348 NITROUS OXIDE DEPRESSES ELECTROENCEPHALOGRAPHIC RESPONSES TO REPETITIVE NOXIOUS STIMULATION

BackgroundAlthough N20 has been used for over a century as an anesthetic adjuvant its effect on electroencephalographic (EEG) activity is poorly understood because it is most often studied in the presence of additional anesthetics, such as potent inhaled anesthetics. We examined the EEG effects of N20 in rats using a hyperbaric chamber that permitted N20 to be the sole anesthetic.MethodsRats (n = 8) were anesthetized with isoflurane and EEG activity was recorded from screws placed into the skull. The rats were placed into a hyperbaric chamber and mechanically ventilated. The isoflurane was eliminated while the chamber was pressurized with N20. The minimum alveolar concentration (MAC) was determined in five rats by adjusting the chamber pressure and N20 concentration and applying a tetanic noxious stimulation to the tail via an electrical pass-through at the end of the chamber. EEG responses to noxious stimulation (20 electrical pulses applied to the tail at 0.1, 1, and 3 Hz and 50 Hz tetanic stimulation applied for 30 sec) were determined at 1.5 and 2.0 atm N20.ResultsThe N20 MAC was 1.7 ± 0.1 atm. No discernible EEG activation was noted during the noxious electrical stimulation at either partial pressure of N20, although spontaneous EEG activation often occurred. The electrical stimulation (3 Hz and 50 Hz) increased blood pressure.ConclusionsThese data indicate that N20 at peri-MAC partial pressures prevents EEG activation resulting from noxious electrical stimulation. The fact that spontaneous activation occurred suggests that the N20 is blocking the EEG activation at least in part by preventing impulses from reaching the brain.