Effect of remifentanil on plasma propofol concentration and bispectral index during propofol anaesthesia.

BACKGROUND Propofol and remifentanil are commonly administered together in clinical anaesthesia, but the effect of remifentanil on the plasma concentration of propofol has yet to be established. The aim of the present study was to investigate the effect of remifentanil on plasma propofol concentrations (Cp) in the absence of surgical stimulation. METHODS Thirty-eight patients undergoing elective gynaecologic surgery were randomly assigned to receive one of the three remifentanil doses (0, 0.5, or 1.0 µg kg⁻¹ min⁻¹). Anaesthesia was induced by a target-controlled infusion of propofol. After tracheal intubation, saline or remifentanil infusion was administered for 15 min. Mean arterial pressure (MAP), heart rate (HR), and bispectral index (BIS) were recorded and cardiac index (CI), blood volume, and indocyanine green disappearance ratio (K-ICG) were measured using a dye densitogram analyser before and 15 min after saline or remifentanil infusion. Cp was measured using high-performance liquid chromatography. RESULTS HR, K-ICG, and BIS were significantly decreased in the remifentanil 0 µg kg⁻¹ min⁻¹ group. The decrease in MAP, HR, CI, and K-ICG was significantly lower in the remifentanil 0.5 and 1.0 µg kg⁻¹ min⁻¹ groups compared with the remifentanil 0 µg kg⁻¹ min⁻¹ group. Cp was significantly increased after remifentanil administration, but this had no influence on BIS. CONCLUSIONS Remifentanil reduced the CI and increased the Cp, which may be related to a decrease in the K-ICG, but had no significant effect on the BIS.