Ketamine and thiopental sleep responses in guinea pigs in hyperbaric helium-oxygen.

One-hundred twenty three guinea pigs, randomly divided into four groups, were exposed to the following normoxic environments: Group I, 1 atmosphere absolute (ATA) air; Group II, 1 ATA helium-oxygen (heliox); Group III, 20 ATA heliox; and Group IV, 31 ATA heliox. Each group was divided into five subgroups; each subgroup received a different and graduated intravenous dose of ketamine in one of these environments. An additional 172 guinea pigs were exposed either to 1 ATA air or 31 ATA heliox and given thiopental. The duration of sleep, determined by failure of the animals to right themselves, was then used to construct dose-response curves for the two drugs in the hyperbaric environments. Results demonstrated less pressure antagonism of anesthesia with ketamine than with thiopental. For both drugs, pressure not only increased the dose required to induce sleep but also decreased the duration of sleep, when it occurred, by as much as 50% compared to effects of the same doses at surface pressure.