High venous urea concentrations in the opposite arm. A consequence of hemodialysis-induced compartment disequilibrium.

Resistance to urea diffusion among body fluid compartments diminishes the therapeutic effectiveness of hemodialysis. Cell membrane or capillary wall resistance is thought to be responsible for hemodialysis-induced urea disequilibrium. The authors examined the possibility that reduced blood flow might contribute to urea disequilibrium in the arm opposite the blood access site. Blood samples were taken simultaneously from a vein in the arm opposite the access site and from the arterial port after occluding the access graft between the needle sites for 1 min. Venous urea nitrogen levels from the opposite arm averaged 10% higher after 5 min, 26% higher after 60 min, and 36% higher after 120 min of dialysis. A three-compartment model of urea kinetics that includes a blood flow term accurately predicted all measured urea nitrogen concentrations in both arms. These data suggest that the opposite arm often behaves as a compartment with high resistance to urea diffusion. Slow diffusion from this compartment is partially due to reduced blood flow/compartment volume, and results in a delayed fall in venous blood urea nitrogen (BUN).