Antinociceptive effects of three escalating dexmedetomidine and lignocaine constant rate infusions in conscious horses.

The study explored the pain-relief effects, plasma concentrations, and sedative effects of various doses of dexmedetomidine and lignocaine in horses. The researchers found that both drugs increased pain thresholds, though individual responses to dexmedetomidine varied considerably.

Research Objectives

• The study aimed to investigate the impact of different doses of dexmedetomidine and lignocaine, both clinically used for analgesia in horses. The focus was on their antinociceptive (pain relief) effects, changes in plasma concentrations, and sedative effects.
• To provide a comprehensive picture, researchers tested three escalating doses for each drug administered intravenously using a continuous rate infusion (CRI).

Methodology

• Three dosage rates for dexmedetomidine and lignocaine were administered to 10 Swiss Warmblood horses using CRI, each preceded by a bolus, which is an initial large dose intended to quickly elevate plasma drug concentration.
• Pain reactions or thresholds were evaluated using electromyography following single and repeated stimulation. The withdrawal reflexes of horses to the given pain stimuli were quantified using an ingenious method – electrically induced nociceptive withdrawal reflex.
• In addition to evaluating nociceptive thresholds (pain sensitivity), the researchers also looked at the nociceptive tolerance, or how much pain a horse can tolerate before eliciting a pain response.
• Plasma drug concentrations and levels of sedation were also monitored and recorded. The plasma concentration was analyzed using liquid chromatography and mass spectrometry, which are standard methods for detecting and quantifying substances in a mixture. Sedation levels were measured using a visual analogue scale.

Findings

• All doses of dexmedetomidine increased pain thresholds to both single and repeated stimulation, exceptions being at the lowest dose where an increase in pain threshold to single stimulation did not occur.
• Dexmedetomidine enhanced tolerance of the horses to the induced pain at every dose rate. However, a notable amount of individual variability was recorded in dexmedetomidine plasma concentrations and the levels of sedation.
• On a similar note, lignocaine enhanced pain thresholds and tolerance at the two higher dosages. At a lower dosage, it only increased tolerance to repeated stimulation.
• The study identified that the lowest doses to result in consistent antinociception were dexmedetomidine at 4 µg/kg/h and lignocaine at 40 µg/kg/min.
• Unlink dexmedetomidine, lignocaine did not induce significant sedation in the horses.