Control of HPNS in humans during rapid compression with trimix to 650 m (2131 ft).

In 1978 a series of deep trimix (He-N2-O2) dives was initiated to establish the relationship between a given nitrogen percentage and the rate of compression required to prevent the high pressure nervous syndrome (HPNS) at 460 m (1509 ft) and to determine the effects of inspired gas density, hydrostatic pressure, and narcosis o various circulatory and respiratory parameter, including the presence of dyspnea. In 1979, three human subjects were compressed to 460 m in 12 h 20 min with 5% N2 in He-O2. This resulted in nausea, vomiting, fatigue, tremors, and other signs and symptoms of HPNS that were especially prominent on arrival at that depth but had much improved by Day 2. In March 1980 the same profile was repeated but with 10% N2 in He-O2. The divers arrived at 460 m with virtually no symptoms of HPNS, but the psychometric performance, as for Atlantis I, still was decreased by some 40% on Day 1 and recovered to some 15% by Day 2. After 5-6 days at 460 m further extension of the dive to 650 m (2132 ft) with a 7.7% N2 mixture for 24 h showed similar control of symptoms of HPNS, although inspiratory resting dyspnea was present in one subject. The results are discussed in relation to the interactions of nitrogen percentage and rate of compression.