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Increased Urea Kinetic Modeling Volume: Possible Mechanism and Its Significance

The predence of access recirculation reduces delivered urea clearance and produces an increased volume/weight (V/M) ratio in three-point kinetic modeling. We measured nR in 20 patients receiving conventional hemodialysis and correlated results with normalized intra-access venous pressure (P1A) and with angiographic or color-flow Doppler studies. Twenty patients were equally divided into those with and without persistently elevated modeled V/W ratios (0.64 vs 0.53), and subdivided into those with native and synthetic bridge graft accesses, Kinetic modeling parameters (Kt/V) and P1A did not difer between the two V/W groups. Modeled volume was quite accurately predicted by the equations in the normal group but deviated by 7.3 ±2.1 L in the high V/W ratio group. Three of 10 native and 4 of 10 graft accesses had trivial and hemodynamically insignificant abnormalities by colourflow Doppler or angiography. Recirculation was independent of V/W group and when measured by the slow flow/clamp technique was negligible (<2.0%). Access flow always exceded prescrivbed dialyzer blood flow by more than 300 ml/min. Therefore, access recirculation was unlikely. In many of the high V/W patients, alternative explanations for falsely high modeled volume were found on follow-up modeling. Only one patient appeared to have a true high volume. The authors conclude that high urea volumes during kinetic modeling are unlikely to occur from access recirculation, but arise from other factors affecting the delivered urea clearance.