Effects of the α ₂-Adrenoceptor Agonist Dexmedetomidine on Bronchoconstriction in Dogs

Background: Tracheal intubation can elicit reflex bronchoconstriction in patients with asthma or chronic obstructive pulmonary disease, complicating mechanical ventilation and weaning from mechanical support. In vitro studies of human and animal bronchial tissue indicate that α ₂-adrenoceptor stimulation can lead to smooth muscle relaxation and prevention of bronchoconstriction. Dexmedetomidine is a selective α ₂-adrenoceptor agonist approved for sedation in the intensive care unit. Whether dexmedetomidine can affect reflex bronchoconstriction is unknown. Methods: After the approval of the institutional animal care and use committee, five mongrel dogs were anesthetized with thiopental, endotracheally intubated, and ventilated, and their airways were challenged with histamine. High-resolution computed tomography was used to measure airway luminal areas at baseline and after nebulized histamine. After recovery to baseline, on separate days, dexmedetomidine (0.5 μg/kg) was administered either intravenously or as an aerosol, and the histamine challenge was repeated. Results: At baseline, histamine constricted the airways to 66 ± 27% (mean ± SD) (P < 0.0001) and 59 ± 30% (P < 0.0001) of maximum on the days dexmedetomidine was administered by intravenous and inhalational means, respectively. After recovery, intravenous administration of dexmedetomidine blocked the histamine-induced bronchoconstriction (87 ± 30.4% of maximum, compared with histamine alone (P < 0.0001), whereas dexmedetomidine administered by inhalation showed no protective effect (45 ± 30% of maximum; P < 0.0001 compared with histamine alone). Conclusion: α ₂-Adrenoceptor stimulation with intravenous dexmedetomidine completely blocked histamine-induced bronchoconstriction in dogs. Therefore, dexmedetomidine might be beneficial to decrease airway reactivity in patients with chronic obstructive pulmonary disease or asthma, particularly during weaning from mechanical ventilation, when neurally mediated airway reflexes may be elicited.