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The major forces responsible for solute transport across the membrane are diffusion and convection. Diffusion is influenced by the concentration gradient of the solute, the solute characteristics (eg, molecular weight and charge), and the membrane characteristics (eg, pore size and number). Removal of solutes by diffusion is enhanced by a large concentration gradient, small solute size, and a membrane with a large surface area and many large pores. The concentration gradient is maximized by using countercurrent flow of blood and dialysate.
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In convection, hydrostatic or osmotic pressure forces water across the membrane. The water transport facilitates the passage of solutes across the membrane. The term ultrafiltration describes the solute and fluid removal via convection.
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In hemodialysis, the predominant mechanism for solute removal is through diffusion, with a smaller amount of solute clearance occurring by convection. Thus, hemodialysis is very effective in removing solutes of small-molecular-weight, but is relatively inefficient in removing solutes of larger sizes. In addition, hemodialysis is an inefficient means of removal of protein-bound substances. Only the free portion of these solutes can diffuse across the membrane and be removed. The removal rate of protein-bound compounds thus depends on the concentration of the unbound solute, the size of the protein, and the replacement rate of the unbound solute.
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In hemofiltration, the predominant mechanism for solute removal is through convection; thus, this procedure has the ability to remove solutes of larger molecular size when coupled with a dialysis membrane with a large pore size.
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Indications for Starting Chronic Dialysis Therapy
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Patients should be considered for initiation of chronic hemodialysis therapy once the estimated glomerular filtration rate (GFR) is less than 15 mL/minute. In most patients, the four variables in the Modification of Diet in Renal Disease (MDRD) equation can be used to estimate the GFR. A 24-hour urine collection for creatinine and urea should be considered in those patients who have reduced muscle mass due to medical conditions such as amputations or limitation on mobility due to congestive heart failure, claudication, chronic lung disease requiring oxygen therapy, etc. There are no randomized trials that suggest an optimal time to initiate chronic dialysis therapy, so clinical judgment is important in making this decision in individual patients.
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Earlier Initiation of Dialysis
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There are specific indications for starting chronic hemodialysis therapy at a level above a GFR of 15 mL/minute. These conditions include intractable fluid overload not responsive to diuretics, hyperkalemia unresponsive to medical therapy, metabolic acidosis not fully corrected by medical therapy, malnutrition or weight loss not ascribed to other medical conditions, or decreasing functional status. It may also be desirable to start home dialysis therapies at a higher level of GFR to minimize training difficulties due to neurologic dysfunction at lower levels of GFR.
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Later Initiation of Dialysis
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Patients can be considered for a ...