Figure 64

Relationships between membrane efficiency and clearance and blood flow rates in hemodialysis. When prescribing the blood flow rate for a hemodialysis procedure the following must be considered: the relationship between the type of dialysis membrane used, blood flow rate, and clearance rate of a given solute. For a small solute such as urea (molecular weight, 60) initially a linear relationship exists between clearance and blood flow rates. Small solutes are therefore said to be flow-limited because their clearance is highly flow-dependent. At higher blood flow rates, increases in clearance rates progressively decrease as the characteristics of the dialysis membrane become the limiting factor. The efficiency of a dialyzer in removing urea can be described by a constant referred to as KoA, which is determined by factors such as surface area, pore size, and membrane thickness. Use of a high-efficiency membrane (KoA >600 mL/min) can result in further increases in urea clearance rates at high blood flow rates. In contrast, at low blood flow rates no significant difference exists in urea clearance between a conventional and a high-efficiency membrane because blood flow, and not the membrane, is the primary determinant of clearance.

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