Antinociceptive effects of levomethadone in standing horses sedated with romifidine

To evaluate the antinociceptive effect of a bolus of intravenous levomethadone administered to horses during romifidine constant rate infusion (CRI).

Prospective, randomized, masked, crossover experimental study.

A group of eight adult Warmblood horses (seven geldings, one mare) aged 6.6 ± 4.4 years, weighing 548 ± 52 kg [mean ± standard deviation (SD)].

Levomethadone 0.1 mg kg^-1 or an equivalent volume of saline (control) was administered intravenously to standing horses 60 minutes after starting a romifidine CRI. Blood samples to quantify romifidine and levomethadone plasma concentrations by capillary electrophoresis were collected up to 150 minutes after levomethadone administration. The nociceptive withdrawal reflex threshold (NWRT) was determined continuously using an automated threshold tracking device. Sedation and cardiopulmonary variables were assessed at regular intervals. A pharmacokinetic-pharmacodynamic (PK-PD) model was elaborated. Data are presented as mean ± SD or median (interquartile range, 25%-75%) where appropriate. Differences between groups were considered statistically significant for p < 0.05.

Horses exhibited higher NWRTs after levomethadone administration than after saline (123 ± 9% versus 101 ± 9% relative to baseline, p < 0.05). The PK-PD model identified a contribution of levomethadone to the NWRT increase. Effect size was variable among individuals. No adverse reactions to levomethadone administration were observed. A slight effect of levomethadone on sedation scores was evident for the 60 minutes following its administration.

A single injection of levomethadone has the potential to increase the NWRT during romifidine CRI in horses and can be administered in combination with α₂-adrencoceptor agonists to enhance antinociception in horses. However, individual variation is marked.