Continuous Veno-venous Hemodialysis
Continuous veno-venous hemodialysis (CVVHD) - CVVHD is
similar to standard dialysis, however, instead of the short (generally
around 3 hour) intense treatment received under standard hemodialysis,
CVVHD is performed continuously over a longer period of time, up to
days. Additionally, the dialysis solution in CRRT generally comes from a pre-mixed bag that hospital staff will hang onto the CRRT machine. Unlike SCUF or CVVHF, CVVHD uses the dialysate to help convey impurities from the blood through the filter and into the effluent. Unlike CVVHF or CVVHDF, no replacement fluid is used. Figure 1 shows a simple, yet accurate, CVVHD setup.
Figure 1 - Diagram of a typical CVVHD setup including fluid pathways and components.
Anticoagulation is not shown in the diagram. At one time heparin was the clear favorite anticoagulant, now however, citrate is gaining and may be a more popular anticoagulant for use with CRRT therapies.
The hemofilter, or dialyzer, has two compartments, one for the patient's blood and one for
a washing fluid called dialysate. A thin membrane separates these two
parts. Blood cells, protein and other large molecules remain in the patient's
blood because they are too big to pass through the membrane. Smaller
waste products in the blood, such as urea, creatinine, potassium and
extra fluid pass through the membrane and into the effluent. In
typical hemodialysis the dialyzer is often reused, however, in CVVHD
and other CRRT therapies, the hemofilter (and all the rest of the
tubing) is generally used once and then disposed of, ensuring that the system is sterile at the start.
In CVVHD (and CRRT in general) the tubing has colored stripes to help identify its purpose.
- Red striped tubing is the access line, it moves blood from the patient to the hemofilter.
- Blue striped tubing is the return line, it is used to convey blood from the hemofilter back to the patient.
- Green striped tubing is the dialysate line, it moves the dialysate from the source into the hemofilter.
- Yellow striped tubing is the effluent line, it removes the effluent from the dialyzer into a waste or effluent bag.
There may be clear tubing that moves the anticoagulant into the tubing circuit. Certain sections of tubing may be clear to fill certain purposes, clear tubing works best for air detectors and blood detectors, so tubing that goes through those is generally clear, the pump tubing sections are made from a more durable material and are usually clear as well.
The various fluids are moved through the tubing set by roller or peristaltic pumps. Special more durable sections of the tubing are inserted into these pumps and the pumps move the fluid through the tubing. The pumps do not contact the blood or other fluids in any way.