Effect-site concentration of remifentanil during patient-controlled analgesia in labour

•Intravenous remifentanil was used in patient-controlled analgesia for labour.•Hypothetical effect-site remifentanil concentration (Minto’s model) was examined.•Higher value before contraction (residual value) resulted in lower pain score.•Monitoring this parameter may be useful in future clinical practice.•Confirmation with a pharmacokinetic model optimized for pregnant patients is required.

BackgroundIntravenous remifentanil has been described for patient-controlled analgesia in labour. Recently, the application of target-controlled infusion pumps with Minto’s pharmacokinetic/pharmacodynamic model has been reported. Hypothetical effect-site remifentanil concentration during patient-controlled analgesia for labour has yet to be examined. The aim of this concept study was to explore characteristics of this parameter.MethodsWe performed a historical cohort study based on our previous randomised cross-over clinical trial and analysed hypothetical effect-site remifentanil concentration. Values at spontaneous vaginal delivery and Apgar scores were tested for correlation. The association between pain score and the corresponding effect-site remifentanil concentration before and after bolus administration, and their relative difference, was examined with a linear mixed-effects model, adjusted for other variables.ResultsA series of 23 parturients with uncomplicated singleton pregnancies were included. On average, effect-site remifentanil concentration was highest during the third quarter throughout our recordings (5.5 ng/mL; maximum 15.8 ng/mL). The mean (median) {IQR} [range] at spontaneous vaginal delivery (n=14) was 2.52 (1.32) {0.95–4.28} [0.65–6.88] ng/mL, all Apgar scores were >7, and no correlation was confirmed. A negative association between effect-site remifentanil concentration before bolus administration and pain score (scale 0–100) was observed (−3.9, 95% CI −5.16 to −2.61, P <0.01).ConclusionsThe residual value of hypothetical effect-site remifentanil concentration before uterine contraction, at the beginning of bolus administration, predicted lower pain scores. Monitoring effect-site remifentanil concentration may be potentially useful when remifentanil is administered for labour analgesia. However, our results need to be confirmed with a pharmacokinetic model optimized for pregnant patients.