Total intravenous anesthesia with propofol-ketamine-xylazine with or without remifentanil in thoroughbred horses undergoing castration

We evaluated the clinical efficacy of total intravenous anesthesia (TIVA) with propofol-ketamine-xylazine (PKX) with or without remifentanil for castration in horses. Twenty-four Thoroughbred horses were premedicated with intravenous (IV) xylazine (1.0 mg/kg) and midazolam (0.02 mg/kg) and anesthetized with IV ketamine (1.5 mg/kg) and propofol (1.0 mg/kg). Surgical anesthesia was maintained with constant infusion of propofol (3.0 mg/kg/hr)-ketamine (3.0 mg/kg/hr)-xylazine (1.0 mg/kg/hr) (group PKX: n=8), PKX combined with remifentanil (3.0 µg/kg/hr) (group PKXR3: n=8), or PKX combined with remifentanil (6.0 µg/kg/hr) (group PKXR6: n=8). During anesthesia, none of the horses showed any limb movements, but five, two, and two horses in the PKX, PKXR3, and PKXR6 groups, respectively, showed cremaster muscle contractions. One horse in the PKX group required doubling the PKX infusion rate to continue surgery. Adverse effects of remifentanil (trembling of the nose tip or tongue) were observed in one and three horses in the PKXR3 and PKXR6 groups, respectively. Heart rate and arterial blood pressure were well maintained in all groups. Ventilation was assisted in four, five, and six horses in the PKX, PKXR3, and PKXR6 groups, respectively. Recovery scores in the PKX group were fair in one horse, good in three horses, and excellent in four horses, whereas recovery in all horses in the PKXR3 and PKXR6 groups was judged to be excellent. TIVA with PKX combined with remifentanil 3.0 µg/kg/hr could provide more sufficient anesthetic depth than PKX with fewer clinically significant adverse effects than that with remifentanil 6.0 µg/kg/hr.