- 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.
- ▪ Dialysis: Removal of waste products and fluid from
- ▪ 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
- ▪ 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.
CVVH circuit showing the use of an outflow resistor to
modify the pressure gradient across the membrane.
CVVHD circuit showing countercurrent dialysis fluid flow
relative to blood flow.
CVVHDF circuit showing the combined use of a resistor
and countercurrent dialysis to remove fluid and soluted.
- ▪ 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.
Insertion of a large bore dilator over a wire in a Seldinger
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