Decrease in minimum alveolar concentration of sevoflurane during anaesthesia and arthroscopy

BACKGROUND AND OBJECTIVE: Although the MACtetanus (minimum alveolar concentration that prevents movement in response to electrical tetanus stimulation in 50% of patients) of isoflurane decreases during anaesthesia and surgery, it is not known whether this occurs to the same extent with other inhalational anaesthetics. We determined the MACtetanus of sevoflurane before and after surgery. METHODS: Eleven patients who underwent arthroscopy of the shoulder underwent inhalational induction of anaesthesia with sevoflurane and tracheal intubation by succinylcholine. MACtetanus was determined in each patient by testing the response to a 10 s, 50 Hz, 80 mA transcutaneous tetanic electrical stimulus to the ulnar nerve at varying concentrations of sevoflurane. The end-tidal concentration of sevoflurane was kept constant for 15 min before each stimulus and the concentration of sevoflurane varied in increments of 0.1% until a sequence of three alternate responses (move, not move, move) or (not move, move, not move) was obtained. After arthroscopy of the shoulder, individual MACtetanus were measured as described above. RESULTS: The mean MACtetanus decreased from 2.22 +/- 0.29% before arthroscopy to 1.82 +/- 0.26% after arthroscopy (P < 0.01). CONCLUSIONS: It is concluded that the MACtetanus of sevoflurane decreases during anaesthesia and surgery.