Evidence that nitrous oxide can maintain anaesthesia after induction with barbiturates

Three cats were prepared under short acting barbiturate methohexital: approximately 30 mg/kg, given i.v., as required, over about 1.5 hr). Each cat was then paralyzed by continuous infusion of gallamine (7.5 mg/kg.hr) and artificially ventilated with 80% N₂O/19% O₂/1% CO₂. The electroencephalogram, electrocardiogram and expired P(CO₂) were monitored throughout the ensuing neurophysiological experiment. Maximum P(CO₂) was held at 4%. Apart from a reduction in E.E.G. spindling there was no discernible change throughout the 30-32 hr of the experiments. Even a firm pinch to the forepaw failed to desynchronize the E.E.G. However, desynchronization did occur if the concentration of N₂O was reduced to 65% but did not if it was raised again to 80%. After 28-29 hr relaxant infusion was stopped: 1-2 hr later each cat was able to breathe the N₂O mixture for itself. In this unparalyzed state the animals were still adequately anesthetized, having very little spontaneous movement and only a weak withdrawal reflex. Finally, after 30-32 hr, a light dose of methohexital (3 mg/kg) did not affect the E.E.G. although larger doses did cause barbiturate spindling.