Sedative and antinociceptive effects of different detomidine constant rate infusions, with or without methadone in standing horses

Abstract

Summary Background Standing surgery avoids the risks of general anaesthesia in horses. Objectives To assess sedation, antinociception and gastrointestinal motility in standing horses after a detomidine loading dose and 2‐h constant rate intravenous (i.v.) infusion, with or without methadone. Study design Blinded, randomised, crossover with seven healthy adult cross‐bred horses, three geldings and four females (404 ± 22 kg). Methods Five i.v. treatments were administered to all horses with 1‐week washout period: saline (SAL), detomidine low (2.5 μg/kg bwt + 6.25 μg/kg bwt/h) (DL) and high doses (5 μg/kg bwt + 12.5 μg/kg bwt/h) (DH) alone or combined with methadone (0.2 mg/kg bwt + 0.05 mg/kg bwt/h), (DLM) and (DHM), respectively. Height of head above the ground (HHAG), electrical (ET), thermal (TT) and mechanical (MT) nociceptive thresholds and gastrointestinal motility were evaluated at predetermined times between 5 and 240 min. A mixed effect model and Kruskal–Wallis test were used to analyse normally and non‐normally distributed data, respectively. Results Sedation (<50% basal HHAG) was achieved for the duration of the infusion, and for an additional 15 min in DH and DHM groups. Nociceptive thresholds were higher than baseline, to the greatest degree and the longest duration, with DHM (ET and TT for 135 min and MT for 150 min). After DH, TT was significantly higher than baseline from 30 to 120 min and MT from 15 to 135 min. After DLM, ET was increased at 90 min, TT at 30 min and MT for 120 min. Gastrointestinal motility was reduced for up to 135 min after DL, 150 min after DLM and 210 min after DH and DHM. Main limitations Nociceptive thresholds are not equivalent to surgical stimuli. Conclusion Methadone with the highest detomidine dose (DHM) may provide sufficient sedation and analgesia for standing surgical procedures and warrants further investigation.

A possible solution to model nonlinearity in elimination and distributional clearances with α2‐adrenergic receptor agonists: Example of the intravenous detomidine and methadone combination in sedated horses

“…Methadone and detomidine have been administered intravenously (i.v.) as a bolus for short‐term procedures (Gozalo‐Marcilla et al, ) or as constant rate infusions (CRIs) for prolonged surgeries (Gozalo‐Marcilla, Luna, Gasthuys, et al, ; Gozalo‐Marcilla, Oliveira, et al, ).…”

“…In STUDY 1, the model including nonlinearity on the elimination clearance (Cl) (Figure , STEP 2) still over‐predicted detomidine concentrations when administering low doses and under‐predicted detomidine concentrations when administering high doses (Gozalo‐Marcilla, Luna, Moreira da Silva, et al, ). In a subsequent PK/pharmacodynamic (PD) study with the same group of horses (STUDY 2) (Gozalo‐Marcilla, Oliveira, et al, ), the PD of methadone/detomidine combinations as CRIs were reported, but not the PK data. Nonlinearity was also suspected as the ratio was 30% higher with the high detomidine dose compared to the low dose.…”

“… The two following studies were included in the pooled population pharmacokinetic model: bolus study (STUDY 1, treatments A, B, C, D and E in Gozalo‐Marcilla et al, ) and bolus + 2‐hr continuous infusion study (STUDY 2, regimens F, G, H and I, in Gozalo‐Marcilla, Oliveira, et al, ). …”

“…Two significant improvements over the classical mammillary two-compartment model were identified. First, the inclusion of an effect of detomidine plasma concentration on the elimination clearances (Gozalo-Marcilla, Luna, Gasthuys, et al, 2019;Gozalo-Marcilla, Oliveira, et al, 2019).…”

Highlights of recent clinically relevant papers

“…This blinded, randomised placebo‐controlled study, by Miguel Gozalo‐Marcilla and colleagues in Brazil and the UK, investigated the sedative and antinociceptive effects of different detomidine constant rate infusions (CRIs), with or without methadone in standing horses .…”