Relationship between forced expiratory flow and tracheal sounds. Possible effect of vortices on flow.

We measured both tracheal sounds and forced expiratory flows simultaneously in 20 normal subjects and 4 patients with stenosis of the main bronchus. Spectral analysis of the tracheal sounds revealed development of discrete line spectra, the frequencies of which ranged between 488 and 942 Hz. There was a positive correlation between these frequencies and the peak flows (r = 0.601). We speculate that the origin of these line spectra are vortices rather than the vibrations of the wall. It is postulated that the pressure drops caused by vortices make the airways more susceptible to collapse and may lead to flow limitation.