Continuous Renal Replacement Therapy

Mark J. Acierno, Baton Rouge, Louisiana

Continuous renal replacement therapy (CRRT) is a relatively new blood purification modality that is rapidly gaining acceptance for the treatment of azotemia as well as acid-base, electrolyte, and fluid imbalances associated with acute kidney injury (AKI). As its name implies, CRRT is a continuous modality, and once a patient begins treatment, therapy continues until renal function returns or the patient is transitioned to intermittent dialysis. Potential advantages of CRRT over intermittent dialysis include better control of acid-base and electrolyte balance. CRRT also is appealing because it does not require a significant investment in water purification facilities and maintenance, as is the case with intermittent dialysis. This chapter provides a brief overview of the principles, methodology, and indications for CRRT in dogs and cats.

Principles and Mechanisms

All extracorporeal blood purification takes places within the dialyzer, where the patient’s blood is divided and directed into thousands of strawlike semipermeable membranes. Traditional hemodialysis primarily is a diffusive modality in which the strawlike semipermeable membranes are bathed in dialysate solution. Substances that are in higher concentration in the blood, such as urea, diffuse across the membrane into the dialysate, whereas substances that are in higher concentration in the dialysate, such as bicarbonate, leave the dialysate and enter the blood. Movement of individual molecules in or out of the blood can be controlled by adjusting the concentration of a given substance in the dialysate.

CRRT uses diffusion as well as convection and adhesion. Convection exposes blood traveling through the strawlike semipermeable membranes to a positive transmembrane pressure. This forces fluid, called ultrafiltrate, out of the blood. Uremic toxins, electrolytes, and other small molecules are carried with the fluid and then discarded. A sterile balanced electrolyte solution is added to the blood to replace the ultrafiltrate. The primary advantage of convection is its ability to remove larger molecules from the blood than can diffusion; however, convection is technically challenging because the fluid removed must be replaced with accuracy or acid-base, electrolyte, and fluid imbalances can occur rapidly.

Adhesion is the tendency of molecules to adhere to the dialyzer’s semipermeable membrane. For example, inflammatory cytokines can be removed by adhesion from the blood of septic patients receiving CRRT; however, it is not clear if this actually improves patient survival.

Continuous Renal Replacement Therapy Modalities

Although CRRT often is thought of as a single treatment modality, it actually combines the principles of diffusion, convection, and adhesion to provide four different therapies: slow continuous ultrafiltration, continuous venovenous hemofiltration, continuous venovenous hemodialysis, and continuous venovenous hemodiafiltration. Many CRRT systems also can perform therapeutic plasma exchange, which can be used in the treatment of autoimmune diseases and may be helpful in removing some protein-bound toxins.

Slow continuous ultrafiltration (SCUF) (Figure 192-1) is a purely convective modality in which a positive transmembrane pressure forces fluid (ultrafiltrate) out of the blood and the hemoconcentrated blood is returned to the patient. In human medicine, SCUF is used widely for the treatment of diuretic-resistant congestive heart failure. Its utility in veterinary medicine currently is under investigation.

Figure 192-1 Slow continuous ultrafiltration (SCUF) is a purely convective modality that generates ultrafiltrate that is not replaced. (Used with permission, Veterinary Learning Systems.)

Continuous venovenous hemofiltration (CVVH) (Figure 192-2) also is a purely convective modality; however, unlike SCUF, the ultrafiltrate is replaced with a sterile balanced electrolyte solution. The solution can be added to the blood before or after it passes through the dialyzer. In a postdialyzer configuration, a positive transmembrane pressure forces fluid out of the blood, which increases hemoconcentration. Before the patient’s blood is returned, replacement fluids are added to restore it to physiologic packed cell volume. This is an extremely effective way to remove uremic toxins; however, as the blood becomes hemoconcentrated, there is a risk that the dialyzer will sludge and clot. This limits the amount of fluid that can be removed by CVVH to approximately 25% of blood plasma volume. Alternatively, replacement fluids can be added to the blood before it passes through the dialyzer. The diluted blood then enters the dialyzer, where the excess fluid is removed by convection. Although this is less likely to result in dialyzer clotting, the prefilter arrangement is significantly less effective in removing uremic toxins.