The pharmacokinetics of dexmedetomidine administered as a constant rate infusion in horses.

The study investigates the pharmacokinetics and sedative effects of dexmedetomidine, a drug increasingly used on horses, when administered as an infusion. The researchers also found individual variation in concentration of the drug over time, and that sedation level corresponded with the drug’s plasma concentration, implying that for clinical use, infusion rates can be adjusted easily to obtain the desired effect.

Research Design and Methodology

The study involved seven horses who received 8 μg dexmedetomidine/kg/h for 150 minutes. The following are key points of the methodology:

• The researchers administered dexmedetomidine to the horses and collected venous blood samples for analysis.
• Using liquid chromatography-mass spectrometry (LC/MS), they analyzed the dexmedetomidine concentrations in the samples.
• They employed noncompartmental pharmacokinetic analysis, a technique that provides an overview of the drug’s pharmacokinetics without assuming the body behaves like a series of compartments.
• Sedation was quantified by measuring the distance from the lower lip of the horse to the ground in centimetres, indicating the level of muscle relaxation and sedation in the horse.

Findings and Interpretation

The researchers found these key results:

• The harmonic mean plasma elimination half-life of dexmedetomidine was approximately 20.9 minutes, indicating the duration of its significant presence and potential therapeutic effects.
• The clearance rate was calculated to be around 0.3 L/min/kg, illustrating the volume of blood from which the drug is removed per unit time.
• The volume of distribution at steady-state was about 13.7 L/kg, providing a measure of how the drug is distributed throughout the body.
• A considerable individual variation in the concentration of dexmedetomidine versus time profile was observed, suggesting a varying response between different horses.
• The level of sedation corresponded with the plasma concentration of dexmedetomidine, suggesting the drug’s effectiveness can be controlled by adjusting the infusion rate.

Implications

The study implies that dexmedetomidine can be administered to horses as a constant rate infusion, and its rate can be adjusted to get the required level of sedation. This form of administration is preferred over an infusion at a predetermined rate, which may not factor in individual variation among horses. This research provides critical insights for improving the clinical use of dexmedetomidine in horses.