Cardiovascular and respiratory effects of the acaricide amitraz

Amitraz is a formamidine compound used as a topical acaricide mainly in dogs and cattle. In an initial attempt to explain some of its side-effects, the actions of amitraz were studied on the cardiovascular and respiratory systems in thiopentone/methoxyflurane-anaesthetized dogs. In separate experiments, amitraz, at doses of 1, 2 and 5 mg/kg i.v., dissolved in DMSO, increased blood pressure for 1 hour. Heart rate decreased initially then showed a dose-related return towards control values. Tidal volume, respiratory rate and respiratory minute volume all showed initial transient depression. Hyperventilation was a feature after high doses of amitraz. Cumulative doses of amitraz of 0.5, 1, 2, 5 and 10 mg/kg i.v., at intervals of 5 min, increased blood pressure. Heart rate decreased at lower doses but increased slightly after higher doses. Five minutes after injection, cardiac index had returned to control values while total peripheral resistance showed a dose-related increase. The mechanism of action of amitraz in dogs cannot be determined from these results; however, other reports have described an alpha 2-adrenoceptor agonist action of this formamidine compound.