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Vet Rec. 2019 Mar 30;184(13):411. doi: 10.1136/vr.105070.

Clinical pharmacokinetics and pharmacodynamics of intravenous alfaxalone in young Thoroughbred horses premedicated with medetomidine and midazolam.

Author information

1
Miho Training Center Racehorse Clinic, JRA, Inashiki, Japan.
2
Laboratory of Racing Chemistry, Utsunomiya, Japan.

Abstract

To investigate the clinical pharmacokinetics and pharmacodynamics of intravenous alfaxalone in young Thoroughbred horses, seven Thoroughbred horses were randomly anaesthetised twice with either 1 or 2 mg/kg of intravenous alfaxalone after premedication with medetomidine (6 µg/kg intravenous) and midazolam (20 µg/kg intravenous). Blood samples were collected at predetermined time points up to two hours after administration. Plasma alfaxalone concentrations were quantified by a liquid chromatography tandem-mass spectrometry method and analysed by non-compartmental pharmacokinetic analysis. Induction and recovery qualities were good to excellent for both doses. Recovery time for the 2 mg/kg (median 90 minutes) was significantly longer than that for the 1 mg/kg (median 50 minutes). Respiratory rate for the 2 mg/kg was significantly lower than that for the 1 mg/kg, resulting in hypoxaemia. The median (range) elimination half-life, total clearance and volume of distribution were 58.2 (42.3-70.7) minutes, 11.6 (10.3-14.5) ml/minute/kg and 0.8 (0.7-0.9) l/kg for the 1 mg/kg and 59.8 (47.5-68.0) minutes, 14.7 (12.1-16.0) ml/minute/kg and 0.9 (0.9-1.2) l/kg for the 2 mg/kg, respectively. Alfaxalone is rapidly eliminated from the plasma in young Thoroughbred horses. Respiratory depression should be especially noted when alfaxalone is used in clinical practice.

KEYWORDS:

Thoroughbred; alfaxalone; horse; pharmacodynamics; pharmacokinetics

PMID:
30926700
DOI:
10.1136/vr.105070

Conflict of interest statement

Competing interests: None declared.

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