HDCN Article Review/Hyperlink

Daugirdas JT, Schneditz D, Leehey DJ

Effect of access recirculation on the modeled urea distribution volume

Am J Kidney Dis (Apr) 27:512-518 1996

While examining the distortion of urea kinetic modeling caused by access recirculation (AR), the authors noted that the error in the postdialysis BUN caused by AR is time-dependent, diminishes in magnitude as time progresses, and disappears at a point in time when the two postdialysis BUN curves (with and without AR) intersect. Regardless of the magnitude of AR, the two curves intersect at approximately the same point on the URR scale, when the URR is about 70%.

At this point, which is close to the usual target, the error caused by the overestimation of clearance is exactly offset by the error caused by the falsely low postdialysis BUN and the modeled single-compartment volume of urea distribution (V) is predicted accurately despite a significant reduction in effective dialyzer clearance. During the interval preceding this point in time, the low measured postdialysis BUN can be detected as a falsely low V while following this point in time, the error in the postdialysis BUN can be detected as a falsely high V. However, when the curves intersect, V is modeled accurately despite a significant overestimation of effective clearance.

This error, which would go undetected because the usual distortion of V is absent, would endanger the patient from underdialysis. The error could be avoided and the AR would be detected as a high modeled V if precautions were made to draw the postdialysis BUN properly to avoid the falsely low concentration caused by recirculation. The authors' mathematical analysis and case example underscore the importance of taking precautions to avoid dilution of the BUN from recirculated venous blood when drawing the postdialysis blood sample.

Comment: As noted by the authors, their observations are analogous to the distortion caused by single-compartment modeling of multicompartment urea kinetics predicted by Gotch and others and the results are the same (K is overestimated) but the mathematics are simpler and more easily conceptualized. It is important to distinguish the authors' new and useful concept of fractional access recirculation (FAR), which they defined as the ratio of the dialyzer inlet BUN to the upstream access BUN (actually the fractional dilution of the access BUN), from AR, which is classically defined as a ratio of flows and doesn't depend on dialyzer clearance. (Thomas A. Depner, M.D., University of California at Davis)