Classic urea modeling assumes that both urea generation rate (G) and residual renal urea clearance (Kru) are constant throughout the week, but this may not be true. Reductions in intradialysis G could be caused by lower plasma amino acid levels due to predialysis/intradialysis fasting and also to losses of amino acids into the dialysate. Intradialytic reductions in Kru could be due to lower intravascular volume, blood pressure, or osmotic load. To determine the possible effects of reduced G or Kru during dialysis on the calculation of the volume of distribution (V) and Kt/Vurea, we modeled 3 and 6/week nocturnal, 6/week short daily, and 3/week conventional hemodialysis. A modified 2-pool mathematical model of urea mass balance with a constant time-averaged G was used, but the model was altered to allow adjustment of the ratio of dialytic/interdialytic G (Gd/Gid) and dialytic/total Kru (Krud/Kru) to vary from 1.0 down to near zero. In patients dialyzed six times per week for 400 minutes per session, when Gd/Gid was decreased from 1.0 to 0.05, the predicted urea reduction ratio (URR) increased from 68.9% to 80.2%. To achieve an increased URR of this magnitude under conditions of constant G (Gd/Gid=1.0) required a decrease in modeled urea volume (V) of 36%. At Gd/Gid ratios of 0.8 or 0.6 (corresponding to 20% or 40% reductions in intradialysis G), the modeled URR was increased to 71.0% or 73.3%, causing a 7% or 15% factitious decrease in V. The error was intermediate for the 3/week nocturnal schedule, and was much less pronounced for the 6/week daily and 3/week conventional treatments. Reductions in intradialytic Kru had the opposite effect, lowering the predicted URR and increasing the apparent V, but here the errors were of much lesser amplitude. The results suggest that, particularly for nocturnal dialysis, the standard "constant G" urea kinetic model may need to be modified.
View details for DOI 10.1111/j.1525-139X.2009.00688.x
View details for Web of Science ID 000274806000007
View details for PubMedID 20331814