• Masaki Sato, Kenichi Masui, Borjigin Sarentonglaga, Mio Yamaguchi, Rika Fukumori, Yoshikazu Nagao, Haruhiko Sago, and Hiroyuki Sumikura.
• Department of Anesthesiology and Pain Medicine, Juntendo University Graduate School of Medicine, Bunkyo, Tokyo, Japan.
• J Anesth. 2017 Aug 1; 31 (4): 517-522.

PurposeMaternal remifentanil infusion is used for minimally invasive fetal surgery or ex-utero intrapartum treatment. The fetal-to-maternal (F/M) ratio of remifentanil concentration at various dosing regimens is useful to manage remifentanil effects. The aim of this study was to investigate the F/M ratio of remifentanil at various concentrations.MethodsFive pregnant ewes received continuous remifentanil infusion under propofol anesthesia. The remifentanil infusion rate was increased by 0.4 µg/kg/min every 15 min. The response to tail clamping in fetuses was assessed immediately before the change of infusion rate. Arterial remifentanil concentrations in the mother and fetus were determined at each tail clamp. After observing a loss of response to tail clamping, remifentanil infusion was terminated and the concentrations were assessed.ResultsThe median remifentanil maximum infusion rate and maternal concentration were 3.0 µg/kg/min (range 2.4-3.6) and 21.6 (range 18.0-29.9) ng/mL, respectively. During continuous infusion, the F/M ratio was 0.15 (0.07-0.17), and the slope of the linear regression for the F/M ratio versus infusion rate in each individual was -0.001 ± 0.012/μg kg min (P = 0.876 vs hypothetical value of 0). The F/M ratio at the first sampling point in the elimination phase [0.33 (0.07-0.65)] was higher (P = 0.033) than at the last sampling point during continuous infusion [0.15 (0.06-0.17)].ConclusionThe F/M ratio was constant at a steady state regardless of the remifentanil concentration up to 29.9 ng/mL, and increased in the elimination phase in pregnant ewes.

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