Hemodialyzer mass transfer-area coefficients for urea increase at high dialysate flow rates

Abstract

The dialyzer mass transfer-area coefficient (KoA) for area is an important determinant of urea removal during hemodialysis and is considered to be constant for a given dialyzer. We determined urea clearance for 22 different models of commercial hollow fiber dialyzers (N = approximately 5/model, total N = 107) in vitro at 37 degrees C for three countercurrent blood (Qb) and dialysate (Qd) flow rate combinations. A standard bicarbonate dialysis solution was used in both the blood and dialysate flow pathways, and clearances were calculated from urea concentrations in the input and output flows on both the blood and dialysate sides. Urea KoA values, calculated from the mean of the blood and dialysate side clearances, varied between 520 and 1230 ml/min depending on the dialyzer model, but the effect of blood and dialysate flow rate on urea KoA was similar for each. Urea KoA did not change (690 +/- 160 vs. 680 +/- 140 ml/min, P = NS) when Qh increased from 306 +/- 7 to 459 +/- 10 ml/min at a nominal Qd of 500 ml/min. When Qd increased from 504 +/- 6 to 819 +/- 8 ml/min at a nominal Qh of 450 ml/min, however, urea KoA increased (P < 0.001) by 14 +/- 7% (range 3 to 33%, depending on the dialyzer model) to 780 +/- 150 ml/min. These data demonstrate that increasing nominal Qd from 500 to 800 ml/min alters the mass transfer characteristics of hollow fiber hemodialyzers and results in a larger increase in area clearance than predicted assuming a constant KoA.

Modeling of Hemodialysis Operation

“…Thus, the insignificant change in the myoglobin overall mass transfer coefficient is an indication that its clearance from the blood is mainly controlled by the membrane. The influence of the blood flow rate on the clearance of the small molecular weight solutes diminishes at high flow rates, since the resistance of the blood side to the mass transfer of the solutes becomes negligibly small which is in agreement with the experimental results reported by Leypoldt et al 21 The experimental data collected by Jaffrin et al 16 have also been used by other researchers to test the validity of their model predictions. Legallais et al Tables 4, 5, and 6 according to the structural properties of Hospal Filtral 12 AN69 dialyzer given in Table 3.…”

“…On the other hand, the clearance of large molecules such as myoglobin is not influenced by the enhancement of the volumetric flux since it is mainly controlled by its transport through the membrane. This is supported by comparing the change in the overall mass transfer coefficient 9 area of the solutes (K o 9 A s ) with respect to blood flowrate calculated from the expression used in the study of Leypoldt et al 21 It was found that, when the blood flow rate values are increased from 100 to 400 mL/min, K o 9 A s of creatinine increases from 179 to 298 cm 3 /s, whereas that of myoglobin shows a relatively inconsiderable increase from 22.8 to 23.1 cm 3 /s consistent with the predictions shown in Fig. 3.…”

Dialysate Flow Rate and Delivered Kt/Vurea for Dialyzers with Enhanced Dialysate Flow Distribution

“…However, Leypoldt et al (2) showed that increasing the dialysate flow rate from the traditional 500 ml/min to 800 ml/min increased K o A for urea by 14% in an in vitro study using a variety of dialyzers. They suggested that this increase in K o A could result from improved flow distribution through the dialysate compartment or a decrease in boundary layer resistance to mass transfer on the dialysate side of the membrane.…”

The Patient as a Limit to Dialysis Technology

“…Formulae are available to calculate Kt/V for urea based on the assumption of a single-pool urea model (spKt/V) (9), a double-pool model accounting for a concentration equilibration among the compartments (eKt/V) (10), or by measuring the ionic dialysance (i.e., the purely diffusive clearance) and calculating the distribution volume from anthropometric data (Kt/V ID ) (11). To get the target value for Kt/V urea, as recommended by different guidelines (12,13), the focus has been on the maximization of the dialyzer clearance K, since little can be done to change urea distribution volume V, and not much flexibility is available in patients in in center facilities to prolong treatment time t. Hence, manufacturers are constantly working on the development of new dialyzer designs that should result in higher clearances, K, and associated higher K 0 A values.Although Michaels (8) initially assumed K 0 A being constant, subsequent in vitro as well as clinical studies in "nonimproved" dialyzers showed that K 0 A increased by 14.7% and 6.7% when dialysate flow was increased from 500 to 800 ml/min, respectively (14,15). This resulted in the use of higher dialysate flow rates to maximize the dialysis dose.…”

“…Although Michaels (8) initially assumed K 0 A being constant, subsequent in vitro as well as clinical studies in "nonimproved" dialyzers showed that K 0 A increased by 14.7% and 6.7% when dialysate flow was increased from 500 to 800 ml/min, respectively (14,15). This resulted in the use of higher dialysate flow rates to maximize the dialysis dose.…”