Intensification of Middle- and High-Molecular-Weight Toxins Removal in Dialysis Process.

A novel extracorporeal circuit for hemodialysis is described, which enhances the removal of middle- and high-molecular-weight toxins. To enhance the removal of especially high-molecular toxins, a recirculation pathway for dialysate flow is added to conventional circuit. The influence of an increase in the ratio of recirculation to dialysate flow rate and the ultrafiltration rate (UFR) on the removal of toxins was investigated to evaluate the removal of different toxins in clinical conditions. Removal of toxins was also modeled by an analytical method and solved by the MATLAB software (The MathWorks, Inc., Natick, MA). A significant increase in removal of urea (up to 31%) and vitamin B12 (11%) was achieved when the UFR is low (≤50 ml/h) or zero. The model showed an excellent agreement with the experimental results, which indicates its applicability for the removal of different toxins in an extracorporeal circuit. Increase in recirculation flow, while adjusting the UFR near zero, improves the mass transfer coefficient and can lead to enhanced especially middle- and high-molecular-weight toxin removal.

Phenomenological characterization of blood's intermediate shear rate: a new concept for hemorheology.

The phenomena of aggregation, breakdown, and disaggregation of the rouleaux of red blood cells (RBCs) in addition to deformability affect the human blood viscosity at different shear rates. In this study, the intermediate shear rate is introduced and defined when the effect of aggregation on the change of blood viscosity is diminished; and afterwards, the alteration in the blood viscosity is dominantly affected by the deformation of RBCs. With this respect, modeling the effective parameters on the blood shear-thinning behavior including hematocrit and plasma viscosity was performed for the two different shear regions discriminated by the proposed intermediate shear rates. The presented rheological model reflects a phenomenological approach to assess the human blood viscosity with an average error of ± 5% compared to experimental data for hematocrits between 0.299 and 0.702, subjected to various shear rates from 0.2 to 680 1/s. The temperature changes as well as biochemical effects on whole blood viscosity are characterized by the introduced plasma viscosity-dependent model. The presented comprehensive model could be used for better understanding of blood flow hemodynamics and analyzing the shear dependence of aggregation and deformability behaviors of RBCs.

The course of hematocrit value along the length of a dialyzer's fiber: Hemoconcentration modeling and validation methods.

OBJECTIVES: Contemporary therapies for chronic kidney disease patients encompass a wide range of hemodialysis treatments, most of which rely greatly on dialyzers and hemofilters. The filtration process taking place in these devices with respect to the hemodynamic characteristics of the flow, has not yet been fully investigated. This study aims at improving the understanding of hemodynamics in a dialyzer by employing experimental methods and mathematical models. METHODS: A semiempirical model has been formulated based on the principles of hemodynamics, considering the dominant phenomena of filtration-backfiltration and the corresponding driving forces. An in vitro hemodialysis circuit was accordingly assembled for experimental data acquisition, and subsequently for model validation. The circuit consisted of two dialyzers arranged in sequential order, in pursuance of increasing the number of sampling points. Fresh, heparinized porcine blood was used throughout the course of this study. Pressure and flow data obtained from in vitro investigations with the hemodialysis circuit were used as an input for the semiempirical model. FINDINGS: The model predicted a substantial divergence in the course of hematocrit value along the length of the hollow fibers, which is corroborated by the experimental data. Particularly in certain operational conditions, hematocrit rose from 25% at the inlet to 65% halfway along the dialyzers' length, to end at 30% at the outlet. CONCLUSION: Validation of the model's predictions with experimental data demonstrated a very good agreement, confirming the model's accuracy. Potential implementation of the model in clinical practice in the future might contribute greatly to an improved hemodialysis experience.

A new approach for semiempirical modeling of mechanical blood trauma.

PURPOSE: Two semi-empirical models were recently published, both making use of existing literature data, but each taking into account different physical phenomena that trigger hemolysis. In the first model, hemoglobin (Hb) release is described as a permeation procedure across the membrane, assuming a shear stress-dependent process (sublethal model). The second model only accounts for hemoglobin release that is caused by cell membrane breakdown, which occurs when red blood cells (RBC) undergo mechanically induced shearing for a period longer than the threshold time (nonuniform threshold model). In this paper, we introduce a model that considers the hemolysis generated by both these possible phenomena. METHODS: Since hemolysis can possibly be caused by permeation of hemoglobin through the RBC functional membrane as well as by release of hemoglobin from RBC membrane breakdown, our proposed model combines both these models. An experimental setup consisting of a Couette device was utilized for validation of our proposed model. RESULTS: A comparison is presented between the damage index (DI) predicted by the proposed model vs. the sublethal model vs. the nonthreshold model and experimental datasets. This comparison covers a wide range of shear stress for both human and porcine blood. An appropriate agreement between the measured DI and the DI predicted by the present model was obtained. CONCLUSIONS: The semiempirical hemolysis model introduced in this paper aims for significantly enhanced conformity with experimental data. Two phenomenological outcomes become possible with the proposed approach: an estimation of the average time after which cell membrane breakdown occurs under the applied conditions, and a prediction of the ratio between the phenomena involved in hemolysis.

Simulation of blood oxygenation in capillary membrane oxygenators using modified sulfite solution.

Blood oxygenation is the main performance characteristic of capillary membrane oxygenators (CMOs). Handling of natural blood in in vitro investigations of CMOs is quite complex and time-consuming. Since the conventional blood analog fluids (e.g. water/glycerol) lack a substance with an affinity to capture oxygen comparable to hemoglobin's affinity, in this study a novel approach using modified sulfite solution is proposed to address this challenge. The solution comprises sodium sulfite as a component, simulating the role of hemoglobin in blood oxygenation. This approach is validated by OTR (oxygen transfer rate) measured using native porcine blood, in two types of commercially available CMOs. Consequently, the number of complicated natural blood investigations in the evolution procedure of newly developed oxygenators would considerably decrease. Moreover, the reassessing of failed devices, in clinics, would be performed more precisely using a modified sulfite solution than simple water/glycerol testing.

A theoretical model for evaluation of the design of a hollow-fiber membrane oxygenator.

Geometric data are fundamental to the design of a contactor. The efficiency of a membrane contactor is mainly defined by its mass-transfer coefficient. However, design modifications also have significant effects on the performance of membrane contactors. In a hollow-fiber membrane oxygenator (HFMO), properties such as priming volume and effective membrane surface area (referred to as design specifications) can be determined. In this study, an extensive theoretical model for calculation of geometric data and configuration properties, and, consequently, optimization of the design of an HFMO, is presented. Calculations were performed for Oxyphan(®) hollow-fiber micro-porous membranes, which are frequently used in current HFMOs because of their high gas exchange performance. The results reveal how to regulate both the transverse and longitudinal pitches of fiber bundles to obtain a lower rand width and a greater number of windings. Such modifications assist optimization of module design and, consequently, substantially increase the efficiency of an HFMO. On the basis of these considerations, three values, called efficiency factors, are proposed for evaluation of the design specifications of an HFMO with regard with its performance characteristics (i.e. oxygen-transfer rate and blood pressure drop). Moreover, the performance characteristics of six different commercial HFMOs were measured experimentally, in vitro, under the same standard conditions. Comparison of calculated efficiency factors reveals Quadrox(®) is the oxygenator with the most efficient design with regard with its performance among the oxygenators tested.