Commercial diving in compressed air is not feasible to depths greater than 50 m.s.w. (i.e. 6 bars of absolute pressure) since the nitrogen in the air becomes narcotic at partial pressure of about 3 to 4 bars and induces nitrogen narcosis constituted by behavioural problems and changes in mental processes (Table 2 of previous chapter). In order to enable man to dive deeper than 50 to 70 m.s.w., researchers have tried to replace the nitrogen by another inert gas which has fewer narcotic properties. The inert gases are not metabolized by the organism and thus will be inactive. Signs and symptoms of the high pressure nervous syndrome in rats and monkeys in helium-oxygen mixture.

HPNS in animal models (Rats and Monkeys) in helium-oxygen mixture Up to 101 ATA

Behavioral Changes

Electrophysiological Disruptions

Tremor (10–14 Hz)

Muscular hypertonus

EEG changes: Increase of slow waves,


paroxystic activity, epileptic seizure.

Muscular spasms

Changes in evoked potentials, cortical


excitability and reflexes (monkeys)