Small dose midazolam or droperidol reduces the hypnotic dose of propofol at the induction of anaesthesia

We investigated the effect of a small dose of midazolam, ketamine, droperid ol or lidocaine on the propofol dose required for hypnosis during induction of general anaesthesia. These drugs were randomly administered to 100 pati ents about to undergo scheduled surgery. Propofol was then infused at a rat e of 250 mu g kg(-1) min(-1) and the hypnotic dose to produce hypnosis was evaluated. Midazolam (20 mu g kg(-1)) and droperidol (20 mu g kg(-1)) signi ficantly reduced the mean hypnotic dose of propofol (mean) S.D.) compared w ith the placebo (43.7 +/- 17.8 mg, 61.9 +/- 10.6 mg and 72.5 +/- 27.7 mg af ter pretreatment with midazolam, droperidol and placebo, respectively), whe reas ketamine (0.1 mg kg(-1)) and lidocaine (1 mg kg(-1)) did not significa ntly affect the hypnotic dose of propofol (63.1 +/- 25.6 mg and 65.1 +/- 24 .8 mg, respectively). Only midazolam when compared with saline administrati on, (176 +/- 66 s and 298 +/- 126 s, respectively), shortened the time to a chieve hypnosis. The changes in blood pressure (non-invasive) and heart rat e were not significantly different in all groups during the induction of an aesthesia and oro-tracheal intubation. These results raise the possibility that new combinations of central nervous system drugs, such as droperidol a nd propofol, have a potential to reduce the dose of intravenous anaesthetic s, including propofol, that produce hypnosis without significant adverse ef fects.