Synergistic interaction between the effects of propofol and midazolam with fentanyl on phrenic nerve activity in rabbits

Background: It has been shown that the depressive effects of both propofol and midazolam on consciousness are synergistic with opioids, but the nature of their interactions on other physiological systems, e. g. respiration, has not been fully investigated. The present study examined the effect of propofol and midazolam alone and in combination with fentanyl on phrenic nerve activity (PNA) and whether such interactions are additive or synergistic. Methods: PNA was recorded in 27 anaesthetised and artificially ventilated rabbits. In three groups, propofol, fentanyl and midazolam were administered intravenously in incremental doses to construct dose‐response curves for the depressant effects of each one on PNA. In another two groups, the effect of pretreatment with either fentanyl 1 μg · kg−1 i. v. or midazolam 0.05 mg · kg−1 i. v. on the effects of propofol and fentanyl respectively on PNA were studied. Results: Propofol and fentanyl caused a dose‐dependent depression of PNA with complete abolition at the highest total doses of 16 mg · kg−1 i. v. and 32 μg · kg−1 i. v., respectively. In contrast, midazolam in incremental doses to a total of 0.8 mg · kg−1 reduced mean PNA by 63%, but approximately 12% of PNA remained at a total dose as high as 6.4 mg · kg−1. The mean ED50s, calculated from dose‐response curves, were 5.4 mg · kg−1, 3.9 μg · kg−1 and 0.4 mg · kg−1 for propofol, fentanyl and midazolam, respectively. Initial doses of either fentanyl 1 μg · kg−1 i. v. or midazolam 0.05 mg · kg−1 i. v. acted synergistically with subsequent doses of either propofol or fentanyl to abolish PNA at total doses of 8 mg · kg−1 and 8 μg · kg−1, respectively. Conclusion: Fentanyl has a synergistic interaction with both propofol and midazolam on PNA and hence potentially on respiration.