Is intercompartmental urea clearance during hemodialysis a perfusion term? A comparison of two pool urea kinetic models.

Analyses of intradialytic and postdialytic urea profiles call for models that consider delayed urea transfer from different parts of the body to the blood. There are two different approaches to the problem. In the classical cell membrane model it is assumed that the two compartments refer to the serial (s) arrangement of extracellular and intracellular volumes, whereas in the regional blood flow model the two compartments are identified as parallel (p) organ systems with high or low perfusion. In the cell membrane model, delayed urea removal from peripheral body compartments is governed by intercompartmental clearance (Kc) which is a function of cell membrane permeability, whereas in the regional blood flow model delayed urea removal is related to low perfusion (QL) of the large muscle/skin/bone compartment. Both models were compared in a set of 16 high-efficiency hemodialysis treatments. Modeled volumes (Vm,s = 31.2 +/- 9.5 L; Vm,p = 30.0 +/- 8.3 L) and modeled dose of hemodialysis (Kt/Vm,s = Kt/Vm,p = 1.12 +/- 0.33) were the same for both models. However, volumes modeled by either technique were significantly lower than anthropometric volumes (V alpha = 35.0 +/- 6.4 L). These data suggest that at this point the two models are experimentally indistinguishable. Moreover, the main system parameters of both models, Kc (0.54 +/- 0.16 L/min) and QL (0.63 +/- 0.15 L/min) showed a strong linear dependence (QL = 0.921 Kc + 0.139, r2 = 0.884), whereas no relation could be found between Kc and Vm. Therefore, delayed transport that has up to now been characterized by membrane permeability may also be explained by peripheral perfusion.