Exercise Induced Asthma
Based on P239, Essentials of Exercise Physiology 2nd Edition, McArdle, Katch & Katch
Asthma, a chronic, obstructive respiratory disease, affects 12 million Americans, mostly children. A high fitness level does not confer immunity from this ailment. About 11% of US athletes in the 1984 Olympics had asthma. Hyper-irritability of the pulmonary airways usually manifested by coughing, wheezing and shortness of breath characterise an asthmatic condition.
With exercise, catecholamines released from the sympathetic nervous system promote a relaxation effect on smooth muscle that lines the pulmonary airways. Everyone experiences initial bronchodilation with exercise. For the asthmatic, however bronchospasm and excessive mucus secretion follow normal bronchodilation. An acute episode of airway obstruction often appears 10 minutes after exercise; recovery usually occurs spontaneously within 30 to 90 minutes. One technique for diagnosing exercise-induced asthma (EIA) utilises progressive increments of exercise on a treadmill or ergometer. During a 10 to 20 minute recovery after each exercise bout a spirometer evaluates FEV1.0/FVC. A 15% reduction in pre-exercise values confirms the diagnosis of EIA.
Sensitivity to Thermal Gradients
An attractive theory to explain EIA relates to the rate and magnitude of alterations in pulmonary heat exchange as ventilation increases in exercise. As the incoming breath of air moves down the pulmonary pathways, heat and water transfer from the respiratory tract as air warms and humidifies. This form of "air-conditioning" cools and dries the respiratory mucosa; an abrupt airway rewarming occurs during recovery. The thermal gradient from cooling and subsequent rewarming (and loss of water from muscosal tissue) stimulates the release of pro-inflammatory chemical mediators that cause bronchospasm.
Environment Makes a Difference
Exercising in a humid environment, regardless of ambient air temperature, blunts the EIA response. This is perplexing because conventional belief maintains that a dry climate best suits the asthmatic. In fact, inhaling ambient air fully saturated with water vapour in exercising patients totally abolished the bronchospastic response. This also explains why asthmatics tolerate walking or jogging on a warm humid day or swimming in an indoor pool, where as outdoor winter sports usually trigger an asthma attack. An asthmatic should perform 15 to 30 minutes of continuous warm-up because it initiates a "refractory period" that minimises the severity of a bronchoconstrictive response during subsequent, more intense exercise.
In the future researchers may better understand additional factors related to EIA. Physicians would then be able to "prescribe" an optimum environment and exercise intensity so asthmatics could benefit physically and psychologically from regular exercise. Currently, medications offer considerable relief from bronchoconstriction for individuals who want to exercise on a regular basis without affecting their performance. Exercise training cannot "cure" the asthmatic condition, but it can increase airway reserve and reduce the work of breathing during physical activity.