Relationship between effective ionic dialysance and in vivo urea clearanceduring hemodialysis

Effective ionic dialysance (EID) can be measured from dialyzer inlet and ou tlet conductivity changes following two steps of dialysate conductivity. Re lationships between EID and In vivo urea clearances were studied four times per hemodialysis treatment in eight patients, each undergoing six hemodial ysis treatments (192 data sets). Dialyzer blood flow was varied from 190 to 500 mL/min. Dialysate flow was constant (751 to 771 mL/min), and a standar d dialyzer (700 HG; Cobe, Lakewood, CO) was used. Double samples were drawn for arterial, venous, and dialysate urea measurements. Two laboratory valu es were missing. Twelve unreliable laboratory values indicated by divergent results were excluded. Urea clearances were calculated by formulae convert ing whole-blood to blood-water urea clearances. EID was measured using Dias can (Gambro-Dasco, Medolla, Italy). Mass balance was checked by comparison of dialysate and blood-water urea clearances. Divergent results between dia lysate and blood-water urea clearance values led to the exclusion of an add itional three laboratory values. A small error (4.2%) in urea mass balance was found (dialysate greater than blood-water urea clearances). A total of 175 data sets were compared. EID showed excellent correlation with blood-wa ter urea clearances (r = 0.92) over the line of identity, with a mean diffe rence of -3.5 mL/min (-1%), and similarly with dialysate urea clearances (r = 0.92; mean difference, -13.4 mL/min; -5%). For both blood- and dialysate -side comparisons, differences Increased with greater clearances. Because E ID is an effective clearance and urea clearance is a measure of dialyzer cl earance, the curves were corrected for cardiopulmonary recirculation; acces s recirculation was zero (Transonic monitor; Transonic Systems Inc, Ithaca, NY). For cardiopulmonary recirculation correction, cardiac output and acce ss flows were assumed to be 6.4 L and 1.46 L/min. Corrected data show EID c orrelates with blood-side urea clearance (r = 0.92), with a mean difference of +7.3 mL/min (3.3%), and is constant over the range of clearances. EID c orrelated with dialysate urea clearance (r = 0.92) with virtually no differ ence. The difference on the blood side is consistent with the urea mass bal ance error found. These data indicate that EID using Diascan can provide an accurate indication of effective urea clearances obtained during hemodialy sis and is of value in monitoring dialysis adequacy. (C) 2001 by the Nation al Kidney Foundation, Inc.