Acinar flow irreversibility caused by perturbations in reversible alveolarwall motion

Mixing associated with "stretch-and-fold" convective flow patterns has rece ntly been demonstrated to play a potentially important role in aerosol tran sport and deposition deep in the lung (J. P. Butler and A. Tsuda. J. Appl. Physiol. 83: 800-809, 1997), but the origin of this potent mechanism is not well characterized. In this study we hypothesized that even a small degree of asynchrony in otherwise reversible alveolar wall motion is sufficient t o cause flow irreversibility and stretch-and-fold convective mixing. We tes ted this hypothesis using a large-scale acinar model consisting of a T-shap ed junction of three short, straight, square ducts. The model was filled wi th silicone oil, and alveolar wall motion was simulated by pistons in two o f the ducts. The pistons were driven to generate a low-Reynolds-number cycl ic flow with a small amount of asynchrony in boundary motion adjusted to ma tch the degree of geometric (as distinguished from pressure-volume) hystere sis found in rabbit lungs (H. Miki, J. P. Butler, R. A. Rogers, and J. Lehr . J. Appl. Physiol. 75: 1630-1636, 1993). Tracer dye was introduced into th e system, and its motion was monitored. The results showed that even a slig ht asynchrony in boundary motion leads to flow irreversibility with complic ated swirling tracer patterns. Importantly, the kinematic irreversibility r esulted in stretching of the tracer with narrowing of the separation betwee n adjacent tracer lines, and when the cycle-by-cycle narrowing of lateral d istance reached the slowly growing diffusion distance of the tracer, mixing abruptly took place. This coupling of evolving convective flow patterns wi th diffusion is the essence of the stretch-and-fold mechanism. We conclude that even a small degree of boundary asynchrony can give rise to stretch-an d-fold convective mixing, thereby leading to transport and deposition of fi ne and ultrafine aerosol particles deep in the lung.