Heterogeneity of mechanisms in exercise induced asthma.

The development of airflow limitation that occurs in 70-80% of people with asthma several minutes after vigorous exercise is known as exercise induced asthma. In most asthmatic patients the airflow obstruction is reversed spontaneously within 60 minutes. There are, however, asthmatic individuals who have more prolonged airways obstruction and who require bronchodilator treatment to recover pre-exercise lung function. After exercise 40-50% of asthmatic individuals with exercise induced asthma will be less responsive to an identical exercise task performed within one hour.' 2 This is known as the refractory period.3 In some patients, particularly children, exercise induced asthma will be followed three to nine hours later by a further decrease in lung function, termed the late reaction.4' In most asthmatic subjects with exercise induced asthma the administration of sodium cromoglycate or a I2 sympathomimetic aerosol immediately before exercise will prevent the development of the initial8 and late phase asthmatic reactions.6 Although exercise induced asthma has been recognised for centuries there is still considerable debate about its pathogenesis. Some of the controversy may relate to the attempt to seek a single mechanism to explain a condition in which several mechanisms may exist. The recognition that exercise induced asthma is only sometimes followed by a refractory period, that the inhibitory effect of sodium cromoglycate and the anticholinergic agent ipratropium bromide is not universal, and that bronchoactive mediators cannot be detected in the circulation of all patients with the condition suggests that there may be several different mechanisms causing exercise induced asthma.