A Pseudo-One Compartment Model of Phosphorus Kinetics During Hemodialysis: Further Supporting Evidence

A pseudo-one compartment model has been proposed to describe phosphorus kinetics during hemodialysis and the immediate post-dialysis period. This model assumes that phosphorus mobilization from tissues is proportional to the difference between the pre-dialysis serum concentration (a constant) and the instantaneous serum concentration. The current study is exploratory and evaluated the ability of a pseudo-one compartment model to describe the kinetics of phosphorus during two short hemodialysis treatments separated by a 60-min inter-treatment period without dialysis; the latter is the post-dialysis rebound period for the first short hemodialysis treatment. Serum was collected frequently during both hemodialysis treatments and the inter-treatment period to assess phosphorus kinetics in 21 chronic hemodialysis patients. Phosphorus mobilization clearance and pre-dialysis central distribution volume were previously estimated for each patient during the first hemodialysis treatment and the inter-treatment period. Assuming those kinetic parameters remained constant for each patient, serum phosphorus concentrations during the second treatment were used to estimate the driving force concentration (C_df ) for phosphorus mobilization from tissues during the second treatment. Treatment time (117 ± 14 [mean ± standard deviation] vs. 117 ± 14 min), dialyzer phosphorus clearance (151 ± 25 vs. 140 ± 32 mL/min), and net fluid removal (1.44 ± 0.74 vs. 1.47 ± 0.76 L) were similar during both short hemodialysis treatments. Measured phosphorus concentration at the start of the second hemodialysis treatment (3.3 ± 0.9 mg/dL) was lower (P < 0.001) than at the start of the first treatment or C_pre (5.4 ± 1.9 mg/dL). Calculated C_df was 4.9 ± 2.0 mg/dL, not significantly different from C_pre (P = 0.12). C_df and C_pre were correlated (R = 0.72, P < 0.001). The results from this study demonstrate that the driving force concentration for phosphorus mobilization during hemodialysis is constant and not different from that pre-dialysis, providing further evidence supporting a fundamental assumption of the pseudo-one compartment model.