HPNS of baboons during helium-nitrogen-oxygen slow exponential compressions

From the results obtained with the experimental series CORASIN (fast compression with He-N2-O2), a method of compression has been developed for the baboon (Papio papio) to dive deeper than 600 m. This method utilizes an exponential compression profile with stages of 40 min every 100 m and with the introduction of N2 before each stage from 200 m onward to maintain a concentration of 5.5%. Between 0 and 800 m, this procedure did not produce myoclonus or epileptic seizures; tremor appeared beyond 400 m (578 +/- 109 m) but remained slight. If N2 was not introduced, the tremor appeared earlier (266 +/- 52 m) and became severe; between 600 and 800 m, muscular hypertonus, myoclonus, and muscular cramps occurred. The modifications of the electroencephalogram were slight; the increase in slow activity did not exceed 300% with or without N2. Beyond 800 m, the compression procedure with N2 injections revealed new phenomena. There was a general depression of EEG activity starting at 800 m; from 1,000 m and deeper, there were periods of motor disturbances (hypertonus, spasms, and shaking), palpebral clonus, and eye movements associated with peak EEG activities localized in the posterior region of the skull that sometimes evolved toward an epileptic seizure localized in this region. These symptoms differed from the classical description of high-pressure nervous syndrome, which comprises an increase in tremor followed by convulsions. These differences may perhaps be linked to our compression procedure using N2 injection, to the effect of the pressure itself, or to a combination of the two.