The paper deals with the value of Trimix (i.e. addition of 10% nitrogen added to heliox) in prevention of the incapacitating effects of the High Pressure Nervour Syndrome (HPNS). First reported in 1967 by the author, this effect of pressure causes tremors, dizziness, nausea, vomiting and lapses of consciousness in heliox dives at depths greater than 600 ft and becomes worse the faster the compression and the deeper the depth. During an author directed 3 day compression to a record depth of 1500 ft in 1970, it was proved that slow compression and the stages was beneficial and COMEX made dives with similar methods to 2001 ft in 1972 and 1974 with 7- 10 days compression but incapacitating HPNS remained. In 1974 at Duke it was shown that with Trimix (10% N2) it is possible to reach 1000ft in only 33 minds without HPNS. A new series of dives (Atlantis) began in 1979 to investigate the value of TRIMIX deeper than 1000ft. The first to 1509 ft (460m) with 5% N2 in heliox in a 12 hr 20 min compression still showed some HPNS during compression. In March 1980 the dive was repeated with 10% N2 which prevented incapacitating HPNS and the dive was extended to a record 2132 ft (650m) with a N2 of 7.7% and a compression rate twice that of the 1979 USN dive to 1800 ft which produced severe HPNS. Again incapacitating HPNS was prevented and the men functioned amazingly well. The breakthrough in prevention of HPNS provides commercial opportunities for use of divers below 2000ft in the offshore oil industry.
In 1965 men were first compressed to 600 ft (183 m) for 4 hours and 800 ft (244 m) for 2 hrs at rates of 91 ft/min (27.7 m/min) to produce the first report of signs and symptoms, initially attributed to the helium but later to the pressure itself, and called the High Pressure Nervour Syndrome or HPNS (1,2,3). It is now well recognized that rapid compression to pressures greater than 500ft (152m) induces HPNS with dizziness, nausea, vomiting, postural and intention tremors, fatigue and somnolence, myoclonic jerking, stomach cramping, increased amounts of slow wave and decreased amounts of fast wave activity in the electrical activity of the brain (EEG), decrements in intellectual and psychomotor performance, poor sleep with nightmares and in animals, convulsions (4,5,6,7). The faster the rate of compression and the deeper the depth, the more severe are the signs and symptoms so that human exposures between 1500 ft (457m) and 2001 ft (610 m), even with perhaps operationally unrealistic 3 to 10 days compression times, often have resulted in incapacitating HPNS (8,9). Various strategies therefore have been utilized to ameliorate the signs and symptoms of HPNS (10), including selection of less sensitive divers, slow exponential compression with stages for adaption, use of excursions from a shallow saturation exposure, adaptation with time at depth and the use of narcotics to antagonize the effects of HPNS.
