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

The research experiment aimed to understand the effect of intravenously (i.v.) administered fentanyl, an opioid, on the minimum alveolar concentration (MAC) which is a measure of the potency of inhaled anesthetic Isoflurane in horses. The researchers hypothesized and found that fentanyl does decrease the MAC of Isoflurane in a dose-dependent manner.

Methods:

• Initially, the baseline MAC of Isoflurane was determined.
• Then, fentanyl was administered intravenously (i.v.) to target plasma concentrations of 1, 8, and 16 ng ml(-1) in the horses.
• Each horse was randomly assigned two out of three target concentrations administered in ascending order. The loading and infusion doses for each horse were determined from previously noted individual pharmacokinetic values.
• After 45 minutes of each level of fentanyl administration, MAC determination was undertaken.
• Venous blood was collected for measurement of plasma fentanyl concentrations.

Results:

• The study noted that mean plasma concentrations for the targeted fentanyl concentrations (ng ml(-1)) 1, 8, and 16 were 0.72, 8.43, and 13.31 respectively instead of the expected values.
• The corresponding MAC values of Isoflurane (%) for these concentrations were 1.51, 1.41, and 1.37 compared to a baseline of 1.57.
• Fentanyl concentrations of 0.72 and 8.43 ng ml(-1) did not significantly alter the MAC of Isoflurane. But there was an 18% ISO-MAC reduction at the 13.31 ng ml(-1) concentration.

Conclusions:

• The research encourages further study of fentanyl as a potential opioid anaesthetic adjunct to inhalant anaesthesia in horses.
• It is noted that the decrease in the MAC of Isoflurane was found to be dose-dependent on the administered fentanyl.