Bronchodilation and bronchoprotection by deep inspiration and their relationship to bronchial hyperresponsiveness

Bronchial hyperresponsiveness (BHR) is a cardinal feature of asthma. Airway inflammation and BHR are probably linked, but the mechanisms underlying this relationship remain elusive. BHR is closely associated with defects in the beneficial responses to lung inflation. These responses, which become apparent by the fact that healthy individuals can develop severe airway obstruction if they are exposed to methacholine in the absence of deep inspirations, include bronchodilation and bronchoprotection. Bronchodilation refers to the effect of lung inflation after the induction of airway smooth muscle tone, while bronchoprotection is used to indicate the effect prior to inhalation of a spasmogen. Mild asthmatics who manifest BHR lack bronchoprotection by lung inflation. In contrast, many of them are able to bronchodilate. In more severe disease, both functions are impaired. The lack of bronchoprotection is also found in individuals with rhinitis and BHR, but no asthma. These and other observations suggest that the mechanisms of bronchodilation and bronchoprotection may be distinct, although overlap is possible. We believe that the loss of bronchoprotection is pertinent to the phenomenon of hyperresponsiveness, but that both the bronchodilatory and bronchoprotective functions of deep inspiration interact to produce the asthmatic phenotype. In this review, we describe the phenomena of lung inflation-induced bronchodilation and bronchoprotection and detail potential mechanical and neurohumoral mechanisms accounting for these physiologic functions. In addition, possible mechanisms leading to the impairment of these functions in subjects with BHR are discussed.