CNS reactions at 51 ATA on trimix and heliox and during decompression.

Two groups of divers (Group A and B) were compressed to 500 msw with trimix (n = 3) and heliox (n = 3). After 4 d at saturation depth Group A had a gas change to heliox. Both groups were followed with repeated neuropsychological and neurological tests during compression, at stable intermediate depths and at saturation depth. There were marked high pressure nervous syndrome effects during compression for both groups. Only tremor was inhibited by the nitrogen. In addition Group A was impaired due to nitrogen narcosis. During trimix and heliox saturation there was only some recovery in the EEG. Group B had a sustained high tremor during the saturation. On visuomotor and cognitive functions Group B performed up to predive level on the 3rd d at saturation while Group A was heavily impaired during the whole trimix saturation period. Although dizziness and tremor were the main symptoms in Group B, Group A reported concentration trouble, euphoria, and upset stomach during the saturation phase. Minor changes occurred in the EEG during the gas change. There was, however, a marked increase in postural tremor and recovery on cognitive tests relating to the elimination of the nitrogen. Up to the completion of the gas change no severe symptoms were reported. Six hours after the gas change, severe symptoms occurred with visual and auditory hallucinations and myoclonic jerks as the dominant characteristics. Some symptoms lasted for 12 h. During decompression there was a steady normalization in both groups.