Sound transmission between 50 and 600 Hz in excised pig lungs filled with air and helium

This study measured transit time (TT) and attenuation of sound transmitted through six pairs of excised pig lungs. Single-frequency sounds (50-600 Hz) were applied to the tracheal lumen, and the transmitted signals were monit ored on the tracheal and lung surface using microphones. The effect of vary ing intrapulmonary pressure (Pip) between 5 and 25 cmH(2)O on TT and sound attenuation was studied using both air and helium (He) to inflate the lungs . From 50 to similar to 200 Hz, TT decreased from 4.5 ms at 50 Hz to 1 ms a t 200 Hz (at 25 cmH(2)O). Between similar to 200 and 600 Hz, TT was relativ ely constant (1.1 ms at upper and 1.5 ms at lower sites). Gas density had v ery little effect on TT (air-to-He ratio of similar to1.2 at upper sites an d similar to1 at lower sites at 25 cmH(2)O). Pip had marked effects (depend ing on gas and site) on TT between 50 and 200 Hz but no effect at higher fr equencies. Attenuation was frequency dependent between 50 and 600 Hz, varyi ng between -10 and -35 dB with air and -2 and -28 dB with He. Pip also had strong influence on attenuation, with a maximum sensitivity of 1.14 (air) a nd 0.64 dB/cmH(2)O (He) at 200 Hz. At 25 cmH(2)O and 200 Hz, attenuation wi th air was about three times higher than with He. This suggests that sound transmission through lungs may not be dominated by parenchyma but by the ai rways. The Linear relationship between increasing Pip and increasing attenu ation, which was found to be between 50 and similar to 100 Hz, was inverted above similar to 100 Hz. We suggest that this change is due to the transit ion of the parenchymal model from open to closed cell. These results indica te that acoustic propagation characteristics are a function of the density of the transmission media and, hence, may be used to locate collapsed lung tissue noninvasively.