RECIRCULATION MEASURES WITH UREA AND MANNITOL DURING HEMODIALYSIS

A simple, accurate, and reproducible method of measuring recirculation in grafts during hemodialysis is essential for improving the efficien cy of dialysis. In our studies, plasma samples for plasma urea nitroge n (PUN) were taken from the arterial line of the dialyzer at blood flo ws (A) of 200, 300, and 400 ml/min, preceded by a 5-min period of equi libration, and at 15 s and 2 min after turning the flow down to 100 ml /min (S), the latter serving as systemic samples. Recirculation was ca lculated as (S - A)/(S - V). Total blood flow (Q(b)) through the graft s was measured by color Doppler ultrasound. We found a significant, in verse relationship between recirculation and total flow through the gr aft at dialyzer Q(b) of 400 but not 300 or 200 ml/min. The magnitude a nd prevalence of recirculation was always greater when the 2 min sampl e was used as S compared to the 15 s sample and as dialyzer Q(b) incre ased. As a qualitative, urea-independent measure of recirculation, we assayed the appearance of mannitol in the arterial line in blood drawn 15 s after initiating a mannitol push into the venous line. Blood obt ained just prior to the mannitol push was used as the zero blank. Thir teen of 18 patients had a measurable, but low, level of mannitol, 5 di d not, and 2 had inconsistent results in studies done on separate days . We conclude that the majority of patients receiving chronic hemodial ysis have a low degree of recirculation and that methods relying on ur ea must be suspected of exaggerating the true degree of recirculation.