A new scintigraphic method to characterize ultrafiltration in hollow fiber dialyzers

Abstract

Ultrafiltration and pressure profiles in hollow fiber dialyzers with different hydraulic permeabilities have been investigated with a new scintigraphic method. Radiolabelled albumin macroaggregates, used as a nondiffusible marker molecule, were added to the blood in an in vitro circuit and circulated through cuprophan and polysulphon dialyzers. Since the marker molecule was too big to cross the dialysis membrane, its changes in concentration were assumed to occur in response to the variation of the blood water content (filtration or back-filtration). These changes in concentration, recorded by a gamma camera, were evaluated to establish the cumulative values of filtration and back-filtration and their relevant profiles along the length of the dialyzer. The achieved data were compared with the experimental values of ultrafiltration empirically measured and with the theoretical values predicted by a classic linear method. Two conditions were analyzed: A) the minimal filtration rate necessary to avoid back-filtration (critical filtration); and B) the condition of zero net filtration in which filtration equals back-filtration. The nuclear method proved to be extremely precise in predicting the ultrafiltration values and significantly more precise than the linear method, especially for the highly permeable dialyzer. The reason for that probably depends on the non-linear pressure and ultrafiltration profile observed with the scintigraphic pattern of the dialyzer. Viscosity changes and local variations in blood flow may in fact interfere with the pressure drop inside the hollow fibers and result in such a complex behavior. The other interesting aspect of this method is the possibility of accurate measurement of the amount of back-filtration that wouldn't be possible with simple calculations. In conclusion, the complex nature of the phenomena regulating the water fluxes in hollow fiber dialyzers requires more complex calculation than a simple linear model to achieve an accurate range of predictability.

Protein-Leaking Membranes for Hemodialysis

“…Because convective transfer occurs at the dialysate outlet end of the dialyzer, convected solutes are swept into the dialysate outflow and effectively cleared. This phenomenon may produce internal filtration rates of up to 30 ml/min (23). Internal filtration is ubiquitous with high-flux membranes and increases in magnitude as the hydraulic permeability of the membrane increases; it may be the primary mechanism of large solute removal in high-flux hemodialysis.…”

“…With high-flux membranes and proteinleaking membranes, the pore size distribution is shifted toward larger diameter pores, allowing some limited diffusive transfer of low molecular weight proteins. This diffusive transfer is supplemented by uncontrolled convection arising from a process of internal filtration and backfiltration (23). Internal filtration and backfiltration occur when membranes of high water permeability are used with a volume-control system because the low mean transmembrane pressure required to remove the water needed to achieve a patient's dry weight and the countercurrent nature of blood and dialysate flows in a dialyzer create local pressure gradients that favor filtration from blood to dialysate at the blood inlet end of the dialyzer and filtration from dialysate to blood at the blood outlet end of the dialyzer.…”

The Rise of Expanded Hemodialysis

“…According to our previous studies [23,24,25,26,27,] we could model the theoretical behavior of a 1.3-1.5 m 2 dialyzer equipped with an HRO membrane in a reduced inner diameter configuration. A 1.5 dialyzer should be capable of generating an internal filtration of 40 mL/min at zero net filtration.…”

Treatment Policy rather than Patient Characteristics Determines Convection Volume in Online Post-Dilution Hemodiafiltration

“…In HDF, diffusion, which is the main removal mechanism in low-flux HD, is combined with convection. Whereas the estimated amount of convective transport during high-flux HD is <10 liters/session [10], in online post-dilution HDF 25 liters or more can be achieved [11]. …”