Loss of volition and pain response during induction of anaesthesia with propofol or sevoflurane

We compared the time to reach two anaesthetic end-points duringinduction of anaesthesia with a potent inhalation agent (sevoflurane)and an i.v. agent (propofol). We used a method to ensure steadybreathing during inhalation induction, and measured loss oftone in the outstretched arm and loss of response to a painfulstimulus. Thirty-eight female patients (age 39 (9) yr, weight65 (11) kg, and height 165 (8) cm) (mean (SD)) were randomlyallocated to receive either propofol or sevoflurane. The predictedinduction dose of propofol, estimated from age and weight foreach patient, was given at a rate of 1% of the induction doseper second, to a possible maximum of 2.5 times the predictedinduction dose. Sevoflurane was given with an inhaled concentrationof 8%, which was anticipated to cause loss of arm tone within90–120 s. After loss of consciousness, we applied a painfulelectrical stimulus to a finger at 15-s intervals and measuredthe time to loss of motor response. The median times and interquartilevalues for loss of arm tone were 105 (88–121) s for sevofluraneand 65 (58–80) s for propofol. This was equivalent to0.65 of the ED₅₀ of propofol. The time to loss of response topain was 226 (169–300) s for sevoflurane. The variancesof these three measurements were not significantly different,indicating that these dose–response relationships weresimilar. In contrast, only 11 of the patients given propofollost the response to pain after 2.5xED₅₀ had been given. Theseresults support previous evidence of substantial differencesbetween anaesthetic end-points, and show that this evidencecan be obtained using a simple and rapid method. Br J Anaesth 2001; 87: 283–6