Alternative respiratory gases to compressed air have been sought because of the restriction to effective diving at depths greater than 40 to 60 metres of sea water (msw) as a result of nitrogen narcosis and the high density of air (see Chapter 15). Projection of knowledge concerning inert gas narcosis, especially lipid solubilities, suggested that substituting helium for nitrogen would prevent severe narcosis until pressures were greater than 40 ATA. However, in the 1960s, difficulties were encountered beyond a depth of 200 metres (pressure of 20 ATA) when using a helium-oxygen breathing mixture. This syndrome, which is also characterized by a disturbance of the nervous system, is quite different from the effects of nitrogen (i.e. inert gas narcosis). Because the most prominent feature noted was tremors, the condition was initially referred to as helium tremors1, although it is now realized that the use of helium is merely an association and that helium itself is not the cause.
In retrospect, a series of experiments in the 1880s recorded abnormal excitement, disturbed locomotion and paralysis in marine animals exposed to a high-pressure environment. During the 1920s, a series of publications dealt with the effects of high hydrostatic pressures. Halsey3 cited a paper published in 1936 in which manifestations that may have been the high-pressure neurological syndrome (HPNS) were reported in vertebrate animals.
In the 1960s, British, US, Russian, and French investigators noted tremors and performance impairment in divers compressed to depths of 200 to 400 msw (20 to 40 ATA)4. Coarse tremors were often associated with other symptoms such as nausea, vomiting, dizziness and vertigo. Decreased ability to carry out fine movements was observed. In animals, similar changes were noted that, under further pressure, progressed to generalized convulsions.
This complex of features has become known as the ‘high-pressure neurological syndrome’ or the ‘high-pressure nervous syndrome’, abbreviated HPNS in either case. HPNS should not be confused with inert gas narcosis (see Chapter 15), neurological decompression sickness (see Chapter 11), central nervous system oxygen toxicity (see Chapter 17) or other gas toxicities.