High Pressure Nervous Syndrome


During the 1930’s the U.S. Navy tested other gases as a substitute for nitrogen. Their scientists conducted experiments using rare gases such as helium, neon, and argon. After numerous trials, helium was selected as the most suitable gas to dilute oxygen for deep diving.

Helium is the second lightest element known to man; in fact, only hydrogen is lighter. Helium is one seventh as light as air and our atmosphere only contains 5 part per million of helium. Certain natural sources in the U.S. and Canada contain as much as 2% helium and this is where much of this gas is

Helium is chemically inert; it has no color, taste, or odor. These characteristics make it an almost perfect gas for diving. However, helium has two disadvantages. First, helium is extremely expensive due to its rarity. Secondly, helium has high heat conductivity and will rob body heat from a diver at a rapid rate. There are also other major disadvantages to diving with helium mixes; it is more difficult to decompress, i.e., deeper and longer stops are required as compared to air; and there is a greater risk of a serious case of decompression sickness if stops are not done per plan.

In deep saturation diving, under rapid compression rates, divers sometimes suffer from a phenomenon known as the High Pressure Nervous Syndrome (HPNS).

NOAA has the following to say about HPNS:

“At diving depths greater than 600 fsw (183 msw), signs and symptoms of a condition known as the high pressure nervous syndrome (HPNS) appear and become worse the faster the rate of compression used and the greater the depth or pressure attained. HPNS is characterized in humans by dizziness, nausea, vomiting, postural and intention tremors, fatigue and somnolence, myoclonic jerking, stomach cramps, decrements in intellectual and psychomotor performance, poor sleep with nightmares, and increased slow wave and decreased fast wave activity of the brain as measured by an electroencephalogram (Bennett et al. 1986).

First noted in the 1960’s, HPNS was referred to initially as helium tremors. Since that time, numerous studies have been conducted that were designed to determine the causes of HPNS and to develop means of preventing it (Bennett 1982).

Methods of preventing or ameliorating HPNS include using a slow and steady rate of compression to depth, using a stage compression with long pauses at selected intervals, employing exponential compression rates, adding other inert gases such as nitrogen to helium/oxygen mixtures, and selecting personnel carefully. At present, the data suggest that adding 10 percent nitrogen to a helium/oxygen mixture, combined with the use of a proper compression rate, ameliorates many of the serious symptoms of HPNS (Bennett 1982).”

HPNS can result from diving mixtures that contain helium. Here are three actions that you can take to avoid HPNS. They are:
-Don’t dive Heliox (O2/He) deeper than 400 FSW.
-Don’t dive Trimix (O2/He/N2) deeper than 600 FSW.
-Note: Adding as little as 10% Nitrogen to He/O2 mixes buffers the mix to the point that it can be used to 600 FSW without experiencing HPNS.
-Use (very) slow descent rates. Descending slower than one FSW per minute beyond 400 FSW on Heliox and 600 FSW on Trimix keeps HPNS at bay. Unfortunately, this slow rate of decent is only practical in commercial diving and is of no use in tech diving.


Bennett, P.B. 1982b. The high pressure nervous syndrome in man. In: The Physiology and Medicine of Diving and Compressed Air Work. (P.B. Bennett and D.H. Elliot, eds), Balliere-Tindall, London. pp. 262-296.

Bennett, P.B. 1990. Inert gas narcosis and HPNS. In: Diving Medicine, Second Edition (A.A. Bove and J.C. Davis, eds.).W.B. Saunders Company, Philadelphia. pp. 69-81.

Bennett, P.B, R. Coggin, and J. Roby. 1981. Control of HPNS in humans during rapid compression with trimix to 650 m (2132 ft). Undersea Biomed. Res., 8(2): 85-100.


Ernest S. Campbell, M.D., FACS

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