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Interpretation of Breath Sounds

Although thoracic sound transmission is complex, certain simplifications have helped to correlate physical findings with pathologic processes. Normal breath sounds arise from turbulent air flow in the larger airways (1, 2), and factors that diminish air flow (e.g., splinting, bronchial plugging) or that increase the distance from the source (e.g., hyperinflation) will decrease intensity at the stethoscope. If the source intensity remains constant, bronchovesicular breath sounds may be heard over consolidation because of increased conduction. Egophony results from the change in frequency dependence of sound conduction when fluid replaces air and is heard over consolidation or pleural effusion (3). If egophony appears whether the fluid is inside or outside the lung, why are breath sounds diminished over a pleural effusion but increased over consolidation? Many textbooks simply ignore the apparent contradiction. Two textbooks (1, 2) attribute this phenomenon to differences in sound filtering, which, although true, does not correlate with findings of pathologic processes.

Considering the mechanical coupling of lung and chest wall rationalizes these facts. Under physiologic conditions, pleural fluid exerts a cohesive force normal to the surface and makes the mechanical properties approximate those of a solid in that direction. Because sound is a longitudinal disturbance, the sliding of the pleural surfaces tangentially will have little effect on transmission normally. An effusion in the pleural space uncouples the lung and chest wall, reducing the transmission of sound. It is now easy to understand why an inch of consolidation next to the chest wall will increase transmission of breath sounds, but an inch of pleural effusion will decrease transmission, if we assume that the volume of the thorax and the source sound intensity are the same in both cases. In the case of consolidation, a better conductor of sound replaces air, leaving the coupling of lung and chest wall intact. Sound transmission, therefore, increases. In the case of effusion, fluid also replaces an equal volume of aerated lung, but this fluid uncouples the lung and chest wall. This uncoupling overcomes any contrary effect from replacing air with fluid, and sound transmission decreases. Uncoupling the lung and chest wall has little effect on the frequency dependence of sound transmission (egophony). Although rather simplified, this approach allows the rationalization of physical findings within a framework that can be integrated with pathophysiology.

Frank A. Greco

Veterans Affairs Medical Center Bedford, Massachusetts

FOOTNOTES

Conflict of Interest Statement: F.A.G. has no declared conflict of interest.

REFERENCES

1. Lehrer S. Understanding lung sounds. Philadelphia: WB Saunders; 1984.
2. Wilkins RL, Hodgkin JE, Lopez B. Lung sounds. St. Louis, MO: CV Mosby; 1988.
3. McKusick VA, Jenkins JT, Webb GN. The acoustic basis of the chest examination. Am Rev Tuberc 1955;72:12–34.[Medline]

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