A comparison of the effects of thiopentone and halothane anaesthesia on critical oxygen tension.

RECENT WORK has shown that barbiturate anaesthesia protects tissues against hypoxic damage. ~,2 The mechanism by which this occurs is unknown. ~ However, it is known that barbiturates as well as halothane affect the respiratory enzymes. 4-9 This information has been obtained from tissue culture. Unfortunately it is difficult to extrapolate results from tissue culture to whole animals. It is also difficult to extrapolate to clinically comparable concentrations of anaesthetic agents. TM We have therefore used changes in the critical oxygen tension to compare anaesthetic effects on respiratory enzymes in the whole animal at clinical anaesthetic concentrations. The critical oxygen tension is that oxygen tension at which the respiratory enzymes are first affected by a lack of oxygen. This point was determined from a plot of oxygen tension versus time. Above the critical oxygen tension, oxygen consumption is high and fixed. Below the critical oxygen tension, the oxygen consumption decreases as the oxygen tension decreases. This critical oxygen tension has been determined previously for many tissues including brain, heart and kidney. However, the effect of various anaesthetic agents on critical oxygen tension has not been determined.