Simulating the effect of exercise on urea clearance in hemodialysis.

A two-compartment model of urea kinetics during hemodialysis is used to predict the effect of exercise on hemodialysis dose. It is assumed that the two compartments represent tissues that are perfused by low and high blood flows (initially 1.1 L/min and 3.8 L/min). The effect of changing the distribution of flows between the compartments, emulating the effect of exercise, is simulated using the model equations for a range of dialyzer clearances. Compartmental volumes are assumed constant (33.4 L and 8.6 L for low- and high-flow compartments, respectively). The analysis identifies muscle perfusion as a rate-limiting factor during the later stages of hemodialysis and illustrates the benefit of exercise during this phase in increasing dialysis efficiency. The model suggests that the postdialysis rebound in the blood urea concentration is eliminated by increasing flow to the low-flow compartment from 1.1 L/min to 7.1 L/min and sustaining this for at least 30 min of a 150-min dialysis session, independent of the dialyzer clearance. Additional exercise will not increase the dialysis dose. Experimental studies are required to confirm the analysis.