Sevoflurane for maintenance of anaesthesia in major vascular surgery: using the bispectral index to determine the minimum alveolar concentration

ASA physical status I and II patients scheduled to undergo major vascular surgery are enrolled in this study. Monitoring consists on blood pressure, heart rate, pulse oxymetry, endtidal CO2, entidal sevoflurane and bispectral index. Induction of anaesthesia is standardized in all patients using target controlled infusion of propofol and remifentanil. Muscle relaxation is achieved with cisatracurium. After tracheal intubation, propofol infusion is stopped. When the bispectral index reach 65 sevoflurane inhalation is started at the predetermined concentration. The predetermined endtidal concentration for the first patient is 2%. Sevoflurane is used once only to maintain anaesthesia. The endtidal sevoflurane concentration is adjusted with starting dose of 2% for the first patient and maintained for 10 min to ensure equilibration with the cerebral anesthetic partial pressure followed by assessment of BIS. In the next patients doses are given according to the following rule: if the subject responds positively (BIS<50 after 10 minutes of sevoflurane), the endtidal concentration is decreased by 0.2% step for the next subject, and conversely, if the subject does not respond (BIS>50 after 10 minutes of sevoflurane), the dose is increased by 0.2% step. The ED50 and ED95 of sevoflurane to maintain bispectral index below 50 are calculated using probit transformation.