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.