AVALANCHES AND POWER-LAW BEHAVIOR IN LUNG-INFLATION

WHEN lungs are emptied during exhalation, peripheral airways close up1 . For people with lung disease, they may not reopen for a significant portion of inhalation, impairing gas exchange2,3. A knowledge of the m echanisms that govern reinflation of collapsed regions of lungs is the refore central to the development of ventilation strategies for combat ing respiratory problems. Here we report measurements of the terminal airway resistance, R(t), during the opening of isolated dog lungs. Whe n inflated by a constant flow, R(t) decreases in discrete jumps. We fi nd that the probability distribution of the sizes of the jumps and of the time intervals between them exhibit power-law behaviour over two d ecades. We develop a model of the inflation process in which 'avalanch es' of airway openings are seen-with power-law distributions of both t he size of avalanches and the time intervals between them-which agree quantitatively with those seen experimentally, and are reminiscent of the power-law behaviour observed for self-organized critical systems4. Thus power-law distributions, arising from avalanches associated with threshold phenomena propagating down a branching tree structure, appe ar to govern the recruitment of terminal airspaces.