Effect of fentanyl on thermal and mechanical nociceptive thresholds in horses and estimation of anti-nociceptive plasma concentration.

• Fentanyl analgesic efficacy and duration and minimum anti-nociceptive plasma concentrations in horses were investigated. • Fentanyl administered at 10 μg/kg IV produced significant antinociception for 10–30 min in horses. • Dose-dependent increased locomotor activity was the main side effect. • Fentanyl plasma concentrations of ≥6.1–6.8 ng/mL appeared necessary to induce detectable anti-nociception in horses. There are few investigations relating anti-nociception to plasma concentrations of fentanyl in horses. The study objective was to evaluate analgesic efficacy and duration in horses and determine the minimum anti-nociceptive plasma concentrations. Eight horses were treated with saline (P) and fentanyl (F 2.5 = 2.5 μg/kg; F 5 = 5 μg/kg; F 10 = 10 μg/kg) given IV over 5 min, with a wash-out period of 10 days. To evaluate thermal (°C) and mechanical (N) nociceptive threshold single stimulations were applied prior to (baseline) and 10, 30, 60, 90, 120, 180, 240, 300, 360, 420, 540 min and 22.5 h after treatment. Plasma fentanyl concentrations were measured at specific time points. Locomotor activity, heart rate, respiratory rate and gastrointestinal sounds were recorded. Two-way repeated measures ANOVA and pairwise comparisons were used for data analysis (P < 0.05). With treatment F 10 , there was a significant increase in thermal threshold above baseline (47.2 ö 4.1 °C) at t 10 (53.7 ö 4.2 °C) and t 30 (52.1 ö 5.6 °C), whereas mechanical threshold increased considerably above baseline (3.7 ö 1.3 N) only at t 10 (6.6 ö 3.6 N). Estimated mean minimum anti-nociceptive plasma concentration determined by thermal stimulation was 6.1–6.8 ng/mL. Dose-dependent increased locomotion occurred, but no significant changes in heart rate, respiratory rate and gastrointestinal sounds were observed. Fentanyl IV at 10 μg/kg produced anti-nociception for 10–30 min and fentanyl plasma concentrations of ≥6.1–6.8 ng/mL appear necessary to induce thermal anti-nociception. Dose-dependent increased locomotion was the main side effect observed. [ABSTRACT FROM AUTHOR]