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Placental Passage and Uterine Effects of Fentanyl

Received from the Departments of Anesthesiology and Obstetrics and Gynecology, George Washington University Medical Center, Washington, DC.

Abstract

Using the chronic maternal–fetal sheep preparation, 27 pregnant ewes were studied to determine the effects of intravenous fentanyl on maternal and fetal physiology, with particular reference to its placental passage, and its effects on uterine blood flow and uterine tone. Three doses of fentanyl were studied-50, 75, and 100 µg. Maternal and fetal arterial blood was collected for determination of fentanyl levels. All blood levels, both maternal and fetal, were normalized to the 50-µg dose. The maternal normalized blood levels were found to fit a biexponential equation describing a two-compartment open model. The half-life of the maternal elimination phase was 42 ± 7.0 min with an overall elimination constant (K) of 0.21 min^–1. Maternal plasma fentanyl levels decreased very rapidly in the first 10 min after injection, at which time only 9% of the peak value remained. Fentanyl was detectable in fetal blood as early as 1 min and levels peaked at 5 min. Once equilibrium was established between maternal and fetal blood, the maternal levels remained 2.5 times those of the fetal level from 5 min to 60 min after drug injection. Both maternal and fetal drug levels declined in an approximately parallel fashion. No significant deleterious changes were seen in any maternal or fetal cardiovascular or acid-base parameters, and uterine blood flow and uterine tone were also unaffected (P > 0.05).

Key Words: ANALGESICS: fentanyl • ANESTHESIA: obstetric

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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999