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Introduction

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  • Continuous renal replacement therapies have come into increasing use in intensive care units (ICUs), and a variety of alternatives are available, ranging from fluid-removal approaches to continuous dialysis.

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Definitions and Terms

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  • ▪  Dialysis: Removal of waste products and fluid from the blood.
  • ▪  Ultrafiltration: Removal of excess fluid from the blood.
  • ▪  Convection: Movement of solutes and fluid across a semipermeable membrane across which there is a pressure gradient—effective for removal of fluid and certain molecules.
  • ▪  Diffusion: Movement of (typically small) solutes (like urea) along a concentration gradient from an area of high concentration (the blood) into an area of low concentration (the dialysate).
  • ▪  Continuous venovenous hemofiltration (CVVH): Convective dialysis which is very efficient at fluid and cytokine removal (Figure 50-1).
  • ▪  Continuous venovenous hemodialysis (CVVHD): Diffusive dialysis where the dialysate runs “countercurrent” to the blood (Figure 50-2).
  • ▪  Continuous venovenous hemodiafiltration (CVVHDF): Combination of convective and diffusive dialysis which is common in the ICU and very effective at fluid and solute removal (Figure 50-3).
  • ▪  Continuous arteriovenous hemodialysis: A largely archaic method of continuous dialysis in which arterial blood is processed prior to being reinfused into a vein.

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Figure 50-1.
Graphic Jump Location

CVVH circuit showing the use of an outflow resistor to modify the pressure gradient across the membrane.

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Figure 50-2.
Graphic Jump Location

CVVHD circuit showing countercurrent dialysis fluid flow relative to blood flow.

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Figure 50-3.
Graphic Jump Location

CVVHDF circuit showing the combined use of a resistor and countercurrent dialysis to remove fluid and soluted.

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Techniques

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  • ▪  Prior to initiation of continuous venous dialysis, a special double lumen catheter is placed in a central vein.
  • ▪  Vascular cannulation is performed as per the chapter on central line placement (Chapter 34) with the exception that a larger specialty catheter is placed in the vein (Figures 50-4, 50-5, 50-6, and 50-7).
  • ▪  One lumen of the dialysis catheter is treated as the arterial lumen, for flow proceeding from patient into dialysis machine, whereas the second “venous” lumen is used to return blood to the patient (Figure 50-8).
  • ▪  Blood is routed through a dialysis machine and the selected dialytic method applied (Figure 50-9).
  • ▪  The effluent (equivalent of urine) is measured in the medical record (Figure 50-10).
  • ▪  Blood running through the system is often heated to prevent (Figure 50-11) patient cooling.

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Figure 50-4.
Graphic Jump Location

Insertion of a large bore dilator over a wire in a Seldinger exchange.

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