Airways response of asthmatics after a 30 min exposure, at resting ventilation, to 0.25 ppm NO2 or 0.5 ppm SO2

We compared the effect of inhaled NO2 and SO2 on airway tone and airway responsiveness in 14 nonsmoking mild asthmatics (mean +/- SD age 34 +/- 14 yrs; mean +/- SD baseline forced expiratory volume in one second (FEV1), 86 +/- 17% pred). On 3 separate days, 30 min tidal breathing (average minute ventilation 10.6 l.min-1) of either filtered air, 0.25 ppm NO2, or 0.5 ppm SO2 was followed by an isocapnic hyperventilation test with 0.75 ppm SO2. To determine the provocative ventilation necessary to increase specific airway resistance (sRaw) by 100% (PV100sRaw) ventilation was increased in steps of 15 l.min-1, each step lasting 3 min. Resting ventilation of filtered air, NO2, or SO2 was followed by a slight but significant overall decrease of sRaw from 8.8 to 7.7 cmH20.s-1, with no differences between the study days. Mean +/- SEM PV100sRaw(SO2) was 46.5 +/- 5.1, 37.7 +/- 3.5 and 45.4 +/- 4.2 l.min-1 after tidal breathing of filtered air, NO2, and SO2, respectively. PV100sRaw(SO2) was significantly lower after NO2 as compared to filtered air or SO2 (p less than 0.01). There was a significant correlation (rs = 0.86) between the individual shift of PV100sRaw(SO2) after NO2 and the shift after SO2 as compared to filtered air. From these individual comparisons we suggest that in asthmatics short-term exposure to NO2 at rest enhances airway responsiveness to hyperventilation of SO2 without altering airway tone, whereas short-term exposure with low concentrations of SO2 does not.