HPNS effects among 18 divers during compression to 360 msw on heliox

Heliox compression deeper than 16 ATA can lead to EEG changes associated with confusion and somnolence. In man the symptoms termed the high pressure neurologic syndrome (HPNS) can also include increased tremor, memory problems, dizziness, nausea, and vomiting. In a series of 3 dives at NUTEC, a compression profile developed for operational use down to 360 msw was evaluated. In each dive 6 different divers were compressed to 360 msw on heliox. Neuropsychologic and neurophysiologic testing were performed repeatedly. The HPNS testing revealed only mild effects of the compression. Only 3 divers had impairments of more than 2 SD in peripheral motor function compared to their predive average. Memory was impaired periodically in 2 divers. The same was found for perceptual speed and reasoning. Fifty percent of the divers had an increase of more than 2 SD in postural tremor, but that had minimal effect on their motor performance. Six of the 18 divers had an EEG power spectrum with both alpha band inhibition and theta increase. While the performance impairment was most marked around 240 msw, the EEG changes occurred mainly deeper than 300 msw. In only 1 of the 18 divers marked EEG changes, marked tremor increase, and marked cognitive performance impairment were observed at the same time. Although mild HPNS was observed, the divers were little impaired during the compression to 360 msw. The results confirm that using a compression profile with rates decreasing progressively with increasing depth, and with several intermediate stops, provides fit divers at depth. By using standard batteries of HPNS testing we were able to obtain evidence for the acceptability of this compression profile.