Evaluation of the effect of alfentanil on the minimum alveolar concentration of halothane in horses.

This research explores the impact of different plasma concentrations of a drug called alfentanil on the minimum alveolar concentration of halothane, a type of anaesthetic, in horses. The study found no significant changes in the anaesthetic’s minimum concentration for any level of alfentanil, though blood pressure rose in relation to dosage.

Study Design and Procedure

• Five healthy geldings (male horses) underwent anesthesia three times using halothane, a type of anesthesia agent, in oxygen given via a mask.
• Following induction of anesthesia, horses were equipped for measurement of various health indicators, including blood pressure, airway pressure, and end-tidal concentrations of halothane.
• Blood samples were taken from the facial artery to measure pH and blood gas tensions.
• Positive pressure ventilation was started with an aim to sustain the partial pressure of carbon dioxide (PaCO2) at a specified level.

Determination of Minimum Alveolar Concentration (MAC)

• MAC refers to the concentration of anesthetic in the lungs represented as a percentage that is required to prevent movement in response to a surgical incision in 50% of subjects.
• In this study, the MAC was determined thrice using an extreme electrical stimulus applied to the oral mucous membranes.
• Following this, an infusion of alfentanil was started, with the goal of achieving and maintaining one of the three predetermined plasma concentrations of alfentanil.
• Starting from 30 minutes after the infusion started, the MAC was determined again.

Results and Conclusion

• The measured alfentanil plasma concentrations during the infusion were at three respective levels.
• No significant changes in terms of MAC were observed for any alfentanil concentration.
• The study found that the blood pressure increased as a result of the alfentanil infusion and this increase was directly proportional to the dosage, but there was no noticeable alteration in heart rate.
• Pharmacokinetic variables of alfentanil were determined after its infusion and were not significantly different among the three doses, which suggests that the body processed the drug similarly regardless of the dose.

To conclude, the study highlights that the amount of alfentanil in the system does not noticeably alter the minimum required concentration of this form of anesthetic in the lungs to render the horse unresponsive to surgical stimulants, therefore this study provides no evidence to adjust the dosage of halothane dependent on alfentanil levels.