Disposition, behavioural and physiological effects of escalating doses of intravenously administered fentanyl to young foals.

This study explores the behavioural and physiological reactions in young foals aged 5-13 days after being given increasing doses of fentanyl, a powerful opioid. The results of the study demonstrated that these reactions varied with the dosage, and like mature horses, the range of fentanyl plasma concentrations likely to produce clinically acceptable responses is relatively narrow.

Introduction and Objective

• The main aim of this study was to determine the range of plasma fentanyl concentrations in young foals, which are desirable for clinical use. The study also aimed to note the associated behavioural and physiological responses to the various plasma fentanyl concentrations. Fentanyl is a potent opioid drug, and understanding its effects on foals is vital for equine veterinary practice.

Methods

• The study was experimental in nature, with fentanyl being administered intravenously (i.v.) in escalating doses to foals aged between 5 and 13 days.
• The dosage increments were 2, 4, 8, 16, and 32 μg/kg body weight, given at 10-minute intervals.
• Blood samples were collected before and at various times until 24 hours post-administration. These samples were analysed for fentanyl and its metabolic byproduct concentrations.
• Associated behavioural and physiological observations along with chosen blood analytes were correlated with the blood sample results.

Results

• After administering a 2 μg/kg bodyweight dose of fentanyl, most foals appeared to be unaffected. However, 6 out of 8 foals appeared to show signs of sedation at the 4 μg/kg dose.
• Upon reaching the 8 μg/kg and 16 μg/kg doses, many foals exhibited signs of ataxia (loss of full control of bodily movements), increased locomotor activity, muscle rigidity, and a posture of pressing their heads.
• After reaching the highest dose level of 32 μg/kg, all foals were strongly sedated, and 3 out of 8 became recumbent (lying down).
• The average terminal half-life following the final dose administration was approximately 44.2 minutes.

Conclusion

• The research concluded that the reactions to intravenous fentanyl administration in young foals are dose-related.
• Similar to the trend observed in mature horses, the acceptable range of fentanyl plasma concentrations leading to possible clinically desirable effects appears to be quite narrow.