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 (VDaw), which reflects the volume of the conducting airways. The aim of this study was to evaluate measurements of VDaw 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 VDaw 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 VDaw 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.