Two types of responses to exercise and isocapnic hyperventilation in asthmatics

Seven young adult asthmatics (17-35 years old) with an established diagnosis of mild to severe bronchial asthma were tested twice per day on six different days. On these days, all of the subjects undertook the treadmill exercise test of varying intensity for a duration of 6 minutes. Four hours later a voluntary isocapnic hyperventilation challenge of 6 minutes duration was done with essentially the same level of minute ventilation as achieved during and immediately after the exercise. During exercise and voluntary isocapnic hyperventilation tested patients inhaled dry compressed air of 23 degrees C. Heart rate, minute ventilation and end tidal pCO2 were monitored before, during and after each test. Pulmonary functions were measured before and after each procedure including: spirometry, measurement of airway resistance (Raw) and thoracic gas volume (Tgv). We found that five out of seven patients showed a dose-response-like relationship between a change in pulmonary function (FEF25-75% and FEV1) and total ventilation as measured during treadmill exercise or isocapnic hyperventilation challenge. In two patients we failed to establish this relationship. In these two patients different levels of ventilation (as measured during exercise or isocapnic hyperventilation) induced a similar degree of bronchoconstriction. These two patients had the lowest baseline pulmonary function of the whole group and by definition they had the most reactive airways.