Crackles are intermittent explosive sounds that are associated with a number of pulmonary disorders including Interstitial Pulmonary Fibrosis (IPF), Congestive Heart Failure (CHF), and Pneumonia (Pn). The mechanism underlying expiratory crackles generation is not very well understood. Some authors think that airway closing is responsible for expiratory crackles. Others claim that intermittent airway reopening during expiration is responsible for the crackling sounds. The goal of this research was to gain insights into crackle generation mechanism by systematic examination of the relationship between inspiratory and expiratory crackle characteristics and by testing the crackle patterns, as recorded by multiple microphones, against the predictions of the stress-relaxation quadrupole crackle generation model as developed by Fredberg and Holford.
Fifty five patients with over 2 inspiratory crackles per breath and over 2 expiratory crackles per breath were selected for this study from a pool of nearly 1000 patients who were examined using a multichannel lung sound analyzer (Stethographics, STG1602). Crackle characteristics such as frequency, amplitude, transmission coefficient, and polarity were calculated for each crackle.
The frequency, amplitude, and transmission coefficients of expiratory crackles were very similar to those of inspiratory crackles. The majority of patients had predominantly positive polarity of inspiratory crackles (98% of patients) and predominantly negative polarity of expiratory crackles (81% of patients). Crackle polarity was also found to be dependent on the observation angle, consistent with predictions by the stress-relaxation quadrupole crackle generation model.
The reported findings are consistent with the hypothesis that expiratory crackles are caused by events that are identical in mechanism and opposite in direction to that of inspiratory crackles. The expiratory crackle data can be explained by the closing of airways during expiration in accordance with the stress-relaxation quadrupole crackle generation model.
While there are no immediate clinical benefits to knowing the mechanism of crackles, a clearer understanding of the mechanism of production of lung sounds offers the promise of improving noninvasive diagnosis of lung disorders.
Raymond Murphy, Grant monies (from industry related sources) The research was supported in part by a grant from Stethographics, Inc.; Shareholder Dr. Murphy is founder and CMO of Stethographics, Inc.