Antinociceptive effects of levomethadone in standing horses sedated with romifidine.

The research tested the pain-relieving effects of levomethadone when given to horses that were sedated with romifidine. The study found that levomethadone enhanced the pain threshold in these horses, although the effect varied among individuals.

Methodology

• The experiment was a prospective, randomized, masked, crossover study involving eight adult Warmblood horses, with an average age of 6.6 years and an average weight of 548 kg.
• The horses were given an intravenous injection of 0.1mg/kg levomethadone or an equivalent volume of saline (as a control). This injection was administered 60 minutes after a romifidine constant rate infusion (CRI) had started while the horses were standing.
• Blood samples were taken up to150 minutes after the administration of levomethadone. The blood was tested to quantify the plasma concentrations of romifidine and levomethadone. These tests were carried out using capillary electrophoresis.

Data Collection and Analysis

• The nociceptive withdrawal reflex threshold (NWRT), a measure of the body’s pain response, was continuously monitored using an automated device.
• The state of the horses’ sedation and cardiopulmonary variables were regularly assessed.
• The gathered data helped to produce a pharmacokinetic-pharmacodynamic (PK-PD) model. The data was presented as either the mean ± standard deviation or the median with an interquartile range of 25%-75%, based on its suitability.
• Differences between the groups were classified as statistically significant if the p-value was below 0.05.

Results

• The horses showed higher NWRTs after receiving levomethadone compared to saline. The average increase was by 123 ± 9% relative to the baseline, as against 101 ± 9% in the saline group.
• The PK-PD model showed that levomethadone contributed to the increase in the NWRT, but the effect size varied among individuals.
• No adverse reactions were noted after levomethadone was administered.
• A minor influence of levomethadone on sedation scores was seen for 60 minutes after its administration.

Conclusion

Levomethadone has the potential to enhance the NWRT in horses under romifidine CRI. It can also be combined with α-adrencoceptor agonists to increase pain relief (antinociception) in horses. There is, however, significant variation in the response of individual horses.