The effects of acute bronchoconstriction on respiratory activity in patients with chronic obstructive pulmonary disease.

Attacks of acute airway obstruction often complicate the course of chronic obstructive pulmonary disease (COPD). In asthmatic subjects, bronchospasm triggers an increase in respiratory drive, which results in hyperventilation and hypocapnia. In the present study, we assessed the effects of acute bronchoconstriction induced by aerosolized methacholine on breathing and lung mechanics in 12 patients with stable COPD. Even low doses of methacholine markedly increased airway resistance and caused hyperinflation and decreased inspiratory muscle performance in the patients. Increasing airway obstruction produced a progressive rise in PCO2 despite an increase in minute ventilation. Breathing frequency and average inspiratory flow were greater, but tidal volume decreased because of shortening of the inspiratory duration. The magnitude of CO2 retention during acute bronchoconstriction was inversely related to the changes in tidal volume and inspiratory time (p less than 0.01 for each). In subjects with COPD, the occlusion pressure response to progressive hypercapnia failed to increase during bronchoconstriction. These results show that patients with COPD retain CO2 during acutely increasing airway obstruction induced by bronchoconstriction partly because of a rapid shallow breathing pattern that reduces alveolar ventilation.