Abstract
Background:
Presedation before general anesthesia helps reduce patient anxiety and facilitates anesthetic induction. Acepromazine and dexmedetomidine are widely used in dogs; however, these drugs cause cardiopulmonary alterations that may have a significant physiological impact when combined with the effects of propofol.

Aim:
This study aims to evaluate the effect of premedication with acepromazine or dexmedetomidine associated with methadone on anesthetic induction with propofol administered at a rate of 1 mg/kg/min in healthy dogs.

Methods:
Twenty-four healthy adult dogs were randomly divided into two groups according to the premedication protocol: ACP (0.02 mg/kg acepromazine associated with 0.3 mg/kg methadone) or DEX (2 µg/kg dexmedetomidine associated with 0.3 mg/kg methadone). In both groups, premedication was administered intramuscularly, and the dogs were subsequently induced with propofol at a rate of 1 mg/kg/min using a syringe infusion pump. The sedation level, total propofol dose required for induction, induction time, and cardiopulmonary variables at baseline, pre-induction, and post-induction periods were evaluated.

Results:
The DEX group showed a higher sedation score (p < 0.001), lower propofol requirement (p = 0.005), shorter induction time (p = 0.024), lower heart rate (p = 0.036), lower respiratory rate (p = 0.04), and a higher incidence of post-induction hypercapnia (p < 0.001) compared to the ACP group. The incidence of myoclonus during anesthetic induction was higher in the ACP group (p = 0.005). No animals exhibited apnea, desaturation, or hypotension.

Conclusion:
The use of 2 µg/kg dexmedetomidine associated with 0.3 mg/kg methadone as premedication promotes a higher degree of sedation, reducing the propofol dose and the incidence of myoclonus during anesthetic induction with propofol at a rate of 1 mg/kg/min. However, it has a greater impact on heart rate and respiratory parameters after induction compared to the combination of 0.02 mg/kg acepromazine and 0.3 mg/kg methadone.

Key words: Premedication, Anesthesia, Canine, Continuous infusion