High pressure nervous syndrome

High pressure nervous syndrome (HPNS) is a neurological and physiological diving disorder that results when a diver spends too much time breathing a high-pressure mixture of helium and oxygen (heliox). HPNS is a limiting factor in future deep diving.

"Helium tremors" were first described in 1965 by Royal Navy physiologist Peter B. Bennett, who also founded the Divers Alert Network. Zal'tsman also reported on helium tremors in his experiments from 1961. Unfortunately these reports were not available in the west until 1967.

The term high pressure nervous syndrome was first used by Brauer to describe the combined symptoms of tremor, electroencephalography (EEG) changes, and somnolence that appeared during a 1,189 foot chamber dive in Marseilles.

Symptoms
Symptoms of HPNS include tremors, myoclonic jerking, somnolence, EEG changes, visual disturbance, nausea, dizziness, and decreased mental performance.

The susceptibility of divers and animals to HPNS does depend on the individual.

Suppressive Measures
It is not likely that HPNS can be prevented entirely but there are effective methods to delay or change the development of the symptoms.

Rate of Compression
Utilizing slow rates of compression or adding stops to the compression have been found to prevent large initial decrements in performance.

Breathing Mixture
It can be avoided by including small amounts of other gases in the mix, such as nitrogen (creating trimix) or hydrogen (hydreliox).

Drugs
Alcohol, anesthetics and anticonvulsant drugs have had varying results in suppressing HPNS with animals. None are currently in use for humans.