A preliminary study comparing the sedative, cardiorespiratory, and histaminic-releasing effects of intramuscular and intravenous administration of pethidine (meperidine) with midazolam in healthy cats.

Pethidine is a synthetic opioid that is widely used in cats. However, the sedative, cardiorespiratory, and histaminic effects following administration of pethidine with midazolam in cats remain unclear. The objectives of this study were to evaluate and compare changes before and after intravenous (IV) and intramuscular (IM) administration of pethidine with midazolam in healthy cats. In this prospective randomized blind study, 12 cats were assigned equally to either the IV or IM treatment group. The IV group received pethidine 3 mg/kg and midazolam 0.1 mg/kg. The IM group received pethidine 6 mg/kg and midazolam 0.2 mg/kg. The sedative effects, heart rate, respiratory rate, non-invasive arterial blood pressures, and behavioral signs were recorded before and at 2, 5, 15, 30, 45, and 60 min after the injection. Blood samples were taken for an ELISA histamine assay at baseline and at 5 and 15 min after treatment. Cats that received IV treatment were rapidly induced a moderate degree of sedation but those received IM treatment were only mildly sedated. There was no significant difference in the cardiorespiratory values within and between the treatments over time. Plasma histamine concentrations increased by 3 and 5 times at 5 and 15 min after IV treatment, respectively, compared to baseline values. IM injections induced minimal changes in the plasma histamine concentration. In summary, intravenous pethidine with midazolam induced potentially superior sedative effects without serious side effects in clinically healthy cats. However, further studies with larger sample sizes are required to validate this finding.