S. Sapsed-Byrne

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 tension in arterial blood. Sixty‐seven arterial blood samples were drawn from patients undergoing hypothermic cardiopulmonary bypass with anaesthesia maintained by either isoflurane or desflurane. Anaesthetic tensions in the oxygenator exhaust gas were measured using an infrared analyser and in arterial blood using a two‐stage headspace technique with a gas chromatograph. Both measurement systems were calibrated with the same standard gas mixtures. There was no difference in anaesthetic tension measured in arterial blood and 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 anaesthetic tensions in arterial blood can be accurately monitored by analysis of the gas emerging from the exhaust port of a membrane oxygenator.

Potential Errors in the Measurement of Anesthetic Partial Pressure in Blood

Vials of 2 ml nominal volume (1.95 ml by water displacement) are used with silicone, Teflon-faced septa (Phase Separations Ltd., Deeside, TJnited Kingdom) in their caps. They are prepared shortly before sampling by first flushing with oxygen and then partially evacuating them to a pressure of approximately 150 mmHg using a needle and syringe. This provides sufficient oxygen to saturate any sample, but the pressure within the Vial will remain subatmospheric even after warming, humidification, or movement of nitrous oxide. Exactly 1 ml blood is injected through the septum, and the vial is placed in a motorized roller in a water bath at 37°C. After 30 min, the vial contents are brought to atmospheric pressure by penetrating the septum with a hollow needle and entraining air. This reduces the concentration of anesthetic in the headspace, but its partial pressure is unaffected and remains in equilibrium with the blood. The vial is rotated in the water bath for another minute to disrupt bubbles in the headspace, and then the headspace is sampled and analyzed in a gas chromatograph. A portion (0.5 ml) of the bloodis transferred to a second prepared vial that is then processed in the same way. We calculate the blood gas coefficient (Al, for each anesthetic from the second vial equilibration using a relationship derived from conservation of mass during equilibration:

Nitrous oxide and carbon dioxide have no effect on the blood-gas solubilities of sevoflurane and isoflurane

Nitrous oxide (N2 O) has been shown to decrease the solubility (lambda (B):G) of volatile anesthetics in human blood and, consequently, affect their rate of uptake. If this is true, then carbon dioxide (CO2) may also have an effect, which is important because methods that measure the tension of volatile anesthetics in blood washout CO2 in the process. Blood samples were obtained from fasted, healthy volunteers and patients undergoing major surgery. Each sample was divided into two aliquots: one was equilibrated at 37[degree sign]C in a closed glass tonometer with a mixture of isoflurane 1% and sevoflurane 2% in a test gas mixture of either 50:50 N2 O/O2 or 5:95 CO2/O2; the other aliquot was equilibrated with isoflurane and sevoflurane in O2 alone as a control. Using a two-stage headspace technique using gas chromatography, we measured the lambdaB:G of isoflurane and sevoflurane in the presence and absence of the test gas in each subject. There was no significant difference between the lambdaB:G of sevoflurane and isoflurane obtained from the N2 O group and their controls or between the CO2 group and their controls. We conclude that neither N2 O nor CO2 has an effect on the lambdaB:G of sevoflurane or isoflurane in the concentrations tested. Implications: The blood solubilities of sevoflurane and isoflurane were measured with and without nitrous oxide and carbon dioxide. No differences were found. Nitrous oxide does not affect the kinetics of other anesthetics by altering their solubility. Carbon dioxide tensions need not be controlled when measuring anesthetic tensions in blood. (Anesth Analg 1998;87:1412-5)