Hemodialyzer dialysance

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. 

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... 

In a hollow-fiber-type hemodialyzer of the following specifications, 200 cm min of blood (inside fibers) and 500 cm min of dialysate (outside fibers) flow countercurrently. 

Calculate the overall mass transfer coefficient ffp (based on the hollow-fiber inside diameter) and the dialysance of the hemodialyzer for urea, neglecting the effect of water permeation. 

In a hoUow-fiber-type hemodialyzer of the total membrane area (based on o.d.) A = 1 m , 200 cm min of blood (inside fibers) and 500 cm min of dialysate (outside fibers) flow countercurrently. The overall mass transfer coefficient for urea (based on the outside diameter of the hollow fiber) is 0.030 cm min . Estimate the dialysance for urea. 

Figure 18.3 Schematic of fluid and mass transfer between blood and dialysate compartments in a hemodialyzer.

The capacity for toxin removal of an hemodialyzer is expressed by the dialysance or the clearance which have the same unit as a flow rate. The dialysance D is defined, for each toxin as... 

Parallel-plate hemodialyzers using flat membranes, with several compartments in parallel, separated by plastic plates, are now only available from Hospal Co (Crystal and Hemospal models). Blood circulates between two membranes and the dialysate between the other side of membrane and the plastic plate. These parallel-plate dialyzers have a smaller blood-pressure drop than hollow-fiber ones and require less anticoagulants as flat channels are less exposed to thrombus formation than fibers, but they are heavier and bulkier and thus less popular. A recent survey of the state-of-the-art in hemodialyzers is given in [13]. 

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. 

In a hollow-fiber-type hemodialyzer, 200 ml min-1 ofblood (inside fibers) and 500 ml min-1 of dialysate (outside fibers) flow countercurrently. The urea concentrations of the inlet blood, outlet blood, and outlet dialysate are 100 mg dl x, 80 mg dl-1 and 32 mg dl-1, respectively. Calculate the clearance for urea. 

Thus, in 4 hr, which is a typical treatment time, the urea concentrate has been reduced by 45%. It is left as an exercise at the end of this chapter for a study of the effect on the rate of urea removal of changing the hemodialyzer geometry, the blood and dialysate flow rates to the dialyzer, the rate of waste withdrawal, the volume of the dialysate tank, and the sensitivity of the rate of urea mass transfer to the mass-transfer coefficient. In particular, the above estimate of the coefficient on the shell side may be low because the entry to and exit fi om the hemodialyzer of the dialysate is normal to, rather than parallel to the fibers. This should enhance the shell-side coefficient. ... 

Hemodialyzer performance is analyzed using a number of indices solute transfer efficiency i/ , clearance C, and dialysance Db. The solute transfer efficiency i/ (Michaels, 1966),... 

In Section 4.3.1, we were introduced to a hemodialyzer with blood on one side of the membrane and the dialyzing solution on the other side. Solutes (metabolic waste products) from blood diffused through the liquid filled pores of the membrane to the dialysate side. Using a simple lumped analysis based on the overall solute mass-transfer coefficient Ku, we will develop an expression for the solute removal efficiency of a hemodialyzer in which blood as well as the dialyzing solution are in steady cocurrent flow (Section 8.1.7 treated countercurrent dia-lyzers). The analysis is valid for any other system, not just hemodialysis. 

Leypoldt, J. K., Cheung, A. K., Agodoa, L. Y., Daugitdas, J. T., Greene, T., and Keshaviah, R R. (1997). Hemodialyzer mass transfer-area coefficients for urea increase at high dialysate flow rates. The Hemodialysis (HEMO) Study. Kidney Int. 51, 2013. 

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