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.


METHODS
Prospective experimental study.


METHODS
Five clinically healthy Australian Stock Horse foals of mean ± SD age of 12 ± 3 days and weighing 67.3 ± 12.4 kg.


METHODS
Foals were premedicated with butorphanol (0.05 mg kg(-1) IV) and anaesthesia was induced 10 minutes later by IV injection with alfaxalone 3 mg kg(-1) . 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 (t½) for alfaxalone was 22.8 ± 5.2 minutes. The observed mean plasma clearance (Cl(p) ) and volume of distribution (Vd) were 19.9 ± 5.9 mL minute kg(-1) and 0.6 ± 0.2 L kg(-1) , 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 ± 4.7 minutes, 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.


CONCLUSIONS
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.
School of Veterinary Science, University of Queensland, Gatton, Qld, Australia Jurox Pty Ltd, Rutherford, NSW, Australia. w.goodwin@uq.edu.au
Wendy
W
Goodwin
Helen
H
Keates
Kirby
K
Pasloske
Martin
M
Pearson
Ben
B
Sauer
Millagahamada G
MG
Ranasinghe
D000701:Analgesics, Opioid / Q000008:administration & dosage / Q000494:pharmacology; D000777:Anesthetics / Q000008:administration & dosage / Q000097:blood / Q000493:pharmacokinetics / Q000494:pharmacology; D000818:Animals; D000831:Animals, Newborn; D019540:Area Under Curve; D002077:Butorphanol / Q000008:administration & dosage* / Q000494:pharmacology; D005260:Female; D006207:Half-Life; D006736:Horses; D008297:Male; D011277:Pregnanediones / Q000008:administration & dosage / Q000097:blood / Q000493:pharmacokinetics* / Q000494:pharmacology*
D000701:Analgesics, Opioid; D000777:Anesthetics; D011277:Pregnanediones; C006477:alphaxalone; D002077:Butorphanol