Percutaneous Anesthetic Loss

I have ignored three possible avenues for anesthetic loss: transcutaneous movement, transvisceral movement, and metabolism. Although transcutaneous movement occurs, the movement is small.^{[20] [21] [22] [23]} The greatest loss per alveolar anesthetic percent occurs with nitrous oxide. Loss of nitrous oxide may equal 5 to 10 mL/min with an alveolar concentration of 70%. The movement of anesthetic across visceral or pleural surfaces during abdominal or pulmonary surgery is larger than the movement across skin, but such visceral losses are usually small relative to total uptake.^[24]

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Figure 5-4 In patients, administration of 65% nitrous oxide produces a more rapid rise in the alveolar concentration of anesthetic/concentration of inspired anesthetic (FA/FI ratio) for nitrous oxide than administration of 5% (i.e., concentration effect, two solid lines). The FA/FI ratio for 4% desflurane rises more rapidly when given with 65% nitrous oxide than when given with 5% (i.e., second gas effect, two dotted lines). (Adapted from Taheri S, Eger EI II: A demonstration of the concentration and second gas effects in humans anesthetized with nitrous oxide and desflurane. Anesth Analg 89:774–780, 1999.)