Effect of fentanyl on thermal and mechanical nociceptive thresholds in horses and estimation of anti-nociceptive plasma concentration.
Authors: Echelmeyer J, Taylor P M, Hopster K, Rohn K, Delarocque J, Kästner S B R
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary: Fentanyl Anti-nociception in Horses Understanding the relationship between plasma fentanyl concentrations and pain relief is essential for optimising analgesia in equine clinical practice, yet this area has received limited investigation in horses. Echelmeyer and colleagues administered three fentanyl doses (2.5, 5 and 10 µg/kg) and placebo intravenously to eight horses in a crossover design, measuring thermal and mechanical nociceptive thresholds repeatedly over 22.5 hours alongside plasma drug concentrations and physiological parameters. The highest dose (10 µg/kg) significantly elevated thermal thresholds to 53.7°C at 10 minutes and 52.1°C at 30 minutes compared to baseline (47.2°C), whilst mechanical threshold improvements were briefer and less pronounced, rising to 6.6 N at 10 minutes only. The authors established that plasma concentrations of 6.1–6.8 ng/mL represent the minimum threshold for thermal anti-nociception, with efficacy lasting approximately 10–30 minutes at the highest dose studied. Dose-dependent increases in locomotor activity occurred without significant cardiovascular, respiratory or gastrointestinal effects, indicating fentanyl's analgesic window is narrow in equines—a finding that should inform practitioners' dosing intervals and choice of supplementary analgesia for procedures requiring sustained pain relief beyond 30 minutes.
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Practical Takeaways
- •Intravenous fentanyl at 10 µg/kg provides rapid but short-lived anti-nociception (10-30 min), making it suitable for brief procedures requiring pain relief but requiring redosing for longer interventions
- •Plasma concentrations ≥6.1-6.8 ng/mL are needed to achieve thermal anti-nociception in horses; monitoring or repeat dosing strategies may be necessary to maintain therapeutic levels
- •Expect increased locomotor activity as a dose-dependent side effect when using fentanyl, requiring appropriate restraint and safety precautions during and after administration
Key Findings
- •Fentanyl 10 µg/kg IV produced significant increase in thermal threshold (47.2°C baseline to 53.7°C at 10 min) and mechanical threshold (3.7N baseline to 6.6N at 10 min)
- •Estimated minimum anti-nociceptive plasma concentration for thermal anti-nociception was 6.1-6.8 ng/mL
- •Anti-nociceptive effects persisted for only 10-30 minutes at the 10 µg/kg dose
- •Dose-dependent increased locomotor activity was the primary side effect observed with no significant changes in heart rate, respiratory rate, or gastrointestinal sounds