Computational Pulmonary Edema

We present a microvascular model of fluid transport in the alveolar septa related to pulmonary edema. It consists of a two-dimensional capillary sheet coursing by several alveoli. The alveolar epithelial membrane runs parallel to the capillary endothelial membrane with an interstitial layer in between, making one long septal tract. A coupled system of equations is derived using lubrication theory for the capillary blood, Darcy flow for the porous media of the interstitium, a passive alveolus, and the Starling equation at both membranes. Case examples include normal physiology, cardiogenic pulmonary edema, noncardiogenic edema ARDS and hypoalbuminemia, and the effects of PEEP. COVID-19 has dramatically increased ARDS in the world population, raising the urgency for such a model to create an analytical framework. Overall, the interstitial pressures are found to be significantly more positive than values used in the traditional physiological literature. That creates a steep gradient near the upstream and downstream ends outlets which drives significant flows towards the distant lymphatics. This may provide a possible explanation to the puzzle, noted since the late 19^th century, of how pulmonary lymphatics can function so far from the alveoli: the interstitium can be self-clearing.