Somatic-evoked brain responses as indicators of adaptation to nitrogen narcosis.

Two 2-week experimental pressure chamber exposures to nitrogen-oxygen breathing mixtures afforded an opportunity to study adaptation to nitrogen narcosis. Somatic-evoked brain responses induced by electrical stimulation of the median nerve in the wrist were processed on-line with a signal averager. The N1P2 interval was seen generally to be reduced in amplitude as a result of exposure to increased nitrogen partial pressure. Compressions with air were made from sea level and saturation to 200, 250 and 300 ft of sea water (fsw) equivalent (61, 76, and 91m). The decrement was found to be less, for equivalent exposures, in subjects who had been saturated at the pressure of 90 and 120 fsw (27 and 36 m); we interpret this as evidence of a nonspecific "adaptation." Less adaptation was seen from 30 and 60 fsw (9 and 18 m). These results are consistent with performance tests on the same exposures, and with subjective impressions. Saturation with 3 0r 4 atm of nitrogen may permit somewhat deeper diving without serious narcosis, than is possible from sea level.