Bronchial reactivity to isocapnic hyperventilation: results in an unselected population of outpatients with known or suspected asthma.

Isocapnic hyperventilation with dry air is nearly as effective as with dry cold air, and appears to be a valuable screening test for bronchial hyperresponsiveness. However some incidental factors such as prechallenge bronchoconstriction, level of hyperventilation, age and smoking habits have barely been examined or were investigated in small samples of either normals or well-characterised asthmatics. In an inhomogeneous population of 186 outpatients with known asthma, 286 with suspected asthma and 32 normals, a single isocapnic hyperventilation challenge of 6-min duration was performed. There was a weak, but significant correlation between the degree of prechallenge airway function and the bronchial response, assessed by the change in forced expiratory volume in 1 s (r = 0.27, p = 0.000) in known asthmatics, but not in patients with suspected asthma and in normals. No significant relation was found between the level of hyperventilation and the bronchial response when comparing the bronchial response to the single-dose hyperventilation test between the subjects (NS). Increasing age appears to be associated with an attenuation of the response in known asthmatics (r = 0.21, p = 0.004), but not in patients with suspected asthma. Smoking habits did not affect the bronchial response in this study. In conclusion, the main finding is that there is a weak correlation between baseline airway obstruction and the subsequent response to isocapnic hyperventilation, a slow decline in response with age and no increase in responsiveness in smokers. Hence, isocapnic hyperventilation is a relatively robust test for assessing bronchial reactivity in an inhomogeneous population of outpatients like ours.