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Effects of high plasma fentanyl concentrations on minimum alveolar concentration of isoflurane in horses

Heather K DiMaio Knych1, Eugene P Steffey, Khursheed R Mama

  • 1K. L. Maddy Equine Analytical Chemistry Laboratory, California Animal Health and Food Safety Laboratory, and the Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616, USA. hkknych@ucdavis.edu

American Journal of Veterinary Research
|October 3, 2009
PubMed
Abstract

Insights

This study found that fentanyl did not reduce the required dose of isoflurane anesthetic in horses. Higher fentanyl concentrations also failed to show a significant minimum alveolar concentration (MAC)-sparing effect, questioning its routine use.

Area of Science:

  • Veterinary Anesthesiology
  • Equine Pharmacology

Background:

  • Fentanyl is an opioid analgesic used in veterinary medicine.
  • Its potential to reduce the minimum alveolar concentration (MAC) of isoflurane in horses requires further investigation.

Purpose of the Study:

  • To determine if a target plasma fentanyl concentration of 16 ng/mL spares isoflurane MAC in horses.
  • To assess the effect of higher fentanyl concentrations (24 and 32 ng/mL) on isoflurane MAC.

Main Methods:

  • Eight horses received ascending target plasma fentanyl concentrations (16, 24, 32 ng/mL).
  • Isoflurane MAC was determined at baseline and at each fentanyl concentration.
  • Plasma fentanyl concentrations and recovery behaviors were monitored.

Main Results:

  • Actual plasma fentanyl concentrations averaged 13.9, 20.1, and 24.1 ng/mL.
  • Changes in isoflurane MAC were -3.28%, -6.23%, and +1.14%, none statistically significant.
  • Recovery was variable, with some horses exhibiting undesirable excitatory behavior.

Conclusions:

  • Fentanyl did not demonstrate a significant isoflurane MAC-sparing effect in horses at the tested concentrations.
  • The findings do not support the routine use of fentanyl as an anesthetic adjuvant in adult horses.
  • Variable recovery and excitatory behaviors were noted.

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