Airway Response during Exercise and Hyperpnoea in Non-Asthmatic and Asthmatic Individuals

Abstract

Airway calibre is an important determinant of air flow and respiratory work both at rest and during exercise. While much is understood about control of airway calibre at rest, less is known about the dynamics and changes in airway resistance during exercise, especially in those with asthma. This article provides an overview of the current understanding provided by the literature that has addressed airway resistance during exercise in normal non-asthmatic individuals and in those with asthma. There are many interesting studies that provide some insight into this issue. In general, non-asthmatic individuals appear to have near maximally or maximally dilated airways at rest and, thus, have little change associated with exercise, at least during short duration exercise of <15 minutes. However, potent bronchodilating influences are in operation during the exercise as shown by exercise data from normal subjects with pharmacologically pre-constricted airways. In contrast, dynamic exercise has consistently demonstrated improved airway calibre in asthmatic individuals. Again, the exercise has typically been <15 minutes. Data from longer duration exercise (20-30 minutes) are lacking in normal subjects but suggest declining pulmonary function over time during exercise in asthmatic individuals after the initial bronchodilation. However, the lack of non-asthmatic controls and small subject numbers in these studies leave much remaining to be studied in this regard. Handgrip exercise in asthmatic individuals also elicits bronchodilation raising interesting questions as to potential mechanisms. Isocapnic voluntary hyperpnoea matched to exercise hyperpnoea levels induces bronchodilation in asthmatic individuals during both short- and longer-term bouts up to 20 minutes. This result in longer-term isocapnic hyperpnoea apparently deviates from the response of asthmatic individuals to the hyperpnoea of longer-term dynamic exercise raising interesting questions. Voluntary hyperpnoea is important to this discussion as this technique is often used to assess the prevalence of exercise-induced asthma. While much is yet to be understood, dynamic, and possibly isometric, exercise is a powerful bronchodilator, at least over a short period of up to 15 minutes. It remains to be determined how airways respond to dynamic exercise of >15 minutes in both normal and asthmatic individuals and the mechanisms operating in the various circumstances. Additionally, attention to resting pulmonary function in asthmatic individuals must be given as those with constricted airways may differ in response to exercise, or to voluntary hyperpnoea, from those with normal airway function at rest.