Occurrence of high-pressure nervous syndrome at constant pressure during change of mixture.

Three professional divers have performed a dive to 450 msw. From 200 msw and during the first 64 h on the bottom, they breathed a H2-He-O2 mixture with 54-56% H2. At this time a switch was performed to a mixture with 30% H2, and 8 h later a second switch was performed to 0% H2. In the H2-He-O2 mixture the clinical symptoms of high-pressure nervous syndrome (HPNS) were not present and the electroencephalogram changes were slight. The switch of the mixture induced an isobaric HPNS of high intensity. Twenty-four hours later the HPNS decreased, but the clinical symptoms persisted throughout the stay in the He-O2 mixture. The appearance of isobaric HPNS during the switch might be due to the disappearance of the narcotic substance which suppressed or masked the clinical symptoms; it might also be due to the sudden increase in the partial pressure of He, which was equivalent to a fast compression.