The article that's been most cited from this journal, published more than 45 years ago, provides the basis for much of how we practice anesthesia today. (Eger EI,2nd, Saidman LJ, Brandstater B: Minimum alveolar anesthetic concentration: a standard of anesthetic potency. Anesthesiology 1965; 26: 756-63
) It continues to be frequently referenced, most recently this month. Though some may doubt the clinical relevance of studies not performed in humans, this seminal work was a study of dogs. Also, although halothane, the anesthetic that was being tested, is rarely used now, the concept behind what was demonstrated, is still quite relevant.
Unlike previous studies where pentothal was used to induce anesthesia, in this one, anesthesia was induced with the gas that was studied. Alveolar concentration was held constant for 15 min before stimulation. The stimulus was usually applied for a minute. A positive response was a gross purposeful muscular movement. MAC was defined as the concentration midway between the highest concentration allowing and the lowest concentration preventing a response.
The most striking finding was the stability of MAC values. MAC was the same at the beginning, end and during intervening times of anesthesia. It did not vary two weeks later when the dogs were again anesthetized.
MAC did, however, vary depending on the type of stimulus. It was highest when the tail was clamped and was lowest when the paw rather than the tail was clamped. Increasing the stimulus beyond a certain point did not increase MAC. MAC decreased after the dogs were given ammonium chloride, making them acidotic. Respiratory acidosis and alkalosis had little effect on MAC. MAC also changed little during hypoxia until the PO2
was allowed to dip below 30. MAC was additionally shown to decrease when the dogs were allowed to hemorrhage.
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Posted by J. Lance Lichtor, M.D.