The pharmacokinetics and pharmacodynamics of intravenous hydromorphone in horses.

This research article studies how intravenous hydromorphone works in horses, its effects on certain physical parameters, and the resulted changes over time. The most effective dose was identified with minimal negative imprints.

Methodology

• This study was carried out with a masked, randomized layout following a so-called ‘Latin square design’. The experiment was carried out on a group of eight healthy adult horses.
• Four treatment groups were created with varying intravenous hydromorphone doses: 0.02 mg/kg (LD), 0.04 mg/kg (MD), 0.08 mg/kg (HD), and one with saline (P).
• An 8-day washout period was provided between each treatment.
• For a duration of a day following every treatment, blood samples were collected at regular intervals. The amount of hydromorphone in the plasma was quantified and other pharmacokinetic parameters were estimated using noncompartmental analysis.
• In the same time frame, certain pharmacodynamic information was collected such as the thermal nociceptive threshold (essentially pain threshold), heart rate, respiratory rate, and rectal temperature. The collected data was analyzed with mixed-effects linear models.

Findings

• The half-life, systemic clearance, and apparent volume of distribution at steady state (Vd) of hydromorphone was calculated for each of the treatment groups.
• All the hydromorphone treatments had a significant increase in the thermal threshold compared to the baseline for up to 12 hours, indicating that the drug effectively increases the pain threshold in horses.
• HR, respiratory rate, and rectal temperature were found to have increased above the baseline in the LD, MD, and HD treatments. The duration for which these rates stayed high varied between the groups.
• The 0.04 mg/kg dosage offered clinically relevant thermal antinociception with minimal side effects.

Conclusion

• The study concluded that hydromorphone has a short lifespan and is rapidly cleared from a horse’s body, yet its presence reflects in a large Vd.
• It was also determined that the drug increases thermal threshold, heart rate, respiratory rate, and the rectal temperature of horses in a dose-dependent manner.
• The most effective dosage was found to be 0.04 mg/kg as it provided substantial increase in thermal threshold (pain tolerance) with minimal side effects.