Alfaxalone for total intravenous anaesthesia in horses

To determine the suitability of alfaxalone total intravenous (IV) anaesthesia in horses and concurrently evaluate infusion rates, cardiovascular effects, pharmacokinetics and the quality of the anaesthetic recovery period.

Prospective, experimental study.

Eight Standardbred horses.

Horses were premedicated with IV acepromazine (0.03 mg kg^-1) and xylazine (1 mg kg^-1) and anaesthesia was induced with guaifenesin (35 mg kg^-1) and alfaxalone (1 mg kg^-1). Anaesthesia was maintained for 180 minutes using an IV infusion of alfaxalone at a rate determined by a horse's response to a supramaximal electrical noxious stimulus. Venous blood samples were regularly collected to determine alfaxalone plasma concentrations and for pharmacokinetic analysis. Cardiopulmonary variables were monitored and the quality of the anaesthetic recovery period scored.

The median (range) alfaxalone infusion rate was 3.1 (2.4-4.3) mg kg^-1 hour^-1. The mean ± standard deviation plasma elimination half-life, plasma clearance and volume of distribution for alfaxalone were 41 minutes, 25 ± 6.3 mL minute^-1 kg^-1 and 1.6 ± 0.5 L kg^-1, respectively. During anaesthesia, mean arterial blood pressure was maintained above 70 mmHg in all horses. Cardiac index reached a minimum value (68% of baseline values) immediately after induction of anaesthesia and was maintained between 74% and 90% of baseline values for the remainder of the anaesthetic protocol. Following the cessation of the alfaxalone infusion, six of eight horses exhibited muscle tremors and paddling. All horses stood without incident on the first or second attempt with a median recovery score of 4.5 (good to excellent).

Anaesthesia in horses can be maintained with an infusion of alfaxalone at approximately 3 mg kg^-1 hour^-1. The alfaxalone infusion rates used resulted in minimal haemodynamic changes and good recovery quality. Mean alfaxalone plasma concentration was stable over the infusion period and clearance rates were similar to previously published single-dose alfaxalone studies in horses.