Effect of helium on maximal expiratory flow in patients with asthma before and during induced bronchoconstriction.

The effect of breathing helium on maximal expiratory flow at 50 per cent of vital capacity (V50) was studied in 27 patients with asthma during remission and during induced bronchoconstriction. Nine patients gave a history of asthma induced by exercise; two had asthma due to timothy pollen allergy, and the remaining 16 had asthma due to exposure to western red cedar. Bronchoconstriction was induced by exercise in 10 patients, timothy pollen in 4 patients, methacholine in 4 patients, and red cedar in 16 patients. During remission, the increase in V50 with helium (deltaV50He) was greater than 20 per cent in 22 patients who were classified as responders; deltaV50He was less than 20 per cent in the remaining 5 patients, who were classified as nonresponders. There was a significant correlation between the severity of airway obstruction as measured by V50 and the response to helium, both during remission and during induced bronchoconstriction; however, there was no correlation between helium response and specific airway conductance. In general, patients who were responders during remission remained responders during induced bronchoconstriction, and nonresponders remained nonresponders, regardless of the method of challenge or the type of reaction (immediate verus late). There were a few exceptions in which a responder became a nonresponder during severe bronchoconstriction. The results of this study suggest that in most patients with asthma, the site of airway obstruction is likely to be in the large airways and, in most cases, remains constant in an individual asthmatic; however, an asthmatic who is a helium responder may become a nonresponder during severe bronchoconstriction.