Effects of high plasma fentanyl concentrations on minimum alveolar concentration of isoflurane in horses.
OBJECTIVE
To verify the isoflurane anesthetic minimum alveolar concentration (MAC)-sparing effect of a previously administered target plasma fentanyl concentration of 16 ng/mL and characterize an anticipated further sparing in isoflurane MAC associated with higher target plasma fentanyl concentrations.


METHODS
8 horses.


METHODS
Horses were assigned 2 of 3 target plasma fentanyl concentrations (16, 24, and 32 ng/mL), administered in ascending order. Following determination of baseline MAC, horses received a loading dose of fentanyl followed by a constant rate infusion; MAC determination was performed in triplicate at baseline and at each fentanyl concentration. Venous blood samples were collected throughout the study for determination of actual plasma fentanyl concentrations. Recovery from anesthesia was monitored, and behaviors were rated as excellent, good, fair, or poor.


RESULTS
Mean + or - SD fentanyl plasma concentrations were 13.9 + or - 2.6 ng/mL, 20.1 + or - 3.6 ng/mL, and 24.1 + or - 2.4 ng/mL for target concentrations of 16, 24, and 32 ng/mL, respectively. The corresponding changes in the MAC of isoflurane were -3.28%, -6.23%, and +1.14%. None of the changes were significant. Recovery behavior was variable and included highly undesirable, potentially injurious excitatory behavior.


CONCLUSIONS
Results of the study did not verify an isoflurane-sparing effect of fentanyl at a plasma target concentration of 16 ng/mL. Furthermore, a reduction in MAC was not detected at higher fentanyl concentrations. Overall, results did not support the routine use of fentanyl as an anesthetic adjuvant in adult horses.
K. 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
Heather K DiMaio
HK
Knych
Eugene P
EP
Steffey
Khursheed R
KR
Mama
Scott D
SD
Stanley
D000762:Anesthesia Recovery Period; D000769:Anesthesia, Inhalation / Q000662:veterinary; D018685:Anesthetics, Inhalation / Q000493:pharmacokinetics*; D018686:Anesthetics, Intravenous / Q000097:blood / Q000493:pharmacokinetics*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D005260:Female; D005283:Fentanyl / Q000097:blood / Q000493:pharmacokinetics*; D006736:Horses; D007530:Isoflurane / Q000493:pharmacokinetics*; D008297:Male; D011650:Pulmonary Alveoli
D018685:Anesthetics, Inhalation; D018686:Anesthetics, Intravenous; D007530:Isoflurane; D005283:Fentanyl