Background and objective: Target-controlled infusions of anaesthetic agents have become increasingly available. They can involve the use of propofol in combination with an opioid or a benzodiazepine. The effect site concentration of propofol infusions has been advocated as a method of estimating drug distribution. We investigated the influence of co-induction with remifentanil and midazolam on effect site propofol requirements at induction of anaesthesia using target-controlled infusions.

Methods: Sixty-six consenting adult patients were randomly allocated to three treatment groups. Each group received induction of anaesthesia with a different total intravenous technique. One group was induced with target-controlled propofol alone; another received target-controlled propofol and target-controlled remifentanil (3 ng mL−1); and the last received midazolam (0.03 mg kg−1), target-controlled remifentanil (3 ng mL−1) and target-controlled propofol. Computer simulation was used to calculate effect site concentrations. We recorded propofol dose and effect site concentration at loss of verbal response.

Results: The effect site concentration (Ce50) of propofol alone was 2.19 µg mL−1. This was reduced to 1.55 µg mL−1 during co-induction with remifentanil and further reduced to 0.64 µg mL−1 with midazolam premedication (P < 0.001; ANOVA).

Conclusions: We conclude that co-induction with remifentanil alone or with midazolam can be used to reduce propofol doses at induction of anaesthesia using target-controlled infusions. We believe that using effect site concentration may prove a useful tool in routine clinical practice.