Compartment effects in hemodialysis

Compartment effects in hemodialysis are important because they reduce the efficiency of removal of the compartmentalized solute during dialysis. The dialyzer can only remove those waste products that are presented to it, and then only in proportion to the concentration of the solute in the blood. Classically a two-compartment system has been modeled, with the compartments arranged in series. Because modeling suggests that the sequestered compartment is larger than the accessible compartment, an assumption has been made that the sequestered compartment is the intracellular space. For urea and other solutes that move easily across many cell membranes, compartmentalization may be flow related, that is, related to sequestration in organs (muscle, skin, bone). Although mathematically urea rebound and mass balance can be described with either model, the flow-related model best explains data showing that urea rebound after dialysis is increased during ultrafiltration, diminished during high cardiac output states, and also reduced during exercise. Whether compartmentalization is increased in vasoconstricted intensive care unit patients receiving acute dialysis remains an open question.