The mechanism of exercise-induced asthma is ...

Exercise-induced asthma (EIA) refers to the transient narrowing of the airw ays that follows vigorous exercise. The mechanism whereby EIA occurs is tho ught to relate to the consequences of heating and humidifying large volumes of air during exercise. In 1978 airway cooling was identified as an import ant stimulus for EIA; however, severe EIA also occurred when hot dry air wa s inspired, and there was no abnormal cooling of the airways. In 1986 the t hermal hypothesis proposed that cooling of the airways needed to be followe d by rapid rewarming and that these two events caused a vasoconstriction an d a reactive hyperemia of the bronchial microcirculation, together with ede ma of the airway wall, causing the airways to narrow after exercise. The os motic, or airway-drying, hypothesis developed from 1982-1992 because neithe r airway cooling nor rewarming appeared to be necessary for EIA to occur. A s water is evaporated from the airway surface liquid, it becomes hyperosmol ar and provides an osmotic stimulus for water to move from any cell nearby, resulting in cell volume loss. It is proposed that the regulatory volume i ncrease, after cell shrinkage, is the key event resulting in release of inf lammatory mediators that cause airway smooth muscle to contract and the air ways of asthmatic subjects to narrow. This event may or may not be associat ed with airway edema. The osmotic and thermal theories come together by con sidering that inspiration of cold air not only cools the airways but also i ncreases the numbers of airway generations becoming dehydrated in the humid ifying process.