Cardiovascular Effects of Medetomidine

Medetomidine is an alpha-2 adrenoreceptor agonist known for its sedative and analgesic properties. Dose-dependent cardiovascular depression that includes bradycardia, increased systemic vascular resistance, and decreased cardiac output has been described for single-bolus dosing of medetomidine in the dog. Doses as low as 1 mcg/kg have produced a decrease in cardiac index (a measure of cardiac function) of > 50%, and increasing the dose caused more profound and prolonged effects. This study investigated the cardiovascular effects of medetomidine administered as a constant rate infusion at 1, 2, and 3 mcg/kg per hour for 60 minutes in 6 conscious dogs. Decreased heart rate and cardiac output, as well as increased temperature, pulse, and respiration, have been described with single-bolus dosing, but these changes were not demonstrated in this study. The authors reported that there was a lack of a biphasic (early peripheral vasoconstriction and late central vasodilation) response in treatments 1 and 2, probably because of the slow increase in plasma medetomidine concentrations; however, an initial increase in pressure followed by a decrease below baseline values did occur in treatment 3. An increase in vascular resistance in the higher-dosing treatments (2 and 3) was also apparent, similar to what is seen with single-bolus dosing. Bradycardia occurred with all treatments, and stroke volume also decreased, because of the decrease in sympathetic tone (not a direct negative inotropic effect) or the increase in afterload created by an increase in systemic vascular resistance. The cardiac index was decreased significantly with all treatments.

Commentary: Medetomidine causes significant cardiovascular effects when used as a continuous rate infusion, just as with single-bolus dosing. These findings should be considered when medetomidine is used as a sedative or analgesic in this manner. However, lower doses seem to have lesser effects. Further studies to compare the analgesic effects with the hemodynamic effects are warranted. Also needed are comparisons between medetomidine and dexmedetomidine (the active isomer of medetomidine), which is now being used more commonly in private practice. Our hospital finds constant rate infusions of dexmedetomidine to be a safe and effective way to provide analgesia and sedation to anxious dogs or those in moderate to severe pain, but it is always used with careful consideration of the patient’s comorbid conditions.

The hemodynamic effects of medetomidine continuous rate infusions in the dog. Carter JE, Campbell NB, Posner LP, Swanson C. VET ANAESTH ANALG 37:197-206, 2010.