Increased urea kinetic modeling volume. Possible mechanisms and its significance

The presence of access recirculation reduces delivered urea clearance and produces an increased volume/weight (V/M) ratio in three-point kinetic modeling. We measured R in 20 patients receiving conventional hemodialysis and correlated results with normalized intra-access venous pressure (PIA) 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 differ 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 color-flow 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 exceeded prescribed 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.