Plasma pharmacokinetics and pharmacodynamics of alfaxalone in neonatal foals after an intravenous bolus of alfaxalone following premedication with butorphanol tartrate

Objective To determine the pharmacokinetics and pharmacodynamics of the neurosteroid anaesthetic, alfaxalone, in neonatal foals after a single intravenous (IV) injection of alfaxalone following premedication with butorphanol tartrate. Study design Prospective experimental study. Animals Five clinically healthy Australian Stock Horse foals of mean±SD age of 12±3days and weighing 67.3±12.4kg. Methods Foals were premedicated with butorphanol (0.05mgkg IV) and anaesthesia was induced 10minutes later by IV injection with alfaxalone 3mgkg. Cardiorespiratory variables (pulse rate, respiratory rate, direct arterial blood pressure, arterial blood gases) and clinical signs of anaesthetic depth were evaluated throughout anaesthesia. Venous blood samples were collected at strategic time points and alfaxalone plasma concentrations were assayed using liquid chromatography-mass spectrometry (LC/MS) and analysed by noncompartmental pharmacokinetic analysis. Results The harmonic, mean±SD plasma elimination half life (t1/2) for alfaxalone was 22.8±5.2minutes. The observed mean plasma clearance (Cl) and volume of distribution (Vd) were 19.9±5.9mLminutekg and 0.6± 0.2Lkg, respectively. Overall, the quality of the anaesthetic inductions and recoveries was good and most monitored physiological variables were clinically acceptable in all foals, although some foals became hypoxaemic for a short period following recumbency. The mean durations of anaesthesia from induction to first movement and from induction to standing were 18.7±7 and 37.2±4.7minutes, respectively. Conclusions The anaesthetic protocol used provided a predictable and consistent plane of anaesthesia in the five foals studied, with minimal cardiovascular depression. In foals, as in the adult horse, alfaxalone has a short elimination half life. Clinical relevance Alfaxalone appears to be an adequate anaesthetic induction agent in foals and the pharmacokinetics suggest that, with continuous infusion, it might be suitable to provide more prolonged anaesthesia. Oxygen supplementation is recommended. © 2012 The Authors. Veterinary Anaesthesia and Analgesia