CMR(glu)) was autoradiographically assessed in nine anatomic structures with the [¹⁴C]-2-deoxyglucose technique. No differences between anesethetic groups were observed in any structure. (CMR(glu) values ranged between 12-33 μmol/100 g/min). The authors concluded that N₂O has no metabolic stimulatory effect in rats deeply anesthetized with methohexital. In addition, the presence or absence of N₂O in the respiratory gas mixture (given a constant FI(O₂)) does not alter outcome from focal ischemia in rats undergoing deep barbiturate anesthesia.