Hemodialyzers

The main form of artificial kidney is the hemodialyzer, which uses semipermeable membranes to remove urea and other metabolic wastes, as well as some water 

In this system, the blood flows through the fibers, and the dialysate outside the fibers. 

The blood of the patient, withdrawn from an artery near the wrist, is allowed to flow through the blood circuit, which includes the dialyzer, usually a blood pump plus monitoring instruments, and is returned to a nearby vein. The connections to the blood vessels are made via the so-called subcutaneous arteriovenous shunt this involves an artificial tube that connects the artery and vein underneath the wrist skin. 

A dialysate solution of a composition appropriate to the patient is prepared by diluting with water one of concentrated dialysates of standard compositions that are available commercially. Typical compositions of diluted dialysates are as follows Na 130-140mEql , K 2-2.5 mEql , Ca 2.5-3.5 mEql , Mg + 1.0-1.5mEql , Cl 100-110 mEql HCOj 30-35 mEql b glucose 0 or 1-2 gl , osmolarity 270-300 mOsm 1 T Electrolytes are added to the dialysate mainly to prevent electrolytes in the body fluid from moving into the dialysate, and sometimes to control the concentration of some ions such as Na in the body fluid at an appropriate level. 

The dialysate solution is recirculated through the hemodyalyzer system. In hospitals where multiple patients are treated, central dialysate supply systems are normally used. The flow rates of blood and dialysate through a hollow-fiber-type 

A dialysate solution of a composition appropriate to the patient is first prepared by diluting with water one of concentrated dialysates of standard compositions that 

The dialysate solution is recirculated through the hemodialyzer system. In hospitals where multiple patients are treated, central dialysate supply systems are normally used. The flow rates of blood and dialysate through a hollow-fiber-type dialyzer are approximately 200-300 ml min-1 and 500 ml min-1, respectively. The more recently developed hemodialyzers have all been disposable that is, they are presterilized and used only once. Normally, a patient will undergo dialysis for 4—5 h per day, for three days each week. 

Operationally, dialysis (cf. Section 8.2) utilizes differences in the diffusion rates of various substances across a membrane between two liquid phases. The diffu-sivities of substances in the membrane and liquid phases (particularly the former) decrease with the increasing molecular sizes of the diffusing substances. Thus, with any hemodialyzer, the rates of removal of uremic toxins from the blood will decrease with increasing molecular size, though a sharp separation at a particular molecular weight is difficult. In contrast, proteins (e.g., albumin) should be retained in the patient s blood. In the human kidney, small amounts of albumin present in the glomerular filtrate are reabsorbed in the proximal tubule. 

Hemodialysis which involves some transfer of water due to differences in the hydrostatic or osmotic pressure is often referred to as hemodiafiltration (HDF). 

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Figure 4 is a schematic of a typical hemodialyzer. Although other geometries are stiH employed, the preferred format is a hoUow fiber hemodialyzer about 25 cm in length and 5 cm in diameter. Devices typically contain 6,000 to 10,000 capillaries, each with an inner diameter of 200 p.m and a wall thickness of around 10 p.m. Mean total membrane surface area is 1.1 0.4. Well over 60 million hemodialyzers were produced in 1992. Because of... 

Fig. 4. Schematic of a hemodialyzer. The design of a dialyzer is close to that of a sheU and tube heat exchanger. Blood enters through an inlet manifold, is distributed to a parallel bundle of fibers, and exits into a coUection manifold. Dialysate flows countercurrent in an external chamber the blood and dialysate are separated from the fibers by a polyurethane potting material. Housings are typically prepared from acrylate or polycarbonate. Production volume is...

The flow behavior in miniaturized hemodialyzer modules with two types of biocompatible membrane materials, SMC and SPAN, was investigated by using doubly distilled water as the flowing fluid in both compartments, subsequently termed membrane side (M) and dialysate side (D), respectively (Figure 4.6.1 (c, d)) [12], SMC stands for Synthetically Modified Cellulose and SPAN for Special PolyAcryloNitrile-based copolymer (Akzo Nobel, Membrana GmbH), both types representing standard membrane material. The capillaries made from this hollow... 

Fig. 4.6.1 (a) Di agram ofthe counterflow principle of membrane and dialysate flow in a hemodialyzer module, (b) Schematic depiction ofthe module cross section, (c-d) Photographs ofthe cross sections ofthe mini-hemodialyzer modules used in this study. 

Fluid Flow and Trans-membrane Exchange in a Hemodialyzer Module 459... 

Fig. 4.6.2 ID velocity profiles of counterflow through the mini-hemodialyzer modules of type SMC (a) and SPAN (b), where velocity along z is plotted versus the x position axis (total width 9 mm), with z representing the flow direction. Positive velocities correspond to membrane-side flow (M) and negative velo-...

Fig. 4.6.3 2D spin density images (a, c) and corresponding 2D velocity maps (b, d) along the cross section of miniaturized hemodialyzer modules of the type SMC and SPAN. The applied flow rate on the SMC module is... 

We would like to thank Peter Blunder and Simone Laukemper-Ostendorf who initially established the collaboration between the company Membrana and the RWTH. They started NMR flow imaging studies to characterize filtration in hemodialyzer modules, and Volker Gobbels measured the first 2D VEXSY data of counterflow in such applications. All experimental work has been accomplished in the Magnetic Resonance Center (MARC) directed by Bernhard Bliimich, whose support and leadership is greatly acknowledged. 

A. Frank, G. G. Lipscomb, M. Dennis 2000, (Visualization of concentration fields in hemodialyzers by computed tomography),/. Membrane Sci. 175, 239-251. 

Cordis-Dow hemodialyzer, 16 14 Cordium, molecular formula and structure, 5 97t, 118t Cord making, 11 287... 

Renal function impairment A pharmacokinetic study in subjects with renal failure undergoing hemodialysis showed that voriconazole is dialyzed with clearance of 121 mL/min. The IV vehicle, SBECD, is hemodialyzed with clearance of 55 mL/min. A 4-hour hemodialysis session does not remove a sufficient amount of voriconazole to warrant dose adjustment. 

Formaldehyde and glutaraldehyde are used for disinfection or sterilization of instruments such as fiberoptic endoscopes, respiratory therapy equipment, hemodialyzers, and dental handpieces that cannot withstand exposure to the high temperatures of steam sterilization. They are not corrosive for metal, plastic, or rubber. These agents have a broad spectrum of activity against microorganisms and viruses. They act by alkylation of chemical groups in proteins and nucleic acids. Failures of disinfection or steri... 

Hollow fiber refers to a membrane tube of very small diameter (e.g., 200 pm). Such small diameters enable a large membrane area per unit volume of device, as well as operation at somewhat elevated pressures. Hollow-fiber modules are widely used in medical devices such as blood oxygenators and hemodialyzers. The general geometry of the most commonly used hollow-fiber module is similar to that of the tubular membrane, but hollow fibers are used instead of tubular membranes. Both ends of the hollow fibers are supported by header plates and are connected to the header rooms, one of which serves as the feed entrance and the other as the retentate exit. Another type of hollow-fiber module uses a bundle of hollow fibers wound spirally around a core. 

The hemodialyzer, also known as the artificial kidney, is a device that is used outside the body to remove the so-called the uremic toxins, such as urea and creatinine, from the blood of patients with kidney disease. While it is a crude device compared to the exquisite human kidney, many patients who are unable to receive a kidney transplant can survive for long periods with the use of this device. 

In the situation where the effect of filtration - that is, water movement across the membrane due to the difference in hydrostatic pressure and/or osmolarity - can be neglected, the overall resistance for mass transfer in hemodialyzers with flat membranes is given as... 

Many membrane materials have been developed and are used for hemodialyzers. Today, these include regenerated cellulose, cellulose acetate, polyacrylonitrile, poly(methylmethacrylate), vinyl alcohol-ethylene copolymer, polysulfone, polyamide, and others. 

The relative magnitudes of the three terms on the right-hand side of Equation 15.25 vary with the diffusing substance, the flow conditions of both fluids, and especially with the membrane material and thickness. With the hollow-fiber-type hemodialyzers that are widely used today, membrane resistance usually takes a substantial fraction of the total resistance, and the fraction increases with increasing molecular weight of the diffusing component. 

One widely used performance index of hemodialyzers is that of clearance, defined similarly to that of the human kidney. The clearance of a hemodialyzer is the conceptual volume of blood (cm inin ) from which a uremic substance is completely removed by hemodialysis. Let Qg (cm min ) be the blood flow rate through the dialyzer, Qjj fern min ) the dialysate flow rate, and Cg and Cj3 (mgem ) the concentrations of a uremic substance in the blood and the dialysate, respectively, with the subscripts i and o indicating values at the inlet and outlet, respectively. The rate of transfer of the substance in the dialyzer w (mgmin ) is then given as... 

Another index of hemodialyzer performance is the dialysance, (cm min ), defined as... 

In the above relationships, the effect of the so-called filtration - that is, the permeation of water across the membrane - on the clearance and dialysance has been neglected. In the case where the Qp (cnf min ) of water moves from the blood phase to the dialysate across the membrane, the clearance of a hemodialyzer with respect to a uremic substance Cl is given as... 

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