Effect of methacholine induced bronchoconstriction on the spectral characteristics of breath sounds in asthma.

Background: Analysis of breath sounds by digital techniques offers an attractive non-invasive method of monitoring changes in airway calibre. Asthmatic breath sounds have been analysed and related to changes in forced expiratory volume in one second (FEV1).
Methods: Bronchoconstriction was induced with methacholine in six asthmatic subjects on two occasions and changes in FEV1 and breath sound spectra were measured.
Results: Audible wheeze appeared after a mean (SE) fall in FEV1 of 35% (6.3%) but the level was not reproducible within patients. The mean and median frequency of the spectra of breath sounds correlated with the percentage of predicted FEV1 (r = -0.5 and -0.6 respectively; p < 0.001). Inclusion of the quartile frequencies in a stepwise multiple regression reduced the residual variance by a further 9%.
Conclusion: Detecting changes in airway calibre by this method of sound analysis so far produces qualitative data only and will not yield quantitative data in individual patients.