Background: Indications for surgery during pregnancy have increased. Specifically fetal interventions have increased from conditions that were considered lethal like twin-twin transfusion syndrome and severe fetal anemia to non-lethal conditions like myelomeningocele. The optimal anesthetic agent for in utero surgery is yet to be determined. Success of the procedure is often dictated by the efficacy of the anesthetic to immobilize the fetus without over-sedating mom. Remifentanil is used as preferred agent due to its short half-life however pharmacokinetics in pregnancy is unknown. Objective: To determine the pharmacokinetic parameters of remifentanil in a mid-trimester pregnant patient population undergoing fetal intervention. Study Design: A validated liquid chromatography assay with ultraviolet absorbance was employed to estimate maternal serum remifentanil levels. Blood samples were obtained at baseline and at selected time points: 5, 15, 30, 45, 60 min after the beginning of the remifentanil infusion and at 15, 30, and 60 min post end of infusion. Results: Ten pregnant patients were enrolled in the study however only eight patients had sampling obtained at all time points. The mean gestational age was 22.2 (±2.7) weeks, maternal age was 27.8 (±5.1) years and body mass index was 29.6 (±6.3). After receiving a continuous infusion of remifentanil, mean total dose was 975.3 μg, C_min was 2.0 ng/mL and C_max was 8.4 ng/mL. A two-compartment model best described the plasma remifentanil data. Mean pharmacokinetic parameters were: volume of distribution (Vd_c) = 124.6 L (16.2-530.8 L), maternal remifentanil total clearance (Cl_t) = 170.7 L/h (17.7-486.9 L/h), and half-life (t_½) = 0.6 h (0.2-0.9 h). The maternal remifentanil area under the curve (AUC) ranged from 2.7 to 21.7 μg/L^*h. The mean alpha-acidic glycoprotein was 124.8 mg/dL (81.3-149.8). Conclusion: The pharmacokinetic profile of remifentanil in pregnant women is similar to previously reported general population profiles. This data did provide potential rationale for the clinical observations why when remifentanil is dosed based on non-pregnant guidelines, it did not uniformly provide adequate fetal immobilization as per anecdotal perception of operating fetal surgeons. These findings are important for the development of further clinical studies to optimize dosing for surgery during pregnancy including the estimation of placental transfer and total fetal exposure.

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