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ANESTHETIC PHARMACOLOGY

Low and Moderate Remifentanil Infusion Rates Do Not Alter Target-Controlled Infusion Propofol Concentrations Necessary to Maintain Anesthesia as Assessed by Bispectral Index Monitoring

Lars P. Wang, FANZCA^*, Peter McLoughlin, FANZCA^*, Michael J. Paech, FANZCA, DM WAust, Irina Kurowski, FANZCA^*, and Emma L. Brandon, FRCA^*

From the *Department of Anaesthesia and Pain Medicine, Royal Perth Hospital, Perth, Western Australia, Australia; and the Pharmacology and Anaesthesiology Unit, School of Medicine and Pharmacology, University of Western Australia, Perth, Western Australia, Australia.

Address correspondence and reprint requests to Lars Wang, FANZCA, Department of Anaesthesia and Pain Medicine, Royal Perth Hospital, Wellington St, Perth WA 6000, Australia. Address e-mail to lpwang{at}iinet.net.au.

BACKGROUND: We investigated whether the EC₅₀ (the effective concentration that is required to achieve a response in 50% of patients) of propofol necessary to lower the Bispectral Index (BIS) value to 50 or less was reduced by coadministration of different remifentanil infusion rates.

METHOD: Seventy-two adult ASA I or II patients undergoing endotracheal intubation and target-controlled infusion (TCI) propofol anesthesia were allocated to six groups by stratified randomization. Group B received remifentanil 0.1 µg · kg^–1 · min^–1, Group C 0.15 µg · kg^–1 · min^–1, Group D 0.2 µg · kg^–1 · min^–1, Group E 0.3 µg · kg^–1 · min^–1 and Group F 0.4 µg · kg^–1 · min^–1. Group A served as control and received no remifentanil. The response of the first patient to propofol TCI at 4 µg/mL determined the effect-site concentration of propofol for the next patient in the same remifentanil group (Dixon's "up-and-down" method). If BIS was >50, the next patient received more propofol, and if BIS was 50, the next patient received less propofol. The hemodynamic effects of the combinations were also studied.

RESULTS: The EC₅₀ varied from 2.4 to 2.9 µg/mL. No additive effect of remifentanil on the EC₅₀ of propofol was observed. However, there was a wider variation in the response to propofol when the patients received no remifentanil. There was a decrease in heart rate in the remifentanil groups.

CONCLUSION: Infusion of remifentanil did not reduce propofol requirements in the unstimulated anesthetized patient. Propofol TCI levels should not be reduced because remifentanil is coinfused.

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