Deep divers are exposed to three types of environmental stresses • the high-pressure nervous syndrome (HPNS), which is related to the effects of the hydrostatic pressure on the central nervous system These appear at around 200 m and increase with depth, causing motor disturbances that may impair divers' efficiency • stress caused by the density of the breathing mixture, which increases with depth Breathing dense gas requires increased ventilatory efforts that may reduce divers' work capacity • the fatigue of long saturation periods (confinement, thermal stress, bad sleep, lack of appetite, etc ) that is generally related to body-weight losses To overcome these environmental stresses, scientists have used the properties of the different constituents of the breathing mixtures The first deep dives were conducted using helium as a diluent Helium has a low molecular weight but no specific anti-HPNS properties The only way of controlling HPNS was to slow down the compression In 1972, helium permitted 610 m to be reached during the Comex Physalie VI dive In 1975, Duke University introduced the concept of the 'pressure-reversal effect' (Simon et al, 1975) A significant reduction in HPNS was obtained by adding to the helium/oxygen mixture a given amount of a narcotic gas, in that case nitrogen The new mixture was called 'trimix', but we would rather call it 'nitrogen trimix' to be more specific The possibilities of nitrogen trimix were further investigated by different research centres The two milestones of these experiments were the Comex Janus IV open-sea dive to 500 m, in 1977, and the Duke University Atlantis III simulated dive to 685 m, in 1983 However, the benefit of using nitrogen to reduce HPNS is counterbalanced by a significant increase in gas density (nitrogen is seven times heavier than helium) During the Janus IV dive, the breathing gas, which contained 5% nitrogen, had a density of 105 g/litre It would have been 82 g/litre with heliox (22% less) For this reason, a large amount of nitrogen cannot be used without affecting the diver's ventilatory function.
Imbert, J. P., Gortan, C., Fructus, X., Ciesielski, T., & Gardette, B. (1987). Hydra 8: Pre-commercial hydrogen diving project. In Submersible Technology: Adapting to Change: Proceedings of an International Conference (SUBTECH 1987 - Adapting to Change) (pp. 107–116). Society of Underwater Technology (SUT). https://doi.org/10.1007/978-94-009-1299-1_13