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

Abstract

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.

Comparison of Recovery Quality Following Medetomidine versus Xylazine Balanced Isoflurane Anaesthesia in Horses: A Retrospective Analysis

“…In a clinical setting, the exact assessment of tissue perfusion and oxygenation is unfortunately still impossible. A recent clinical and well-standardised study comparing healthy horses undergoing elective surgery with medetomidine or xylazine PIVA did not find a difference in oxygenation between both groups, but minimally higher mean arterial blood pressures with xylazine, despite a lower mean dose of dobutamine (medetomidine 36 µg·kg −1 ·h −1 , xylazine 24 µg·kg −1 ·h −1 ) [ 18 ]. In the present study, incidence of hypotension was similar between groups, despite the fact that, in the MED group, fewer horses received acepromazine, probably as a consequence of more laparotomy surgeries in that group.…”

“…Vettorato et al found similar agreement among the four RQSSs tested and suggested that the choice of recovery score should be rather based on the applicability [ 41 ]. The clinic’s in-house numerical scoring system was used for the present study, because of its simple application, the familiarity of all observers with this particular score, and the fact that the power analysis of the present study was based on a study using the same scoring system [ 18 ]. Even though this scoring system has not yet been validated, it has previously been applied in many other studies [ 18 , 25 , 37 , 44 ].…”

“…The clinic’s in-house numerical scoring system was used for the present study, because of its simple application, the familiarity of all observers with this particular score, and the fact that the power analysis of the present study was based on a study using the same scoring system [ 18 ]. Even though this scoring system has not yet been validated, it has previously been applied in many other studies [ 18 , 25 , 37 , 44 ]. Menzies et al used this same score alongside a VAS and found excellent agreement between both scores [ 44 ].…”

“…On the basis of previous in-house publications, it was assumed that the incidence of bad recoveries (defined as scores of 4 or 5 on our in-house recovery scoring system of 1–5) in our population treated with xylazine was 12% [ 18 ]. A power analysis revealed that 496 comparable recoveries would be necessary to detect if medetomidine led to a reduction in the incidence of bad recoveries to 5% (α = 0.05, power 0.8).…”

“…Conversely, during constant rate infusion (CRI) of medetomidine and xylazine, these effects are mitigated, and, at steady-state plasma levels, cardiopulmonary depression is minimal [ 16 , 17 ]. Both drugs are used for PIVA in horses and were compared in a recent study by Wiederkehr et al, who observed a quicker recovery from anaesthesia following xylazine PIVA as opposed to medetomidine PIVA [ 18 ]. However, the study concluded that the investigation of a greater number of recoveries was necessary to detect any significant difference in recovery quality between the two PIVA regimes.…”