Midazolam acts synergistically with fentanyl for induction of anaesthesia

The induction dose-response of midazolam was compared with the dose-response of its combination with fentanyl and with that of fentanyl alone in three groups of 60 unpremedicated, ASA physical status I or II women undergoing minor gynaecological surgery. The end-point of induction of anaesthesia was inability to open eyes upon command. Dose-response curves were determined for each group with a probit procedure and compared with an isobolographic analysis. Midazolam was found to act in synergism with fentanyl for induction of anaesthesia. Twenty-five percent of the ED₅₀ of fentanyl was required in combination with 23% of the ED₅₀ for midazolam to achieve the ED₅₀ of the combination. This degree of synergism may explain mutual potentiation between opioids and benzodiazepines reported previously