in Table 22.1. The blood:gas partition coefficient is defined as the ratio of the concentration of the drug in the blood to the concentration of the drug in the gas phase (in the lung), at equilibrium. The volatile anesthetic is inhaled into the lungs, diffuses into the blood, and when equilibrium is reached, it diffuses into tissues. For the drug to have a quick onset, the solubility in the blood should be low, thus saturation will occur quickly, and the drug can then move into the tissue compartment. Recovery is also expected to be faster for those drugs with a low blood:gas partition coefficient as the drug will be eliminated quicker if it has a low solubility in the blood and quickly passes into the lungs for exhalation.
When a patient is exposed to the volatile gas for prolonged procedures, greater than 5 hours, the solubility of the drug in the tissues will also effect the recovery period. In these cases, it is necessary to consider the solubility of the drug in fat and lean organs as well as the body mass of the individual patient to determine how recovery could be affected by drug solubility. Most inhaled anesthetics have similar solubilities in lean organs, but their solubilities in fat vary as predicted by their oil:gas partition coefficients listed in Table 22.1.3 Obese patients may have increased recovery times if an inhaled anesthetics with high-fat solubility is used for a prolonged period.
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