Hemodialyzer clearance

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. 

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... 

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... 

Hollow Fibers. The general configuration of the hollow-fiber apparatus is similar to that of hemodialyzers and blood oxygenators. Hepatocytes or microcarrier-attached hepatocytes are cultured either inside the hollow fibers or in the extra-fiber spaces, and the patient s blood is passed outside or inside the fibers. A bioartificial liver of this type, using 1.5 mm o.d. hollow fibers with 1.5 mm clearance between them, and with tissue-like aggregates of animal hepatocytes cultured in the extra-fiber spaces, can maintain liver functions for a few months [20]. 

In a hollow-fiber-type hemodialyzer, 200 cm rniri of blood (inside fibers) and 500 cm rniro of dialysate (outside fibers) flow countercurrently. The urea concentrations of the inlet blood, outlet blood, and outlet dialysate are 100, 20, and 32 mg dH, respectively. Calculate the clearance for urea. 

Figure 12.5 Clearance, a measure of membrane permeability, as a function of molecular weight for hemodialyzers and the normal kidney [7]...

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... 

The clearance can be easily calculated from Equation 18.4 and the toxin mass balance on the hemodialyzer, which is... 

Decreasing the fiber inner diameter while increasing the number of fibers to keep the total area constant will both decrease the blood diffusive resistance RB and increase diffusive clearance and also raise the blood pressure drop and the ultrafiltration in the upstream part of the membrane [14]. In fact, it is possible by creating a high U F in the first half of the filter and a back filtration in the second half to simulate HDF conditions in a hemodialyzer without the need of reinjection fluid. The fluid... 

Tsuruoka S, Sugimoto KI, Hayasaka T, et al. Ranitidine clearance during hemodialysis with high-flux membrane comparison of polysulfone and cellulose acetate hemodialyzers. Eur J Clin Pharmacol 2000 56 581-583. 

Cheung AK, Agodoa LY, Daugirdas JT, et al. Effects of hemodialyzer reuse on clearances of urea and beta2-microglobulin. The Hemodialysis (HEMO) Study Group. J Am Soc Nephrol 1999 10 117-127. 

Researchers at Oregon State University have demonstrated the advantages of microchannel architecture in improving the hemodialysis process. Using microchannel architectare, they were able to show 70-80% reductions in the necessary transfer area relative to commercial hollow fiber systems for the clearance of creatinin (Fig. 7.23) and urea (Fig. 7.24) from a simulated blood stream [285]. The microchannel advantage, as has been seen in other applications, comes in the form of well-defined and narrow channels that facilitate rapid mass transfer into and out of the fluid media. This approach is expected to change the current paradigm in hemodialysis from clinical treatment to at-home use, and may allow for the creation of a wearable hemodialyzer [286]. 

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),... 

Figure 19.3 shows a typical hemodialyzer. Device properties such as the fiber length, membrane surface area, number of fibers, hollow fiber packing density, and header design aU affect solute clearances. 

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