Microvascular fluid flow in ex vivo and engineered lungs

• Published in: Journal of applied physiology (1985) Vol. 131; no. 5; pp. 1444 - 1459
• Main Authors: Raredon, Micha Sam Brickman, Engler, Alexander J, Yuan, Yifan, Greaney, Allison M, Niklason, Laura E
• Format: Journal Article
• Language: English
• Published: United States American Physiological Society 01.11.2021
• Series: Deconstructing Organs: Single-Cell Analyses, Decellularized Organs, Organoids, and Organ-on-a-Chip Models
• Subjects: Alveoli; Capillaries; Capillary pressure; Edema; Fluid dynamics; Fluid flow; Lung; Lung transplantation; Lungs; Mathematical models; Membrane permeability; Microvasculature; Microvessels; Organ culture; Parenchyma; Perfusion; Recruitment; Robustness (mathematics); Transplantation; Starling coefficient; alveolar barrier; lung physiology; decellularized; microvasculature
• ISSN: 8750-7587; 1522-1601
• DOI: 10.1152/japplphysiol.00286.2020

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. We present a validated model of pulmonary microvascular fluid mechanics and apply this model to study the effects of increased capillary permeability in decellularized and regenerating lungs. We find that decellularization alters microvascular steady-state mechanics and that re-endothelialization partially rescues key biologic parameters. The described model provides powerful insight into intraorgan microvascular dynamics and may be used to guide regenerative engineering experiments. We include all data and derivations necessary to replicate this work.