Determinants of Hemodialysis Performance: Modeling Fluid and Solute Transport in Hollow-Fiber Dialyzers

Hemodialysis constitutes the lifeline of patients with end stage renal disease, yet the parameters that affect hemodialyzer performance remain incompletely understood. We developed a computational model of mass transfer and solute transport in a hollow-fiber dialyzer to gain greater insight into the determinant factors. The model predicts fluid velocity, pressure, and solute concentration profiles for given geometric characteristics, membrane transport properties, and inlet conditions. We examined the impact of transport and structural parameters on uremic solute clearance by varying parameter values within the constraints of standard clinical practice. The model was validated by comparison with published experimental data. Our results suggest solute clearance can be significantly altered by changes in blood and dialysate flow rates, fiber radius and length, and net ultrafiltration rate. Our model further suggests that the main determinant of the clearance of unreactive solutes is their diffusive permeability. The clearance of protein-bound toxins is also strongly determined by blood hematocrit and plasma protein concentrations. Results from this model may serve to optimize hemodialyzer operating conditions in clinical practice to achieve better clearance of pathogenic uremic solutes.

Acetylsalicylic acid decreases clotting in combination with enoxaparin during haemodialysis in vitro

BACKGROUND

Anticoagulation is a cornerstone in haemodialysis (HD) therapy to avoid clotting of blood when it comes into contact with the dialysis membrane. Although heparins are usually administered as anticoagulants, they are not always sufficient to maintain adequate HD. We investigated the additional effect of acetylsalicylic acid compared with standard anticoagulation on maintaining adequate flow properties during HD in vitro.

METHODS

We collected blood from 42 healthy volunteers, between 18 and 60 years of age, into bags filled with 1, 1.5 or 2 mg enoxaparin, with (treatment group) or without (control group) 100 mg of aspirin. Blood was evaluated before, during and at the end of each experiment to determine coagulation parameters, whole blood aggregation and thromboelastogram measurements. Transmembrane pressure was recorded as indirect estimate of dialysis patency. The primary endpoint was time to filter clotting.

RESULTS

Addition of acetylsalicylic acid significantly prolonged the time to circuit clotting from 120 (105-150) min to >180 min (120-180) min (P = 0.047) and allowed lowering the enoxaparin concentration from 2 mg per circuit to 1 mg without an increase in clotting. Furthermore, it reduced the transmembrane pressure from 46 to 4 mmHg (P < 0.001) after 4 h of dialysis. Acetylsalicylic acid better preserved the platelet count (128 versus 116 × 10E9/L, P = 0.01) and improved platelet aggregation at the end of the dialysis procedure.

CONCLUSION

Adding acetylsalicylic acid to HD circuits lowered the transmembrane pressure, better preserved platelet function and prolonged the time to circuit clotting, which in sum increases haemodialyser performance and may facilitate a more effective HD.

The Influence of Colloid Osmotic Pressure on Hydrostatic Pressures in High- and Low-Flux Hemodialyzers

The aim of this study was to examine the relationship between hydrostatic trans-membrane pressure (TMP_h ) and colloid osmotic pressure (COP) in low-flux (LF) and high-flux (HF) dialyzers. Hydrostatic pressures were measured in dialyzers distinguished by their ultrafiltration coefficient K_uf (16 and 85 mL/h/mm Hg) under constant dialysate flow and variable blood flow (Q_b ) ranging from 0 to 400 mL/min using (i) alginate (70 kDa) dissolved in dialysate, (ii) diluted, undiluted, and concentrated plasma, or (iii) whole blood at different hematocrit, all in absence of ultrafiltration (UF). For a given fluid, TMP_h linearly increased with increasing Q_b . The intercept of the linear TMP_h to Q_b relationship correlated with measured COP with an average bias of 1.00 ± 2.26 mm Hg and a concordance correlation coefficient of 0.98. The slope of the linear TMP_h to Q_b relationship increased with increasing sample viscosity and was much larger in HF dialyzers under otherwise identical operating conditions, most likely because of increased internal filtration. The TMP_h to Q_b relationship measured in dialyzers in absence of UF can be described by the intercept related to measured COP and the slope related to internal filtration. This relationship could be of interest to estimate internal filtration and COP under in vivo conditions.

Internal filtration in a high-flux dialyzer quantified by mean transit time of an albumin-bound indicator

Internal filtration in high-flux (HF) dialyzers significantly contributes to convective solute removal of molecules with poor diffusibility, but it is difficult to quantify. The aim of this study was to present the theory and to develop a method for measuring internal filtration and backfiltration in HF dialyzers, which also could be applied to patient studies. In a series of lab-bench experiments, the mean transit times (τd) of indocyanine green (ICG) passing the dialyzer were optically measured under different operating conditions and compared with mean transit times calculated from the known volume of the blood compartment (τV) using a mathematical model. τd was always larger than τV. The relative difference in mean transit times (1 - τV/τd) was related to the average cumulative filtration rate (Qfil). The internal filtration fraction Fb = Qfil/Qb was largely independent of blood flow (Qb) and not different from theoretical predictions obtained from a mathematical model. The dispersion of a nondiffusible indicator such as ICG can be used to quantify the magnitude of internal filtration and backfiltration in HF dialyzers using available technology. This information could be useful for testing the HF dialyzers in everyday situations.