Abstract
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:
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