Objective - To evaluate the respiratory effects occurring during administration of sevoflurane or isoflurane to the upper airway in dogs. Study Design - A prospective, randomized study. Animals - Twelve healthy adult beagles (6 males, 6 females). Methods - At least 2 weeks after undergoing permanent tracheostomy, dogs were premedicated with acepromazine-buprenorphine, and anesthesia was induced with thiopental and maintained with a-chloralose. The upper airway was functionally isolated so that the inhalant could be administered to the upper airway while dogs were breathing 100% O2 via the tracheostomy. Respiratory reflexes in response to the administration of sevoflurane or isoflurane at concentrations of 1.2, 1.8, and 2.4 times the minimal alveolar concentration (MAC) (administered in 100% O2 at a flow rate of 5 L/min) were recorded. Reflexes in response to administration of each anesthetic were also recorded following upper-airway administration of lidocaine. Results - Respiratory reflexes elicited by upper-airway administration of each anesthetic were characterized by a dose-dependent increase in expiration time, with a resultant decrease in respiratory minute ventilation and increase in end-tidal PCO2. The magnitude of these responses was greater with isoflurane than with sevoflurane at 1.8 and 2.4 MAC. These reflexes were abolished after lidocaine nebulization into the upper airway. Conclusion - Isoflurane induces greater reflex inhibition of breathing than does sevoflurane when the anesthetic is inhaled into the upper airway at concentrations used for mask induction.
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