Airway cooling as the stimulus to exercise-induced asthma--a re-evaluation

Exercise-induced asthma (EIA) was recorded in 20 of 22 asthmatic children who cycled for 8 min while inspiring hot (32-40 degrees C) dry (3-10 mg H2O L-1) air. The mean +/- 1SD reduction in forced expiratory volume in one second was 39.8% +/- 22.3 of the pre-exercise value. To determine the relationship between respiratory heat loss (RHL) and retrotracheal temperature (RTT) under these inspired air conditions we studied 11 non-asthmatic adults who performed steady-state and incremental bicycle exercise. Exercise tests were also carried out while inspiring cold dry air. At the equivalent RHL, RTT was reduced by only 0.1 +/- 0.35 degrees C breathing hot air compared with 1.0 +/- 0.81 degrees C when cold air was inhaled. These data suggest that abnormal cooling of the airways is not occurring during the inhalation of hot dry air. Thus an additional stimulus to airway cooling must have been acting to induce asthma in the children. We propose that water loss and not heat loss is the stimulus to EIA under these inspired air conditions.