Continuous Renal Replacement Therapy: Modalities and Their Selection

Endogenous toxins accumulate in blood as a result of many biochemical processes. If their concentration exceeds certain levels, they cause illness. Some toxins are volatile (CO ₂ , ketones) and can be excreted by the lungs through ventilation, others are lipophilic (bile acids, bilirubin) and can be excreted by the liver via the biliary system, and others are water soluble and nonvolatile and are excreted by the kidneys ( Table 166.1 ). When the kidneys fail acutely, removal of such water-soluble toxins requires acute artificial renal replacement therapy (RRT).

Water-soluble toxins exist in blood at various concentrations. Blood is a complex fluid containing cells and plasma. Plasma is a complex solution, and it is the plasma compartment of blood that is available for purification by RRT. Plasma contains myriad solutes (electrolytes, proteins, lipids, carbohydrates, vitamins, amino acids), which are dissolved in plasma water (the solvent). Only those solutes that are water soluble and not protein bound (free solutes) are available for removal by classic RRT. This is because the conventional biosynthetic membranes used for RRT have an in vivo cutoff point of about 15 to 20 kilodaltons (kD), which does not allow the passage of anything beyond small proteins (such as β ₂ -microglobulin). Accordingly, the following discussion of blood purification principles and techniques relates to free solutes of relatively small to small-medium molecular weight (<15 kD). The chapter does not discuss peritoneal dialysis, in which the dialyzing membrane is the peritoneum and some larger proteins are removed during the blood purification process. Also not discussed in this chapter are different forms of plasma therapies in which protein-bound solutes can be removed through high-porosity membranes.

Extracorporeal techniques are broadly named RRT and include continuous or intermittent hemofiltration, hemodialysis, or hemodiafiltration, each with its own technical variations. All of these techniques rely on the principle of removing unwanted solutes and water through a membrane separation process.