Nineteen stable asthmatic subjects and 10 nonasthmatic control subjects were subjected to a dry-air tachypnea challenge. Nine of the 19 asthmatic subjects, but none of the control subjects, responded with a > 10% drop in FEV1 values from baseline following this challenge (Fig 2
). The mean maximum drop in FEV1 was 12.1% in the asthmatic responder group vs 4.5% in the asthmatic nonresponder group (p < 0.0001). There was no significant change in FEV1 from baseline in the control group, with a mean maximum change of 1.8% (Fig 2). During the tachypnea challenges, there was no difference in tidal volume (p = 0.27), respiratory rate (p = 0.25), and end-tidal CO2 levels (p = 0.18) recorded between the groups (Table 1
), and no difference in minute volume recorded between responders, nonresponders, and control subjects (p = 0.88; Table 1). Comparison between the responder and nonresponder asthmatic subjects, revealed an immediate difference in FEV1 after the dry-air tachypnea challenge (p = 0.0074), which was maximal 5 min after challenge (p = 0.0004), and remained significant up to 20 min (p = 0.0069), but was no longer significant at 30 min after challenge (p = 0.1163; Fig 3
, top, A). The responders had significantly greater bronchial hyperreactivity to inhaled histamine (mean, 0.7 mg/mL; CI, 0.1 to 1.3 mg/mL) vs nonresponders (mean, 3.7 mg/mL; CI, 1.6 to 5.9; p = 0.009), and Global Initiative for Asthma symptom scores: responders, 1.9 mg/mL (CI, 1.4 to 2.3 mg/mL) vs nonresponders, 1.2 mg/mL (CI, 0.9 to 1.5 mg/mL; p = 0.03). The dry-air tachypnea challenge caused a similar reduction in the relative humidity of the exhaled air from baseline values in all groups (Fig 3, center, B), and also a similar reduction in the expired air temperature from baseline (mean, 32.9°C; CI, 32.8 to 33°C) to postchallenge values (mean, 32.4°C; CI, 32.1 to 32.8°C), with no difference between the groups (p = 0.96; data not shown). To investigate whether the drying of the airways during the tachypnea challenge was responsible for the drop in FEV1 in the responder group, eight of the nine responders were subjected to a humidified-air tachypnea challenge. The ninth subject was excluded due to β-agonist use on the morning of the challenge. This experiment revealed that humidifying the air used during the tachypnea maneuver abolished the drop in FEV1 at all times over the 30-min postchallenge period (Fig 3, bottom, C). When the air used during the tachypnea challenge was humidified, no drop in humidity from baseline values was observed in the exhaled air immediately after the tachypnea maneuver (data not shown). Together, these results show that while the dry-air tachypnea maneuver consistently results in reduced humidity of exhaled air, this is only associated with reduced FEV1 in those asthmatic subjects with more severe symptoms and bronchial hyperreactivity to histamine.