BACKGROUND: The aim of this study was to evaluate the analgesic potency dose of remifentanil to maintain Surgical Pleth Index (SPI) values at less than 50 after intubation in patients undergoing general anesthesia with target-controlled infusion of propofol and remifentanil. METHODS: We randomly allocated 120 patients to receive one of three remifentanil target effect-site concentrations (5, 7, or 9 ng×mL-1) during intubation. The target effect-site concentrations of propofol were adjusted within a range of 2.5-3 μg×mL-1 to maintain bispectral index values at less than 60 during anesthesia induction. A reusable SPI sensor was placed on the index finger of the arm, and the SPI values were continuously recorded. The predicted probability for maintaining the SPI values at less than 50 after intubation against the cumulative amount of remifentanil was analyzed using logistic regression. The measurands were the baseline SPI value in patients without pain scheduled for surgery, and the maximal SPI value after intubation in patients receiving remifentanil with a target effect-site concentration of 7 ng×mL-1. RESULTS : The estimated cumulative amount of remifentanil associated with a 50% and 95% probability of maintaining the SPI values at less than 50 after intubation were 135.0 μg and 330.4 μg, respectively. The estimated expanded uncertainty for the baseline and maximal SPI values after intubation in patients scheduled for surgery were 54.9±44.4 and 54.1±37.9, respectively, which corresponded to a confidence level of approximately 95%. CONCLUSIONS: The analgesic potency dose of remifentanil to maintain SPI values at less than 50 after intubation was 135.0 μg.
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