Cardiorespiratory effects of intramuscular alfaxalone combined with low-dose medetomidine and butorphanol in dogs anesthetized with sevoflurane.

Background: The intramuscular (IM) administration of 7.5-10 mg/kg of alfaxalone produces anesthetic effects that enable endotracheal intubation with mild cardiorespiratory depression in dogs. However, the effects of IM co-administration of medetomidine, butorphanol, and alfaxalone on cardiorespiratory function under inhalation anesthesia have not been studied.

Aim: To assess the cardiorespiratory function following the IM co-administration of 5 μg/kg of medetomidine, 0.3 mg/kg of butorphanol, and 2.5 mg/kg of alfaxalone (MBA) in dogs anesthetized with sevoflurane.

Methods: Seven intact healthy Beagles (three males and four females, aged 3-6 years old and weighing 10.0-18.1 kg) anesthetized with a predetermined minimum alveolar concentration (MAC) of sevoflurane were included in this study. The baseline cardiorespiratory variable values were recorded using the thermodilution method with a pulmonary artery catheter after stabilization for 15 minutes at 1.3 times their individual sevoflurane MAC. The cardiorespiratory variables were measured again following the IM administration of MBA. Data are expressed as median [interquartile range] and compared with the corresponding baseline values using the Friedman test and Sheff's method. A p < 0.05 was considered statistically significant.

Results: The intramuscular administration of MBA transiently decreased the cardiac index [baseline: 3.46 (3.18-3.69), 5 minutes: 1.67 (1.57-1.75) l/minute/m² : p < 0.001], respiratory frequency, and arterial pH. In contrast, it increased the systemic vascular resistance index [baseline: 5,367 (3,589-6,617), 5 minutes:10,197 (9,955-15,005) dynes second/cm⁵/m² : p = 0.0092], mean pulmonary arterial pressure, and arterial partial pressure of carbon dioxide.

Conclusion: The intramuscular administration of MBA in dogs anesthetized with sevoflurane transiently decreased cardiac output due to vasoconstriction. Although spontaneous breathing was maintained, MBA administration resulted in respiratory acidosis due to hypoventilation. Thus, it is important to administer MBA with caution to dogs with insufficient cardiovascular function. In addition, ventilatory support is recommended.