The effects of i.v. fentanyl administration on the minimum alveolar concentration of isoflurane in horses

Background. Fentanyl decreases the minimum alveolar concentration(MAC) of inhaled anaesthetics and has been used clinically toreduce the requirements of other anaesthetic drugs in humansand small animals. We hypothesized that i.v. fentanyl woulddecrease the MAC of isoflurane in horses in a dose-dependentmanner. Methods. Following determination of baseline MAC of isoflurane,fentanyl was administered i.v. to target plasma concentrationsof 1, 8 and 16 ng ml^–1. Each horse was randomly assignedtwo of three target concentrations administered in ascendingorder. Loading and infusion doses for each horse were determinedfrom previously derived individual pharmacokinetic values. IsofluraneMAC determination began 45 min after fentanyl administrationat each target fentanyl concentration. Venous blood was collectedat fixed intervals during the infusion for measurement of plasmafentanyl 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^–1for the target concentrations of 1, 8 and 16 ng ml^–1,respectively. The corresponding isoflurane MAC values were abaseline 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 and8.43 ng ml^–1 did not significantly alter the MAC of isoflurane,but an 18 (7)% ISO-MAC reduction was observed at the 13.31 ngml^–1 concentration. Conclusions. These results cautiously encourage further studyof fentanyl as an opioid anaesthetic adjunct to inhalant anaesthesiain horses.