The effects of medetomidine, an alpha 2-adrenergic agonist, on ventilatory drive in the dog.

alpha 2-Adrenergic agonists have been used as potent adjuncts to anesthesia and have anesthetic properties themselves. For this reason, we studied the effects of medetomidine, and isoflurane (1 MAC) on ventilatory drive in dogs. Six chronically tracheotomized mongrel dogs were studied during spontaneous ventilation. Arterial blood samples were analyzed for pH, PaCO2, and PaO2. Airway O2, CO2, N2, and isoflurane were continuously monitored using a mass spectrometer; respiratory rate was determined. The hypercapnic ventilatory response was assessed using the Read rebreathing technique. Control measurements were made under isoflurane anesthesia. Fifteen minutes after the medetomidine (20 ug/kg) was given and the isoflurane discontinued, all measurements were repeated. Isoflurane levels were 1.38 volume % during the isoflurane test period and had declined to 0.3 volume % by the time the medetomidine measurements were obtained. The slope of the CO2 response curve was significantly steeper after medetomidine (0.582 vs 0.269 1.min-1.mmHg), suggesting less respiratory depression when compared to the measurements under isoflurane. PaCO2 and endtidal CO2 were significantly lower in the medetomidine group. No other significant differences were found. Under these conditions, medetomidine (20 ug/kg) resulted in normal blood gas values with less depression of the hypercapnic response curve than under isoflurane anesthesia.