Sound speed in pulmonary parenchyma

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Abstract
The time it takes audible sound waves to travel across a lobe of excised horse lung was measured. Sound speed, which is the slope in the relationship between transit time and distance across the lobe, was estimated by linear regression analysis. Sound-speed estimates for air-filled lungs varied between 25 and 70 m/s, depending on lung volume. These speeds are less than 5% of sound speed in tissue and less than 20% of sound speed in air. Filling the lung with helium or sulfur hexafluoride, whose free-field sound speeds are 970 and 140 m/s, respectively, changed sound speed +/- 10% relative to air filling. Reducing the ambient pressure to 0.1 atm reduced sound speed to 30% of its 1-atm value. Increasing pressure to 7 atm increased sound speed by a factor of 2.6. These results suggest that 1) translobar sound travels through the bulk of the parenchyma and not along airways or blood vessels, and 2) the parenchyma acts as an elastic continuum to audible sound. The speed of sound is given by c = (B/rho)1/2, where B is composite volumetric stiffness of the medium and rho is average density. In the physiologic state B is affected by ambient pressure and percent gas phase. The average density includes both the tissue and gas phases of the parenchyma, so it is dependent on lung volume. These results may be helpful in the quantification of clinical observations of lung sounds.