title: Influence of maternal remifentanil concentration on fetal-to-maternal ratio in pregnant ewes.
abstract: OBJECTIVE
  Maternal 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.
  
  
  METHODS
  Five 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.
  
  
  RESULTS
  The 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)].
  
  
  CONCLUSIONS
  The 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.
authors.affiliation: Department of Anesthesiology and Pain Medicine, Juntendo University Graduate School of Medicine, Bunkyo, Tokyo, Japan.|Division of Obstetric Anesthesiology, National Center for Child Health and Development, Setagaya, Tokyo, Japan.
authors.forename: Masaki
authors.initials: M
authors.lastname: Sato
authors.affiliation: Department of Anesthesiology, National Defense Medical College, Tokorozawa, Saitama, Japan. kenichi@masuinet.com.
authors.forename: Kenichi
authors.identifier: http://orcid.org/0000-0003-4702-9874
authors.initials: K
authors.lastname: Masui
authors.affiliation: University Farm, Facility of Agriculture, Utsunomiya University, Utsunomiya, Tochigi, Japan.
authors.forename: Borjigin
authors.initials: B
authors.lastname: Sarentonglaga
authors.affiliation: University Farm, Facility of Agriculture, Utsunomiya University, Utsunomiya, Tochigi, Japan.|United Graduate School of Agricultural Science, Tokyo University of Agriculture and Technology, Fuchu, Tokyo, Japan.
authors.forename: Mio
authors.initials: M
authors.lastname: Yamaguchi
authors.affiliation: University Farm, Facility of Agriculture, Utsunomiya University, Utsunomiya, Tochigi, Japan.|United Graduate School of Agricultural Science, Tokyo University of Agriculture and Technology, Fuchu, Tokyo, Japan.
authors.forename: Rika
authors.initials: R
authors.lastname: Fukumori
authors.affiliation: University Farm, Facility of Agriculture, Utsunomiya University, Utsunomiya, Tochigi, Japan.|United Graduate School of Agricultural Science, Tokyo University of Agriculture and Technology, Fuchu, Tokyo, Japan.
authors.forename: Yoshikazu
authors.initials: Y
authors.lastname: Nagao
authors.affiliation: Center for Maternal-Fetal, Neonatal and Reproductive Medicine, National Center for Child Health and Development, Setagaya, Tokyo, Japan.
authors.forename: Haruhiko
authors.initials: H
authors.lastname: Sago
authors.affiliation: Department of Anesthesiology and Pain Medicine, Juntendo University Graduate School of Medicine, Bunkyo, Tokyo, Japan.
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Sumikura
mesh_terms: D000758:Anesthesia / Q000379:methods; D018686:Anesthetics, Intravenous / Q000008:administration & dosage*; D000818:Animals; D005260:Female; D005333:Fetus / Q000378:metabolism*; D010880:Piperidines / Q000008:administration & dosage*; D011247:Pregnancy; D015742:Propofol / Q000008:administration & dosage*; D000077208:Remifentanil; D012756:Sheep
chemical_list: D018686:Anesthetics, Intravenous; D010880:Piperidines; D000077208:Remifentanil; D015742:Propofol
keywords: Fetal-to-maternal ratio; Pharmacokinetics; Remifentanil