Microvascular fluid flow in ex vivo and engineered lungs.

In recent years, it has become common to experiment with ex vivo perfused lungs for organ transplantation and to attempt regenerative pulmonary engineering using decellularized lung matrices. However, our understanding of the physiology of ex vivo organ perfusion is imperfect; it is not currently well understood how decreasing microvascular barrier affects the perfusion of pulmonary parenchyma. In addition, protocols for lung perfusion and organ culture fluid-handling are far from standardized, with widespread variation on both basic methods and on ideally controlled parameters. To address both of these deficits, a robust, noninvasive, and mechanistic model is needed which is able to predict microvascular resistance and permeability in perfused lungs while providing insight into capillary recruitment. Although validated mathematical models exist for fluid flow in native pulmonary tissue, previous models generally assume minimal intravascular leak from artery to vein and do not assess capillary bed recruitment. Such models are difficult to apply to both ex vivo lung perfusions, in which edema can develop over time and microvessels can become blocked, and to decellularized ex vivo organomimetic cultures, in which microvascular recruitment is variable and arterially perfused fluid enters into the alveolar space. Here, we develop a mathematical model of pulmonary microvascular fluid flow which is applicable in both instances, and we apply our model to data from native, decellularized, and regenerating lungs under ex vivo perfusion. The results provide substantial insight into microvascular pressure-flow mechanics, while producing previously unknown output values for tissue-specific capillary-alveolar hydraulic conductivity, microvascular recruitment, and total organ barrier resistance.