Scaling behavior in crackle sound during lung inflation

During slow inflation of lung lobes, we measure a sequence of short explosi ve transient sound waves called "crackles," each consisting of an initial s pike followed by ringing. The crackle time series is irregular and intermit tent, with the number of spikes of size s following a power law, n(s)propor tional to s(-alpha), with alpha = 2.77 +/- 0.05. We develop a model of crac kle wave generation and propagation in a tree structure that combines the a valanchelike opening of airway segments with the wave propagation of crackl es in a tree structure. The agreement between experiments and simulations s uggests that (i) the irregularities are a consequence of structural heterog eneity in the lung, (ii) the intermittent behavior is due to the avalanchel ike opening, and (iii) the scaling is a result of successive attenuations a cting on the sound spikes as they propagate through a cascade of bifurcatio ns along the airway tree. [S1063-651X(99)13810-8].