Disposition, behavioural and physiological effects of escalating doses of intravenously administered fentanyl to young foals.
Authors: Knych H K, Steffey E P, Casbeer H C, Mitchell M M
Journal: Equine veterinary journal
Summary
# Fentanyl Dosing in Young Foals: Establishing Safe Plasma Concentrations Fentanyl's clinical application in neonatal foals remains poorly characterised, despite its widespread use in equine anaesthesia and analgesia. Knych and colleagues administered escalating intravenous fentanyl doses (2–32 µg/kg) to eight foals aged 5–13 days at 10-minute intervals, measuring plasma concentrations whilst recording behavioural and physiological responses over 24 hours. Minimal observable effects occurred at 2 µg/kg (mean 1.09 µg/L), but six of eight foals showed sedation at 4 µg/kg (3.07 µg/L), whilst higher doses produced ataxia, muscle rigidity and head pressing at 8–16 µg/kg before heavy sedation and recumbency developed at 32 µg/kg (34.5 µg/L); the drug's terminal half-life in neonates averaged 44.2 minutes. These findings confirm that fentanyl's therapeutic window in young foals is narrow and dose-dependent, mirroring adult horses—information critical for practitioners selecting appropriate sedation protocols and avoiding undesirable dysphoric or rigid responses, particularly important given neonatal foals' vulnerability to complications from prolonged recumbency or compromised posture during recovery.
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Practical Takeaways
- •Fentanyl dosing in neonatal foals (5–13 days old) requires careful titration, as the therapeutic window is narrow and adverse effects appear rapidly with modest dose escalation
- •Doses below 4 μg/kg may be insufficient for clinical sedation, but 8–16 μg/kg produces ataxia and dysphoria rather than desired analgesia or sedation
- •Close monitoring of behavioural and physiological signs is essential when fentanyl is used in young foals, as individual responses vary significantly
Key Findings
- •Foals showed minimal response to 2 μg/kg fentanyl (1.09 ± 0.41 μg/L), but 6 of 8 foals were sedated at 4 μg/kg (3.07 ± 1.11 μg/L)
- •Ataxia, increased locomotor activity, muscle rigidity and head pressing occurred at 8–16 μg/kg doses (7.24–17.4 μg/L)
- •All foals were heavily sedated at 32 μg/kg (34.5 ± 10.3 μg/L) with 3 of 8 becoming recumbent
- •Terminal half-life following final dose was 44.2 ± 9.85 minutes, with a narrow therapeutic window similar to mature horses