A method is described for the direct determination of the volatile anaesthetics halothane and isoflurane in blood by gas chromatography with flame-ionisation detection. The method is accurate and precise and allows rapid measurements of blood levels of anaesthetic agents. Headspace concentrations of anaesthetic agents in the concentration range 0-3% V/V are determined with an accuracy of +/- 0.01% V/V. The relative standard deviation of these results is less than 4.0%. A relatively small volume of blood is required for each determination, a factor of great significance in the treatment of children. The need for separate blood calibration graphs for each patient is discussed, further emphasising the need for a rapid calibration procedure. The results from the clinical application of this method show conclusively its suitability for the management of anaesthetised subjects.
A method for preparing standard gas mixtures of the volatile anaesthetics halothane, enflurane and isoflurane is described. Static mixtures of gases of known concentration can be prepared manometrically by measuring the required pressure of anaesthetic gas into a bulb and diluting to atmospheric pressure with air. Standard gas mixtures in the concentration range 0 4 % V/Vcan be prepared with an accuracy of +0.01% V/V, and the relative standard error of measurements of a single standard concentration is less than 0.8%. Significant adsorptive losses in the gas sampling valve were observed for gas standards prepared in the absence of any diluent gas. These losses were not detected for measurements of standards made up to atmospheric pressure in air. A comparison with calibration procedures currently in practice is presented.
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