Role of Carbon Dioxide in Inert Gas Narcosis.

Abstract : The role of carbon dioxide and oxygen in high pressure narcosis was studied by exposing animals to hyperbaric conditions while maintaining them normoxic and normocapnic. Chickens were the experimental animal used. The unanaesthetized restrained birds were exposed to pressures up to 6 atmospheres absolute (ATA) in a hyperbaric chamber. Narcosis was estimated from changes in the visually evoked response (VER). The VER was recorded from the optic tectum by means of implanted bipolar fine wire electrodes. The VER was initiated by an external strobe light. A depression of the amplitude of the VER is considered to reflect narcosis. All animals showed a strong depression in VER amplitude on exposure to normoxic, normocapnic gas at 6 ATA when the inert component was nitrogen. No depression of VER was seen when the inert diluent was helium. Increasing lung PCO2 to 70 mmHg had no effect on VER. Relatively small changes in the body temperature (1.5 C) depressed VER and may explain some of the depression of VER seen by others with helium. Resistance to the high flow of gas through the bird was usually 7.5-10 cm H2O at a flow rate of 6 L/min ambient temperature and pressure; this resistance remains nearly constant over the 1-6 ATA pressure range investigated.