Although crackles are heard over the chests of patients with a number of common cardiopulmonary disorders, the mechanism of production of these sounds is poorly understood. Fredberg and Holford postulated that crackles were due to a stress relaxation quadrupole associated with sudden airway opening and closing. Their model predicted that the polarity of expiratory crackles would be the reverse of inspiratory crackles. The goal of this research was to examine systematically the relationship between crackle polarity and respiratory cycle.
Patients with pneumonia (PN), congestive heart failure (CHF) and interstitial fibrosis (IPF), with over 2 inspiratory crackles per breath (n=158), or over 2 expiratory crackles per breath (n=89) were examined using a 16-channel lung sound analyzer (Stethographics, Inc, Model STG1602). This device automatically analyzes crackles. Crackle polarity was defined as positive if the largest deflection of the crackle waveform was upward. Crackle polarity was defined as negative if the largest deflection was downward.
The majority of patients had predominantly positive polarity of inspiratory crackles (88% of patients) and predominantly negative polarity of expiratory crackles (83% of patients). Seventy one percent of all inspiratory crackles had positive polarity (the total number of inspiratory crackles examined was 8,249). Seventy five percent of all expiratory crackles had negative polarity (the total number of expiratory crackles examined was 3,485). Inspiratory crackle polarity was significantly different between CHF and IPF (p<0.0008). It was slightly different between Pn and IPF (p<0.02), but not statistically different between PN and CHF. There were no significant differences in expiratory crackle polarity among the groups.
The reported findings are consistent with the hypothesis that sudden airway opening is responsible for inspiratory crackles and airway closing is responsible for expiratory crackles.
While there are no immediate clinical benefits to knowing the polarity of a patient’s crackles, a clearer understanding of the mechanism of production of lung sounds offers the promise of improving noninvasive diagnosis of lung disorders.
R. Murphy, Stethographics,