[Bronchial response to carbachol and isocapnic hyperventilation in asthma].

Bronchial challenge tests with carbachol and isocapnic voluntary hyperventilation (IVH) were performed in 15 asthmatics on two separate occasions. For the IVH test, the subject inhaled dry air at 20-21 degrees C; respiratory frequency and tidal volume were controlled in order to obtain various 3 min levels of ventilation; isocapnia was maintained by adding CO2 to the inspired air. Bronchial response was assessed by measuring airway conductance (Gaw) with a plethysmograph. Individual dose-response curves were constructed, allowing the calculation of two indices: the slope of the dose-response curve and the value of the minute-ventilation (V25, expressed as percent of predicted maximal minute-ventilation), or the dose of carbachol (D25) causing a 25% decrease in Gaw. There was no significant correlation between the slopes of carbachol and IVH dose-response curves and between the values of V25 and D25. These data demonstrate that the IVH and carbachol tests cannot be used indifferently to assess bronchial responsiveness.