Changes in airway dead space in response to methacholine provocation in normal subjects

Measurements of bronchial hyper-responsiveness rely on sensitive techniques for measurement of bronchoconstriction, ideally based on tidal breathing. A potentially useful technique is measurement of airway dead space (V_Daw), which reflects the volume of the conducting airways. The aim of this study was to evaluate measurements of V_Daw with the single breath test for CO₂ (SBT-CO₂), compared to spirometric measurements, as a method of measuring bronchial response to methacholine challenge. Nineteen healthy adults were studied. Dosimetric methacholine challenge tests were performed on two study days. Forced expirations or the SBT-CO₂ were used to assess the response. There were dose-dependent reductions in the spirometric measurements, with a 10 ± 10% reduction from the baseline value of forced expiratory volume at the highest dose of methacholine. There was a dose-dependent reduction from the baseline value of V_Daw by 19 ± 9% at the highest dose. There was also a dose-dependent increase in the slope of the alveolar plateau of the SBT-CO₂. This study provides support for measurement of V_Daw as a means of evaluating bronchial responsiveness after methacholine challenge. In a group of healthy adults, this method shows a greater response but with similar dispersion as measurement of forced expiratory volume after methacholine challenge.