A new diagnostic test to assess airway reactivity in asthmatics

A new, simple test to assess reactivity using a voluntary isocapnic hyperpnea (VIH) is described. The test consists of baseline determinations of pulmonary function, followed by 5 min of voluntary isocapnic hyperventilation of a CO₂-room air mixture (20-22°C and relative humidity of 7-15%) at ventilatory levels between 25-45 L/min^-1. Then pulmonary function tests are repeated. Utilizing this test it was found that, in asthmatics (n = 11), a dose-response-like relationship exists between voluntary isocapnic hyperpnea (VIH) and the bronchoconstrictive response, as measured by pulmonary function tests. Furthermore, it was observed that this response shows relatively good reproducibility. For comparison, airway response to the standardized methacholine inhalation challenge was also examined in normals and asthmatics. The response was evaluated by both PD₃₅ (for SGaw) and the slope of the dose-response relationship. PD₃₅ was defined as the level of ventilation or concentration of methacholine, at which a decrease of 35% in specific airway conductance (SGaw) was observed. Our study revealed that the VIH challenge is more specific in the detection of airway hyperreactivity than the methacholine inhalation challenge. Five normal subjects demonstrated no measurable airway response to VIH but did respond to the methacholine inhalation challenge.