Hyperpnea: The Common Stimulus for Bronchospasm in Asthma during Exercise and Voluntary Isocapnic Hyperpnea

Abstract

Different forms of exercise can induce varying degrees of bronchoconstriction in asthmatic patients. To determine whether the degree of bronchoconstriction is dependent on the level of hyperpnea as recorded during different forms of exercise, 9 asthmatics (10-16 yr of age) performed treadmill walking, arm peddling and leg cycling on 3 separate days. On the 4th day, all patients performed voluntary hyperpnea under isocapnic conditions. Each challenge, i.e., exercise and/or hyperpnea, lasted 10 min. The subject breathed room (19-23.degree. C, 7-14% relative humidity) air through the mouth. The following variables were continously monitored: ventilation (.ovrhdot.VE), end-tidal and mixed expiratory PCO2 [CO2 pressure] and heart rate. The pulmonary functions measured before and at 7-15 min after challenge included spirometry, MEFV [maximal expiratory flow-volume] curve and SGaw [airway specific gravity]. All 4 forms of challenges; when matched for minute ventilation (a mean of 42.3 .+-. 0.2 l/min), induced a comparable degree of bronchoconstriction. This was measured by all pulmonary function variables including .ovrhdot.Vmax [maximum ventilation] at 70% TLC [total long capacity] and SGaw. The voluntary isocapnic hyperpnea is an equally strong stimulus to bronchospasm compared to absolute exercise hyperpnea, regardless of the method of exercise. Ventilatory response and O2 consumption measured during leg cycling were significantly increased in asthmatics compared to normal range. Each individual showed a dose-response-like relationship between minute ventilation measured during exercise and the degree of bronchoconstriction following this challenge, as measured by pulmonary functions.