HPNS or High Pressure Nervous<\/a> Syndrome (also called High-Pressure Neurological Syndrome<\/em>) can onset beyond a depth of 150m\/500ft when using heliox or trimix breathing<\/a> gasses. <\/p>\n\n\n\n The severity of HPNS is determined by the speed of descent<\/a> (rate of compression), depth attained (absolute pressure) and the percentage of helium<\/a> inspired by the diver. There is also a varying level of individual susceptibility, but an individual diver is likely to see no variation in susceptibility on different dives.<\/p>\n\n\n\n High Pressure Nervous Syndrome is a neurological\/physiological disorder, that was first described using the term “helium tremors” by the navy physiologist Peter Bennett in 1965 (they were first discovered by Russian scientist G. L. Zal’tsman in 1961, but this report was not made public until 1967). Numerous studies have been completed since that time but, despite much research, the physiological\/neurological causes of HPNS are still unknown.<\/p>\n\n\n\n