Direct Measurement of Nitrous Oxide MAC and Neurologic Monitoring in Rats During Anesthesia Under Hyperbaric Conditions

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Abstract
The minimum alveolar concentration (MAC) of nitrous oxide necessary to prevent purposeful movement in rats has not been directly measured;rather, it has been extrapolated because the required partial pressure exceeds 760 mm Hg, or 1 atm absolute pressure (ATA). Values reported have ranged from 1.36 to 2.20 ATA (136–220 vol%, or 1034–1672 mm Hg). By maintaining general anesthesia at 2.25 ATA (1710 mm Hg), we directly measured the nitrous oxide MAC in 17 Long-Evans rats during mechanical ventilation and monitoring of two-channel electroencephalogram, compressed spectral array and cortical evoked potentials, electrocardiograph, and respiratory and anesthetic gases by mass spectrometry. After a minimal stabilization period of 30 min during ventilation by 1.8 ATA nitrous oxide and 0.45 ATA oxygen, MAC measurements were begun. Each rat was given up to three noxious electrical stimulations of 50 V by 10-ms-duration pulses at 50/s for 45 s. The partial pressure of nitrous oxide was decreased by approximately 10% after each negative response. The MAC was taken as the nitrous oxide concentration midway between that at which there was no response and that at which the rat moved purposefully. The nitrous oxide MAC in Long-Evans rats was determined to be 1.55 ± 0.16 ATA (mean ± SD). Hyperbaric nitrous oxide decreased electroencephalogram wave frequency to a predominantly theta rhythm of increased amplitude. Cortical evoked potentials had decreased wave amplitudes and increased latencies with increasing partial pressures >0.75 ATA. General anesthesia with nitrous oxide at hyperbaric pressures allows direct measurement of the nitrous oxide MAC in rats and demonstrates neurophysiologic depressant effects on the electroencephalogram and somatosensory evoked potentials.

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