A comparison of anaesthetic tensions in arterial blood and oxygenator exhaust gas during cardiopulmonary bypass

This study evaluates the usefulness of the analysis of gas sampled from the exhaust port of a membrane oxygenator in the estimation of anaesthetic ten sion in arterial blood. Sixty-seven arterial blood samples were drawn from patients undergoing hypothermic cardiopulmonary bypass with anaesthesia mai ntained by either isoflurane or desflurane. Anaesthetic tensions in the oxy genator exhaust gas were measured using an infrared analyser and in arteria l blood using a two-stage headspace technique with a gas chromatograph. Bot h measurement systems were calibrated with the same standard gas mixtures. There was no difference in anaesthetic tension measured in arterial blood a nd gas leaving the oxygenator exhaust (isoflurane: n = 29, range: 0.3-0.8%, 95% limits of agreement: - 0.08% to 0.09%; desflurane: n = 38, range: 1.5- 5.4%; 95% limits of agreement - 0.65% to 0.58%). We conclude that anaesthet ic tensions in arterial blood can be accurately monitored by analysis of th e gas emerging from the exhaust port of a membrane oxygenator.