Efficacy of intramuscular meperidine hydrochloride versus placebo in experimental foot lameness in horses.

REASONS FOR PERFORMING STUDY: There are no peer reviewed, blinded controlled studies regarding the skeletal analgesic efficacy of intramuscularly administered meperidine in horses. OBJECTIVES: Using an adjustable heart bar shoe model of equine foot pain, the objective of this study was to test the hypothesis that meperidine (pethidine) administered intramuscularly would prove more efficacious in alleviating lameness than a saline placebo. STUDY DESIGN: Crossover pharmacodynamic experiment. METHODS: Eight healthy adult Thoroughbred horses randomly underwent weekly i.m. treatments 1 h after lameness induction: saline placebo (1 ml/45 kg bwt) or meperidine hydrochloride (1 mg/kg bwt i.m.). Heart rate (HR) and lameness score (LS) responses were assessed by a blinded observer every 20 min for 5 h after lameness induction and then hourly through 12 h after treatment. Jugular venous blood samples were obtained at -1, 0, 0:05, 1, 2, 4, 6, 8, 10 and 12 h and were subsequently analysed for drug concentrations (lower limit of detection, 1 ng/ml). Repeated measures ANOVA and post hoc Tukey's test were used to identify analgesic effects at a significance level of P<0.05. RESULTS: Mean (± s.e.) HR were lower in meperidine trials at 2.3, 3.3 and 3.7 h post administration (P<0.05). Mean LS were lower in meperidine trials at 2.0, 2.3 and 3.3 h post administration (P<0.05). Mean plasma (meperidine) peaked at 227 ± 52 ng/ml at 1 h post administration and decreased to 2.7 ± 0.3 ng/ml at 12 h post administration. In 3 of 8 subjects, plasma (meperidine) was below the lower limit of detection at 12 h after administration. CONCLUSIONS: Intramuscular meperidine was more effective than the saline placebo but only for 2.0-3.7 h post administration compared with the 8-12 h durations of efficacy reported previously using this same model when horses were treated with nonsteroidal anti-inflammatory drugs (NSAIDs). Meperidine may be a suitable nonNSAID alternative analgesic for acute foot pain with efficacy lasting from 2-3 h after a single i.m. dose.

Phenylbutazone and flunixin meglumine used singly or in combination in experimental lameness in horses.

REASON FOR PERFORMING STUDY: Using an adjustable heart bar shoe model of foot pain, the objective of this study was to test the hypothesis that the combined use of phenylbutazone (PBZ) and flunixin meglumine (FM) would prove more efficacious in alleviating lameness than either drug alone. MATERIALS AND METHODS: One hour after induction of lameness at weekly intervals, 8 healthy adult Thoroughbred horses randomly underwent one of 4 i.v. treatments: saline (SAL) placebo (1 ml/45 kg bwt), PBZ (4.4 mg/kg bwt), FM (1.1 mg/kg bwt) or PBZ+FM (at the same dosages as given individually). Heart rate (HR) and lameness score (LS) responses were assessed in a blinded manner every 20 min for 5 h after lameness induction and then hourly for 12 h after treatment. Jugular venous blood samples were obtained at -1, 0, 0.05, 1, 2, 4, 6, 8, 10 and 12 h and subsequently analysed for drug concentrations. Repeated measures ANOVA and post hoc Tukey's test were used to identify analgesic effects at a significance level of P<0.05. RESULTS: Heart rate was lower in all nonsteroidal anti-inflammatory drug (NSAID)-treated trials from 2 h to 10 h post treatment (P<0.05). Analgesic effects of FM and PBZ+FM, as evidenced by decreases in HR, lasted for 12 h post treatment (P<0.05). Lameness score decreased earlier in PBZ and PBZ+FM trials than in FM trials (P<0.05) and the analgesic effect on LS lasted for 12 h post treatment for all NSAID trials (P<0.05). Peak PBZ plasma concentration was 73.7 ± 6.0 and 77.9 ± 5.5 µg/ml. Peak FM concentration was 12.0 ± 0.8 and 13.7 ± 1.0 µg/ml. CONCLUSIONS: It was concluded that the combination of PBZ+FM was not more effective than either PBZ or FM alone. These data do not support the hypothesis that the combination is more efficacious at these dosages than either drug alone in this model of acute foot pain.