How to determine ionic dialysance for the online assessment of delivered dialysis dose

Background. Ionic dialysance may be equivalent to blood-water urea clearanc e corrected for recirculation (effective urea clearance): however. this is controversial. The aims of our study were (1) to verify in vivo whether the value of ionic dialysance is affected by the method of determination, give n the effect of cardiopulmonary recirculation on inlet plasma water conduct ivity when the inlet dialysate conductivity is changed: and (2) to define t he operative modalities for determining ionic dialysance to obtain an adequ ate estimate of effective urea clearance. Methods. Thirty-three hemodialysis patients were studied during 186 dialysi s sessions with low-flux polysulfone dialyzers using a modified Fresenius M edical Care 4008 B machine equipped with meters to measure inlet and outlet dialysate conductivities. This machine varied inlet dialysate conductivity (Cd-i) according to the following pattern: starting from baseline (step 0) , Cd, was increased by 8% (step 1). After Cd-i had reached the target value , which took 8 to 10 minutes, it was lowered to 8% below the baseline value (step 2). After 8 to 10 minutes. when Cd, had reached the new target, it w as returned to its starting value (step 3). Four values of conventional ion ic dialysance (using the standard formula) and actual ionic dialysance (tak ing into account cardiopulmonary recirculation) were obtained for each cycl e and were compared among them and with effective urea clearance (Kde). Results. Mean conventional dialysance values at steps 0 to 2 and 2 to 3 (1( )0 and 189 mL/min) were similar and higher than those at steps 0 to 1 and 1 to 2 (171 and 181 mL/min). Mean conventional ionic dialysance values under estimated Kde. particularly at steps 0 to 1 (-22.2 mL/min. P < 0.001) and 1 to 2 (-12.6 mL/min. P < 0.001), The actual dialysance values underestimate d Kde by no more than 4.3 mL/min (P < 0.001). In steps 0 to 1 and 1 to 2, t he underestimate of Kde by conventional dialysance increased at higher valu es of Kde. but this relationship did not exist when considering actual dial ysance. Conclusions. The value of ionic dialysance is affected by the method of det ermination. given the effect of cardiopulmonary recirculation on inlet plas ma water conductivity when inlet dialysate conductivity is changed. As a co nsequence, to provide a correct and direct estimate of effective urea clear ance, ionic dialysance must he determined by changing inlet dialysate condu ctivity in such a way as to keep inlet plasma water conductivity constant b y means of two symmetrical high and low dialysate conductivity steps.