BJA Advance Access originally published online on May 23, 2006
British Journal of Anaesthesia 2006 97(2):232-237; doi:10.1093/bja/ael116
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The effects of i.v. fentanyl administration on the minimum alveolar concentration of isoflurane in horses
1 K.L. Maddy Equine Analytical Chemistry Laboratory, California Animal Health and Food Safety Laboratory, University of California Davis, CA 95616, USA.
2 Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California Davis, CA 95616, USA.
3 Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University Fort Collins, CO 80523, USA
*Corresponding author. E-mail: epsteffey{at}ucdavis.edu
Background. Fentanyl decreases the minimum alveolar concentration (MAC) of inhaled anaesthetics and has been used clinically to reduce the requirements of other anaesthetic drugs in humans and small animals. We hypothesized that i.v. fentanyl would decrease the MAC of isoflurane in horses in a dose-dependent manner.
Methods. Following determination of baseline MAC of isoflurane, fentanyl was administered i.v. to target plasma concentrations of 1, 8 and 16 ng ml–1. Each horse was randomly assigned two of three target concentrations administered in ascending order. Loading and infusion doses for each horse were determined from previously derived individual pharmacokinetic values. Isoflurane MAC determination began 45 min after fentanyl administration at each target fentanyl concentration. Venous blood was collected at fixed intervals during the infusion for measurement of plasma fentanyl concentrations.
Results. Mean actual fentanyl plasma concentrations were 0 (baseline), and 0.72 (SD 0.26), 8.43 (3.22), and 13.31 (6.66) ng ml–1 for the target concentrations of 1, 8 and 16 ng ml–1, respectively. The corresponding isoflurane MAC values were a baseline of 1.57 (0.23), and 1.51 (0.24), 1.41 (0.23) and 1.37 (0.09)%, respectively. The fentanyl concentrations of 0.72 and 8.43 ng ml–1 did not significantly alter the MAC of isoflurane, but an 18 (7)% ISO-MAC reduction was observed at the 13.31 ng ml–1 concentration.
Conclusions. These results cautiously encourage further study of fentanyl as an opioid anaesthetic adjunct to inhalant anaesthesia in horses.