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It generally is assumed that the potency of inhalational anesthetics remains unchanged during the course of the administration of an anesthetic. Only one study has indicated a decrease of minimum alveolar concentration with time. In this study, an effect of the duration of anesthesia administration and surgery on the potency of isoflurane was investigated by determining MACtetanus (the minimum alveolar concentration that prevents movement in response to electrical tetanic stimulation in 50% of patients) before and after surgery.Ten patients who underwent removal of a herniated intervertebral disc were anesthetized with isoflurane only. Reaction to a standardized electrical stimulation applied to the forearm was observed and was graded as movement or no-movement. The isoflurane concentration was increased in steps of 0.10 vol% if the patient moved and decreased in steps of 0.10 vol% if no reaction was seen, until a “movement/nomovement/movement” or “no-movement/movement/no-movement” pattern, respectively, was achieved.MACtetanus decreased in all patients from 1.28 ± 0.22 vol% (mean ± SD) before surgery to 1.04 ± 0.22 vol% after surgery (P < 0.01). When the prestimulation arterial blood pressure or the maximal increase in blood pressure caused by stimulation at the individual MACtetanus before surgery were compared to the corresponding values at the individual MACtetanus after surgery, no significant difference could be found. The prestimulation heart rate and the maximal increase in heart rate were significantly lower after surgery, even though the end-tidal isoflurane concentration was 0.24 vol% lower at the individual MACtetanus after surgery.The authors conclude that MACtetanus decreases during the administration of anesthesia and the performance of surgery.