Determinants Of Solute Removal In Dialysis Techniques For Acute Renal Failure

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IHD

CRRT

PD

Small solutes (MW <300)

Diffusion:

Diffusion:

Diffusion:

Qb

Qd

Qd

Membrane width

Convection:

Convection:

Qd

Qf

Qf

Middle molecules (MW 500-5000)

Diffusion

Convection:

Convection:

Convection:

Qf

Qf

Qf

SC

SC

SC

LMW proteins (MW 5000-50,000)

Convection

Convection

Convection

Diffusion

Adsorption

Adsorption

Large proteins (MW >50,000)

Convection

Convection

Determinants of solute removal in dialysis techniques for acute renal failure. Solute removal in these techniques is achieved by convection, diffusion, or a combination of these two. Convective techniques include ultrafiltration (UF) and hemofiltration (H) and they depend on solute removal by solvent drag [6]. As solute removal is solely dependent on convective clearance it can be enhanced only by increasing the volume of ultrafiltrate produced. While ultrafiltration requires fluid removal only, to prevent significant volume loss and resulting hemodynamic compromise, hemofiltration necessitates partial or total replacement of the fluid removed. Larger molecules are removed more efficiently by this process and, thus, middle molecular clearances are superior. In intermittent hemodialysis (IHD) ultrafiltration is achieved by modifying the transmembrane pressure and generally does not contribute significantly to solute removal. In peritoneal dialysis (PD) the UF depends on the osmotic gradient achieved by the concentration of dextrose solution (1.55% to 4.25%) utilized the number of exchanges and the dwell time of each exchange. In continuous arteriovenous and venovenous hemodialysis in most situations ulrafiltration rates of 1 to 3 L/hour are utilized; however recently high-volume hemofiltration with 6 L of ultrafiltrate produced every hour has been utilized to remove middle- and large-molecular weight cytokines in sepsis [12]. Fluid balance is achieved by replacing the ultrafiltrate removed by a replacement solution. The composition of the replacement fluid can be varied and the solution can be infused before or after the filter.

Diffusion-based techniques (hemodialysis) are based on the principle of a solute gradient between the blood and the dialysate. In IHD, typically dialysate flow rates far exceed blood flow rates (200 to 400 mL/min, dialysate flow rates 500 to 800 mL/min) and dialysate flow is single pass. However, unlike IHD, the dialysate flow rates are significantly slower than the blood flow rates (typically, rates are 100 to 200 mL/min, dialysate flow rates are 1 to 2 L/hr [17 to 34mL/min]), resulting in complete saturation of the dialysate. As a consequence, dialysate flow rates become the limiting factor for solute removal and provide an opportunity for clearance enhancement. Small molecules are preferentially removed by these methods. If both diffusion and convection are used in the same technique (hemodiafiltration, HDF) both dialysate and a replacement solution are used and small and middle molecules can both be easily removed.

Dialyste flow, L/h 1.5 1

Dialysis time Ultrafiltrate volume, Cycling Manual

4 h/d 4 h qod L/d treatment time, hrs

40 48

Dialysate inflow, L/wk 160 96

CAVHDF/CVVHDF IHD CAVH PD

FIGURE 19-9

Comparison of weekly urea clearances with different dialysis techniques. Although continuous therapies are less efficient than intermittent techniques, overall clearances are higher as they are utilized continuously. It is also possible to increase clearances in continuous techniques by adjustment of the ultrafiltration rate and dialysate flow rate. In contrast, as intermittent dialysis techniques are operational at maximum capability, it is difficult to enhance clearances except by increasing the size of the membrane or the duration of therapy. CAV/CVVHDF—continuous arteriovenous/venovenous hemodiafiltration; IHD—intermittent hemodialysis; CAVH—continuous arteriovenous hemodialysis; PD—peritoneal dialysis.

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