Clinical Randomized Comparison of Medetomidine and Xylazine for Isoflurane Balanced Anesthesia in Horses

Introduction: To assess drug plasma levels, preanesthetic sedation, cardiopulmonary effects during anesthesia and recovery in horses anesthetized with isoflurane combined with medetomidine or xylazine. Study design: Prospective blinded randomized clinical study. Animals: Sixty horses undergoing elective surgery. Methods: Thirty minutes after administration of antibiotics, flunixine meglumine or phenylbutazone and acepromazine horses received medetomidine 7 μg kg(−1) (group MED) or xylazine 1.1 mg kg(−1) (group XYL) slowly intravenously (IV) and sedation was assessed 3 min later. Anesthesia was induced with ketamine/diazepam and maintained with isoflurane in oxygen/air and medetomidine 3.5 μg kg(−1) h(−1) or xylazine 0.69 mg kg(−1) h(−1). Ringer's acetate 10 mL kg(−1) h(−1) and dobutamine were administered to maintain normotension. All horses were mechanically ventilated to maintain end-tidal carbon dioxide pressures at 45 ± 5 mmHg (5.3–6.7 kPa). Heart rate (HR), invasive arterial blood pressures, inspired and expired gas compositions, pH, arterial blood gases, electrolytes, lactate and glucose were measured. For recovery all horses received intramuscular morphine 0.1 mg kg(−1) and medetomidine 2 μg kg(−1) or xylazine 0.3 mg kg(−1) IV. Recovery was timed and scored using three different scoring systems. Plasma samples to measure medetomidine and xylazine concentrations were collected at predetermined timepoints. Repeatedly measured parameters were analyzed using a two-way repeated-measures analysis of variance for differences between groups and over time; p < 0.05 was considered statistically significant. Results: Mean arterial blood pressures (MAP) stayed within normal ranges but were higher (p = 0.011) in group XYL despite significant lower dobutamine doses (p = 0.0003). Other measured parameters were within clinically acceptable ranges. Plasma levels were at steady state during anesthesia (MED 2.194 ± 0.073; XYL 708 ± 18.791 ng mL(−1)). During recovery lateral recumbency (MED 42.7 ± 2.51; XYL 34.3 ± 2.63 min; p = 0.027) and time to standing (MED 62.0 ± 2.86; XYL 48.8 ± 3.01 min; p = 0.002) were significantly shorter in group XYL compared to group MED. Recovery scores did not differ significantly between groups. Conclusion and Clinical Relevance: In horses anesthetized with isoflurane and medetomidine or xylazine, xylazine maintained higher MAP, reduced the dobutamine consumption and recovery time, whilst overall recovery quality was unaffected.