Effect of helium/nitrogen/oxygen mixtures on HPNS convulsion threshold in euthermic rats

This study compares the effects of various so-called trimix mixtures (10, 20, and 40% N2 in He-O2) on the convulsion threshold pressure (Pc) and EEG activity in 60 adult male Wistar rats with chronically implanted electrodes with those in 20 rats in He-O2 only. Restrained animals were individually compressed with trimix mixtures at 80 or 160 atm per hour to a simulated depth of 138 ATA; colonic temperature was maintained at normal levels. Pc was defined as the initial occurrence of overt sustained generalized tonic-clonic seizures, accompanied by typical "spike and wave" patterns in all EEG leads. As in man, in rats trimix increased the depth of the onset of HPNS tremors and myoclonic jerks in all six groups of rats. However, the Pc of the trimix groups was no different from the Pc of the helium-oxygen group (113 ATA), and at 40% N2, rats showed EEG seizures but no overt convulsions. These results are discussed in relation to those of other studies showing the extension of Pc in mice and monkeys attained by adding narcotics to heliox; the paper also considers the relevance of method of compression, addition of nitrogen, core temperature, and species differences, as well as the need for EEG measurements and direct observation of overt convulsions as indicators of an effective antagonism of HPNS.