Intraocular pressure following four different intravenous sedation protocols in normal horses

BACKGROUND Intravenous sedation is frequently necessary for ophthalmic examination in horses. Common sedation protocols have not been directly compared in terms of relative intraocular pressure (IOP) reduction, duration of IOP reduction and time to maximum IOP reduction. OBJECTIVES To compare the effects of standing sedation protocols on IOP. STUDY DESIGN Randomised cross-over experiment. METHODS Twelve healthy horses received four intravenous sedation protocols with a 48 hours washout: 0.5 mg/kg xylazine and 0.01 mg/kg butorphanol (SED1); 10 µg/kg detomidine and 0.01 mg/kg of butorphanol (SED2); 10 µg/kg detomidine (SED3); 0.5 mg/kg xylazine (SED4). IOP was measured with rebound tonometry before sedation (Tpre) and 5, 10, 15, 30, 45 and 60 minutes post-sedation (Tpost). Post-sedation readings were taken with the head elevated to the Tpre position. Separately, IOP readings were also obtained following sedation with the head not elevated (TpostHeadDown). IOP values were compared using mixed ANOVA and ANCOVA models respectively with significance at P < .05. RESULTS All protocols decreased IOP compared with baseline with greatest reduction at Tpost5. IOP at Tpre (mean ± SD) was 21.8 ± 4.4 mm Hg. At Tpost5, IOP was 16.3 ± 3.8 mm Hg (SED1), 14.5 ± 2.9 mm Hg (SED2), 17.1 ± 3.8 mm Hg (SED3) and 16.9 ± 4.2 mm Hg (SED4). SED2 Tpost5 IOP was lower than other treatments. Considering all time points following sedation, SED3 IOP readings were higher than other treatments. TpostHeadDown IOPs were higher than readings taken with the head elevated (P < .001). MAIN LIMITATIONS Animals with ocular disease were not studied. No animals received mock sedation or equivalent. CONCLUSIONS A combination of detomidine and butorphanol causes greater IOP reduction 5 minutes following sedation than other commonly used sedation protocols. IOP reduction is less pronounced when detomidine is used alone. Consideration of head height is important when performing IOP measurements in horses.