The effect of low density gas breathing on vesicular lung sounds

Turbulent airflow (largely gas density dependent) in larger airways is believed by many lung sound researchers to be the mechanism responsible for the generation of vesicular lung sounds. To test the validity of this concept, we measured the amplitude of lung and tracheal sounds of 6 subjects alternately breathing air and a low density gas mixture (80% helium, 20% oxygen: He-O2). Lung sounds were recorded from 3 chest wall sites: Anterior right upper lobe (RUL), posterior and posterolateral right lower lobe (RLL), and a site over the proximal trachea below the larynx. The subjects rebreathed into an electronic spirometer filled with the test gas, and achieved a peak inspiratory and expiratory airflow of 2-2.5 L/sec. Lung sound amplitude was determined by an automated, flow-corrected measurement procedure. The mean decrease in sound amplitude when breathing He-O2 compared to air was: trachea, inspiration 44%; trachea, expiration 45%; RUL, inspiration 13%; RUL, expiration 25%; RLL, inspiration 15% (expiration at the RLL was too quiet to record). Cross-correlation and frequency analyses of the sounds recorded at the two RLL sites on both test gases revealed no consistent change in frequency or time relationships, indicating absence of effect of gas density on sound transmission between the sound generating airways and chest wall. These data suggest that the mechanism of production of the inspiratory vesicular lung sound is not simply turbulent airflow but some other relatively gas density independent mechanism. The tracheal and expiratory lung sounds do appear to be produced by a more density dependent turbulent mechanism.