EEG and evoked potential changes during gas- and liquid-breathing dives to 1000 msw

To test the hypothesis that compression in helium gas and compression without gas (hydrostatic compression) both produce the same neurological symptoms of high pressure nervous syndrome (HPNS), groups of 4 to 5 dogs were exposed to one of the following: (a) a 2-h surface control breathing He-O2 gas; (b) compression to 700-1000 msw breathing He-O2 gas; (c) a 2-h liquid-breathing control [ventilation with warmed (38 degrees C), oxygenated fluorocarbon liquid, FC-80]; and (d) "near hydrostatic" compression to 700-1000 msw while being ventilated with liquid from a sealed reservoir. Power spectra obtained from scalp-recorded EEG signals revealed a significant compression-related shift of power from the normally dominant 5-8 Hz band to the 8-11 Hz band. This effect was greatest at 600 msw and occurred equally in both dive groups. At very high pressures (greater than 900 msw) power in this 6-10 Hz range became reduced whereas 16-22 Hz activity increased as the EEG flattened. Somatosensory evoked potentials (SEPs) were elicited by subdermal electrical stimulation in the foreleg (n = 120). The latency of P1 (approximately 31 ms) did not change in any group; transmission time to cortex was unaltered by time or pressure. However, the "late-wave" P4 (approximately 250 ms) was slowed by 32-35 ms at 1000 msw in both groups (P less than 0.003 in He-O2). Since these main symptoms were provoked equally in both He-O2 and FC-80 dive groups, helium pressure did not play a significant role in the etiology of HPNS in this animal model. It is concluded that the HPNS is primarily a result of excessive pressure per se or rate of change of pressure per se.