Causes, kinetics and clinical implications of post-hemodialysis urea rebound

The rapid increase in end-dialysis urea concentration (Co) immediately after the end of dialysis (HD), which greatly exceeds that expected as an effect of urea generation and defined as "net rebound," was assessed in 21 chronic HD patients. The curve of serial values of net rebound correlated (r = 0.70) with the theoretical curve predicted by the two pool urea kinetics model (UKM). A mean equilibrium concentration (Ce) was achieved in 48 minutes, with a 7.58% increase in Co. Stabilized rebound (Re) was compared after four different HD procedures, and significant correlations were found between the magnitude of Re and the indexes of HD efficiency, dialyzer clearance (r = 0.75) and Kt/V (r = 0.68). The highest values of Re (8.6% and 8.8%) were observed after the procedures with largest urea removal, irrespective of the biocompatibility conditions (new or reused dialyzers). The single pool UKM applied with the stabilized end-HD urea concentration Ce instead of Co resulted in more physiological values of urea distribution volume (56.1% vs. 50.5% of body wt) and in lower values of Kt/V (0.64 vs. 0.73, P less than 0.001) and protein catabolic rate (1.07 vs. 1.17 g/kg/day, P less than 0.001). A reequilibration process, rather than protein hypercatabolism, seems to be responsible for most rebound, the magnitude of which correlated with the efficiency of the procedure. Only by considering Ce as the true end-HD urea concentration it is possible to minimize the errors arising from the application of a single pool analysis to a two pool system.