• Continuous renal replacement therapy (CRRT) is an extracorporeal blood purification therapy intended to substitute for impaired renal function over an extended period of time for, or attempted for, 24 hours per day.

• CRRT can be accomplished through a variety of methods, with either arteriovenous access or venovenous access. The venovenous access is used almost exclusively because of its less invasive nature.^10,18 The following methods of CRRT are included as listed (details outlined in Table 112-1):

• Basic knowledge is required of the principles of diffusion, ultrafiltration (UF), osmosis, oncotic pressure, and hydrostatic pressure and how they pertain to fluid and solute management during dialysis.

• CRRT uses an artificial kidney (i.e., hemofilter, dialyzer) with a semipermeable membrane to create two separate compartments: the blood compartment and the dialysis solution compartment. The semipermeable membrane allows the movement of small molecules (e.g., electrolytes) and middle-size molecules (creatinine, vasoactive substances) from the patient’s blood into the dialysis solution but is impermeable to larger molecules (red blood cells, plasma proteins).

• Each dialyzer has four ports: two end ports for blood (in one end and out the other) and two side ports for dialysis solution ultrafiltrate (in one end and out the other). In most cases, the blood and dialysate are run through the dialyzer in opposite or countercurrent directions.

• With hollow-fiber dialyzers, the blood flows through the center of hollow fibers, and the dialysis solution (dialysate) flows around the outside of the hollow fibers. The advantages of hollow-fiber filters include a low priming volume, low resistance to flow, and high amount of surface area. The major disadvantage is the potential for clotting as a result of the small fiber size.

• All dialyzers have UF coefficients; thus, the dialyzer selected varies in different clinical situations.1–5 The higher the UF coefficient, the more rapid the fluid removal. UF coefficients are determined with in vivo measurements done by each dialyzer manufacturer.

• Clearance refers to the ability of the dialyzer to remove metabolic waste products or drugs from the patient’s blood. The blood flow rate, the dialysate flow rate, and the solute concentration affect clearance. Clearance occurs by the processes of diffusion, convection, and UF.

• The dialysate (when used during CRRT) is composed of water, a buffer (i.e., lactate or bicarbonate), and various electrolytes. Most solutions also contain glucose. The buffer helps neutralize acids that are generated as a result of normal cellular metabolism and that usually are excreted by the kidney. The concentration of electrolytes is usually the normal plasma concentration, which helps to create a concentration gradient for removal of excess electrolytes. The glucose aids in increasing the oncotic pressure in the dialysate (thus aiding in fluid removal) and in caloric replacement. Although glucose comes in various concentrations, it is usually used in normal plasma concentrations to prevent hyperglycemia.

• Heparin or citrate is often used during CRRT to prevent clotting of the circuit during treatment. Saline solution flushes can be used alone or with other anticoagulants to maintain circuit patency.3–7^,9

• An anticoagulant is used to maintain vascular access patency when CRRT is not in use.^3,10

• If the patient is taking angiotensin-converting enzyme (ACE) inhibitors, contact with certain filters or membranes in the CRRT system can cause an anaphylactic reaction and severe hypotension as a result of increased levels of bradykinin, a potent vasodilator. ACE inhibitors are recommended to be withheld for 48 to 72 hours before treatment, if possible.

• Continuous venovenous renal replacement therapy is achieved with a pumped system.

• The patient’s volume status and serum electrolyte levels are changed gradually so that patients have fewer problems than they do with hemodialysis. Specifics of these therapies are outlined in Table 112-1.^{1,2,4,7,13–15,17}

• SCUF (Fig. 112-1) is a nonpumped system, CVVH (Fig. 112-2), CVVHD (Fig. 112-3), and CVVHDF (Fig. 112-4) use pumped systems. These therapies are used to remove both plasma water and solutes and require venous access, most commonly provided with a double-lumen vascular access catheter (VAC). External arteriovenous (AV) hemodialysis shunts or surgically created AV hemodialysis anastomoses have been used in the past for CRRT; however, because of increased incidence rates of vascular injury, bleeding, and infection, they are not recommended for CRRT access.^1,10