Evaluating the time-varying elastance concept for lymphangions

Abstract

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 a n axial pressure gradient, much like the heart pumps blood. Also, like the heart, lymphangions a r e sensitive to both preload and afterload. T o 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. W e 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 Iymphangion can form a fundamental building block of a large-scale lymphatic system model.

Modelling the lymphatic system: challenges and opportunities

“…The elastance is sensitive to pressure changes, and the nonlinear pressure–volume behaviour of lymphangions limits the applicability of this concept to the study of the lymphatic system [67]. Moreover, there are differences in the behaviour of lymphangions and ventricles [58] and the time-varying elastance model has been criticized as insufficient to describe the ventricle dynamics as well, for which it was initially developed [73,74].…”

“…While simple approaches such as the work by Quick et al [ 57 ] and Venugopal et al [ 71 ] discussed in this section are useful, and can provide useful insight into the function of the lymphatic system when used with caution, their limitations should not be overlooked. The elastance is sensitive to pressure changes, and the nonlinear pressure–volume behaviour of lymphangions limits the applicability of this concept to the study of the lymphatic system [ 67 ]. Moreover, there are differences in the behaviour of lymphangions and ventricles [ 58 ] and the time-varying elastance model has been criticized as insufficient to describe the ventricle dynamics as well, for which it was initially developed [ 73 , 74 ].…”

“…However, the authors used the Thevenin theorem in their analysis, which imposes the requirement that the pressure-flow rate relationship is linear. Depending on the species or lymphatic compartment under consideration, this relationship is, in general, highly nonlinear [32,[65][66][67]. Thus, the applicability of such models is questionable.…”