RESUMEN
OBJECTIVE: To compare anaesthesia induced with either alfaxalone or ketamine in horses following premedication with xylazine and guaifenesin. STUDY DESIGN: Randomized blinded cross-over experimental study. ANIMALS: Six adult horses, five Standardbreds and one Thoroughbred; two mares and four geldings. METHODS: Each horse received, on separate occasions, induction of anaesthesia with either ketamine 2.2 mg kg(-1) or alfaxalone 1 mg kg(-1) . Premedication was with xylazine 0.5 mg kg(-1) and guaifenesin 35 mg kg(-1) . Incidence of tremors/shaking after induction, recovery and ataxia on recovery were scored. Time to recovery was recorded. Partial pressure of arterial blood oxygen (PaO(2) ) and carbon dioxide (PaO(2) ), arterial blood pressures, heart rate (HR) and respiratory rates were recorded before premedication and at intervals during anaesthesia. Data were analyzed using Wilcoxon matched pairs signed rank test and are expressed as median (range). RESULTS: There was no difference in the quality of recovery or in ataxia scores. Horses receiving alfaxalone exhibited a higher incidence of tremors/shaking on induction compared with those receiving ketamine (five and one of six horses respectively). Horses recovered to standing similarly [28 (24-47) minutes for alfaxalone; 22 (18-35) for ketamine] but took longer to recover adequately to return to the paddock after alfaxalone [44 (38-67) minutes] compared with ketamine [35 (30-47)]. There was no statistical difference between treatments in effect on HR, PaO(2) or PaCO(2) although for both regimens, PaO(2) decreased with respect to before premedication values. There was no difference between treatments in effect on blood pressure. CONCLUSIONS AND CLINICAL RELEVANCE: Both alfaxalone and ketamine were effective at inducing anaesthesia, although at induction there were more muscle tremors after alfaxalone. As there were no differences between treatments in relation to cardiopulmonary responses or quality of recovery, and only minor differences in recovery times, both agents appear suitable for this purpose following the premedication regimen used in this study.