Evaluating the time-varying elastance concept for lymphangions

The lymphatic system consists of a network of vessels which return interstitial fluid and protein to the blood circulation. Lymphatic vessels can be divided into smaller units called lymphangions, which contain valves and cyclically contract. Under normal conditions, lymphangions can pump lymph up an axial pressure gradient, much like the heart pumps blood. Also, like the heart, lymphangions are sensitive to both preload and afterload. To describe contraction of the heart independent of preload and afterload, investigators developed the concept of time-varying elastance, which relates chamber pressure and volume in the time domain. To evaluate the applicability of this concept to lymphangions, we analyzed preliminary pressure-volume data from bovine mesenteric lymphangions in vitro. We found that there were significant limitations to the applicability of the time-varying elastance concept-there is a high degree of variability in contraction strength and frequency, the end-diastolic pressure-volume relationship is highly nonlinear, and the values of maximum and minimum elastance are sensitive to pressure. Nonetheless, normalized elastance curves show a remarkable degree of consistency. Just as the lymphangion is the fundamental building block of the lymphatic system, this simple description of a lymphangion can form a fundamental building block of a large-scale lymphatic system model.