Effect of medetomidine on breathing and inspiratory neuromuscular drive in conscious dogs

OBJECTIVE: To evaluate the effects of the alpha2-adrenoceptor agonist medetomidine on respiratory rate (RR), tidal volume (V(T)), minute volume (V(M)), and central respiratory neuromuscular drive as determined by inspiratory occlusion pressure (IOP) during increasing fractional inspired concentrations of carbon dioxide (FiCO2) in conscious dogs. ANIMALS: 6 healthy dogs (3 males and 3 females). PROCEDURE: Dogs were administered 0, 5, or 10 microg of medetomidine/kg i.v. We measured RR, V(T), V(M), and IOP for the first 0.1 second of airway occlusion (IOP0.1) during FiCO2 values of 0%, 2.5%, 5.0%, and 75% at 15 minutes before and 5, 30, and 60 minutes after administration of medetomidine. RESULTS: Increases in FiCO2 significantly increased RR, V(T), and V(M). The i.v. administration of 5 and 10 microg of medetomidine/kg significantly decreased RR and V(M) at 5, 30, and 60 minutes for FiCO2 values of 2.5% and 5.0% and at 30 and 60 minutes for an FiCO2 value of 75%. The IOP0.1 was decreased after 30 minutes only for an FiCO2 value of 7.5% in dogs administered 5 and 10 microg of medetomidine/kg. The IOP0.1 was decreased at 60 minutes after administration of 10 microg of medetomidine/kg for an FiCO2 value of 7.5%. CONCLUSIONS AND CLINICAL RELEVANCE: The i.v. administration of medetomidine decreases RR, V(M), and central respiratory drive in conscious dogs. Medetomidine should be used cautiously and with careful monitoring in dogs with CNS depression or respiratory compromise.