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)*