MINIMUM ALVEOLAR CONCENTRATION
The minimum alveolar concentration (MAC) of an inhaled anesthetic is the alveolar concen-tration that prevents movement in 50% of patients in response to a standardized stimulus (eg, surgical incision). MAC is a useful measure because it mir-rors brain partial pressure, allows comparisons of potency between agents, and provides a standard for experimental evaluations (Table 8–3). Nonetheless, it should be remembered that this is a median value with limited usefulness in managing individual patients, particularly during times of rapidly chang-ing alveolar concentrations (eg, induction).
The MAC values for different anesthetics are roughly additive. For example, a mixture of 0.5 MAC of nitrous oxide (53%) and 0.5 MAC of halothane (0.37%) produces the same likelihood that move-ment in response to surgical incision will be sup-pressed as 1.0 MAC of isoflurane (1.7%) or 1.0 MAC of any other single agent. In contrast to CNS depres-sion, the degree of myocardial depression may not be equivalent at the same MAC: 0.5 MAC of halothane causes more myocardial depression than 0.5 MAC
of nitrous oxide. MAC represents only one point on the dose–response curve—it is the equivalent of a median effective dose (ED50). MAC multiples are clinically useful if the concentration–response curves of the anesthetics being compared are paral-lel, nearly linear, and continuous for the effect being predicted. Roughly 1.3 MAC of any of the volatile anesthetics (eg, for halothane: 1.3 × 0.74% = 0.96%) has been found to prevent movement in about 95% of patients (an approximation of the ED 95); 0.3–0.4 MAC is associated with awakening from anesthe-sia (MAC awake) when the inhaled drug is the only agent maintaining anesthetic (a rare circumstance).
MAC can be altered by several physiological and pharmacological variables (Table 8–4). One of the most striking is the 6% decrease in MAC per decade of age, regardless of volatile anesthetic. MAC is relatively unaffected by species, sex, or dura-tion of anesthesia. Surprisingly, MAC is not altered after spinal cord transection in rats, leading to the hypothesis that the site of anesthetic inhibition of motor responses lies in the spinal cord.
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