A regional blood circulation alternative to in-series two compartment urea kinetic modeling

Assuming that the clearance of urea from total body water (TBW) is flow limited, the authors developed a parallel flow model using physiologic data. Organ systems with a blood flow to water volume ratio of greater than 0.2 min-1 were allocated to the high flow system. Remaining organs were represented in the low flow system. In end-stage renal disease patients with minimal renal blood flow, the high flow system contained 20% TBW and received 70% of the systemic blood flow. The authors used this flow heterogeneity to predict the post-dialysis urea rebound (R) in 12 patients after 1 hr of hemodialysis. Dialyzer clearance was 248 +/- 14.5 ml/min (mean +/- SEM) Access recirculation was obviated by returning cleared blood into a central vein. In these patients, R at 1, 3, 5, 7, 10, and 15 minutes. after slowing dialyzer blood flow (Qb) from 383 +/- 18 to 50 ml/min was 3.8 +/- 2.9, 6.2 +/- 3.4, 7.6 +/- 3.1, 8.8 +/- 3.9, 9.0 +/- 4.1, and 9.9 +/- 4.4%, respectively. CO and QAc were modeled with values of 5.5 and 0.5 L/min, respectively. The modeled TBW was 35 L. Total body water derived by nomogram was 38.1 +/- 2.0 L. Our results suggest that the parallel-flow model for urea transport can be used to explain the amount and time course of post dialysis R on a physiologic basis.