Effects of Propofol and Remifentanil on Phrenic Nerve Activity and Nociceptive Cardiovascular Responses in Rabbits

Background: The effects of propofol, remifentanil, and their combination on phrenic nerve activity (PNA), resting heart rate (HR), mean arterial pressure (MAP), and nociceptive cardiovascular responses were studied in rabbits.

Methods: Basal anesthesia and constant blood gas tensions were maintained with [alpha]-chloralose and mechanical ventilation. PNA, HR, MAP, and maximum changes in HR and MAP ([DELTA]HR, [DELTA]MAP) evoked by electrical nerve stimulation of tibial nerves were recorded. The comparative effects were observed for propofol at infusion rates from 0.05 to 3.2 mg [middle dot] kg-1 [middle dot] min-1 (group I) and remifentanil from 0.0125 to 12.8 [mu]g [middle dot] kg-1 [middle dot] min-1 alone (group II), and during constant infusions of propofol at rates of 0.1 and 0.8 mg [middle dot] kg-1 [middle dot] min-1 (groups III and IV, respectively). Finally, the effect of remifentanil on propofol blood levels was observed (group V).

Results: The infusion rates for 50% depression (ED50) of PNA, [DELTA]HR, and [DELTA]MAP were 0.41, 1.32, and 1.58 mg [middle dot] kg-1 [middle dot] min-1 for propofol, and 0.115, 0.125, and 1.090 [mu]g [middle dot] kg-1 [middle dot] min-1 for remifentanil, respectively. The ratios for the ED50 values of [DELTA]HR and [DELTA]MAP to PNA were 3.2 and 3.9 for propofol, and 1.1 and 9.5 for remifentanil, respectively. Analysis of the expected and observed responses and isobologrms showed that although their combined effects on PNA, resting HR, and MAP, and [DELTA]MAP were synergistic for [DELTA]HR, they were merely additive. Remifentanil had no effect on propofol blood levels.