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

The research investigated if high concentrations of fentanyl in plasma could reduce the minimum concentration of isoflurane, an anesthetic, in the lungs of horses. The study showed no significant difference and therefore, does not encourage the use of fentanyl as an additive to anesthesia in adult horses.

Methodology

• The research was done on eight horses who were exposed to increasing concentrations of fentanyl (16, 24, and 32 ng/mL).
• Each horse was given a loading dose of fentanyl and then a constant infusion to maintain a steady plasma concentration.
• The minimum alveolar concentration (MAC) of isoflurane, an anesthetic gas, was then observed in triple measures for baseline and at each fentanyl concentration level.
• Blood samples from the horses were taken regularly to assess the actual concentration of fentanyl in the plasma.
• The researchers observed and rated how the horses woke up from the anesthesia, recording behaviors as excellent, good, fair, or poor.

Results

• The mean plasma concentrations of fentanyl achieved were 13.9, 20.1, and 24.1 ng/mL at target levels of 16, 24, and 32 ng/mL respectively.
• Changes to the MAC of isoflurane were noted as -3.28% for a target fentanyl concentration of 16 ng/mL, -6.23% for 24 ng/mL, and +1.14% for 32 ng/mL. None of these changes were deemed significant.
• The horses’ reactions to waking from anesthesia varied widely, with some exhibiting potentially harmful levels of excitement.

Conclusions

• Contrary to the researchers’ expectations, the result did not verify that fentanyl has an isoflurane-sparing effect even at plasma concentration of 16 ng/mL.
• There was also no detectable decrease in MAC at higher concentrations of fentanyl.
• The results suggested that using fentanyl as an adjunct to anesthesia in adult horses may not be beneficial and could potentially lead to problematic reactions when the animal recovers from anesthesia.