Fentanyl or midazolam for co-induction of anaesthesia with propofol in dogs

Abstract Objective: Propofol may cause adverse effects (e.g. apnoea, hypotension) at induction of anaesthesia. Co-induction of anaesthesia may reduce propofol requirements. The effect of fentanyl or midazolam on propofol dose requirements and cardiorespiratory parameters was studied. Study design:  Randomized, controlled, blinded clinical study. Animals:  Sixty-six client owned dogs (35 male, 31 female, ASA I-II, age 6–120 months, body mass 4.7–48.0 kg) were selected. Methods:  Pre-medication with acepromazine (0.025 mg kg−1) and morphine (0.25 mg kg−1) was administered by intramuscular injection. After 30 minutes group fentanyl-propofol (FP) received fentanyl (2 μg kg−1), group midazolam-propofol (MP) midazolam (0.2 mg kg−1) injected over 30 seconds via a cephalic catheter and in a third group, control-propofol (CP), the IV catheter was flushed with an equivalent volume of heparinized saline. Anaesthesia was induced 2 minutes later, with propofol (4 mg kg−1minute−1) administered to effect. After endotracheal intubation anaesthesia was maintained with a standardized anaesthetic protocol. Pulse rate, respiratory rate (RR) and mean arterial pressure (MAP) were recorded before the co-induction agent, before induction, and 0, 2 and 5 minutes after intubation. Apnoea ≥30 seconds was recorded and treated. Sedation after pre-medication, activity after the co-induction agent, quality of anaesthetic induction and endotracheal intubation were scored. Results:  Propofol dose requirement was significantly reduced in FP [2.90 mg kg−1(0.57)] compared to CP [3.51 mg kg−1 (0.74)] and MP [3.58 mg kg−1(0.49)]. Mean pulse rate was higher in MP than in CP or FP (p = 0.003). No statistically significant difference was found between groups in mean RR, MAP or incidence of apnoea. Activity score was significantly higher (i.e. more excited) (p = 0.0001), and quality of induction score was significantly poorer (p = 0.0001) in MP compared to CP or FP. Intubation score was similar in all groups. Conclusions and clinical relevance:  Fentanyl decreased propofol requirement but did not significantly alter cardiovascular parameters. Midazolam did not reduce propofol requirements and caused excitement in some animals.