Continuous Renal Replacement Therapy: Who, When, Why, and How

Abstract

Continuous renal replacement therapy (CRRT) is commonly used to provide renal support for critically ill patients with acute kidney injury, particularly patients who are hemodynamically unstable. A variety of techniques that differ in their mode of solute clearance may be used, including continuous venovenous hemofiltration with predominantly convective solute clearance, continuous venovenous hemodialysis with predominantly diffusive solute clearance, and continuous venovenous hemodiafiltration, which combines both dialysis and hemofiltration. The present article compares CRRT with other modalities of renal support and reviews indications for initiation of renal replacement therapy, as well as dosing and technical aspects in the management of CRRT.

Keywords: acute kidney injury; continuous renal replacement therapy; dialysis; hemodialysis; hemofiltration.

Figure 1

A-C, Schematic diagrams of modalities of continuous renal replacement therapy. A, Continuous hemofiltration. Blood flow through the hemofilter is shown from left to right. An ultrafiltrate is generated across the hemofilter membrane, and excess ultrafiltrate above the volume desired for negative fluid balance is replaced with prefilter and/or postfilter replacement solution. B, Continuous hemodialysis. Blood flow through the hemodialyzer is shown from left to right. Dialysate is perfused through the hemodialyzer on the opposite side of the membrane from the blood countercurrent to the direction of blood flow. The effluent consists of spent dialysate plus the volume of ultrafiltrate desired to achieve negative fluid balance. C, Continuous hemodiafiltration. Blood through the hemodiafilter is shown from left to right. As in continuous hemodialysis, dialysate is perfused through the hemodialyzer on the opposite side of the membrane from the blood countercurrent to the direction of blood flow. The effluent consists of spent dialysate plus ultrafiltrate. As in continuous hemofiltration, excess ultrafiltrate above the volume desired for negative fluid balance is replaced with replacement solution. In the figure, replacement solution is shown being infused postfilter; replacement solution can also be infused prefilter.

Figure 2

Convection and diffusion. A, Convection: solute transfer across the membrane occurs via entrainment of solutes in the bulk flow of water during ultrafiltration. Higher molecular weight solutes (larger symbols) and lower molecular weight (< 500-1,500 Daltons) solutes (smaller symbols) are transported across the membrane with equal efficiency until the molecular radius of the solute exceeds the membrane pore size. B, Diffusion: solute transfer across the membrane occurs by movement down a concentration gradient from blood to dialysate. Lower molecular weight (< 500-1,500 Daltons) solutes (smaller symbols) cross the membrane more readily than higher molecular weight solutes (larger symbols).