Peripheral venous blood is not the appropriate specimen to determine the amount of recirculation during hemodialysis

When recirculation is determined by the classic method of comparing the blood urea concentrations in the dialysis inflow and outflow lines with blood obtained from a peripheral vein, any difference in peripheral venous and systemic arterial blood urea concentration will result in an error in recirculation calculation. The purposes of these studies are threefold: 1. To determine the arterial to venous urea, creatinine, albumin, and hematocrit gradients during hemodialysis treatments. 2. To compare recirculation calculated by the standard technique with that calculated by obtaining the systemic sample from the dialysis inflow line after slowing the blood pump to 50 ml/min. 3. To calculate the recirculation values obtained by the two methods when intra-access recirculation is prevented by drawing the inflow blood from the arterial venous access and returning the dialyzed blood directly into the vena cava. During hemodialysis, an arterial venous urea nitrogen gradient of 9.6 +/- 7.0% (standard deviation) was found. The arterial venous creatinine gradient was 10.8 +/- 5.5%, and there were no significant gradients for albumin or hematocrit. In 161 patients at 6 outpatient dialysis centers, the mean recirculation was 6.4% with the systemic sample obtained from the dialysis inflow line compared with 11.9% with the sample obtained from a peripheral vein. When recirculation was determined by the standard technique in 13 patients in whom the possibility of recirculation was prevented by returning the dialyzer outflow blood directly into the central venous system, an average recirculation of 9.8% was found. The blood urea concentration in the dialysis inflow blood line was found to increase for at least 15 min after slowing the blood pump. The author concludes that drawing blood from a peripheral vein for the systemic sample induces a significant error and should be abandoned. An alternate source is the dialysis inflow line after slowing the blood pump to a low value. The blood should be obtained as soon as the previous blood has been cleared from the line.