Continuous veno-venous hemodiafiltration (CVVHDF) - CVVHDF is probably the most common type of CRRT used, it combines aspects of both CVVHD and CVVHF. In CVVHDF, dialysate and replacement fluid are both introduced into the circuit. Dialysate helps convey impurities from the blood through the filter into the effluent. The replacement fluid can be added to the blood pre or post hemofilter, it dilutes the blood and as the name implies, replaces some of the fluids that are being removed from the blood. The replacement fluid is used to help balance the patient's blood fluid level. Figure 1 is a typical hemodialysis set up for comparison to CVVHDF.
Figure 1 - Typical hemodialysis set up.
Not shown in the picture for CVVHDF is the dialysis solution line which connects into the dialyzer and the replacement fluid line which connects into the access line. The dialysis and replacement fluid solutions in CRRT generally comes from a pre-mixed bag that hospital staff hangs onto the CRRT machine. Also not shown, the effluent line connects into the other dialyzer port. Additionally, a couple things may be different depending on which hospital is performing the therapy. 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 blood access is shown connected to the patient's arm, this probably will not be true for CVVHDF, the most typical access device will be a venous catheter, although the principle is still the same.
The dialyzer, or hemofilter, has two paths, 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 CVVHDF
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.
In CVVHDF (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.
- Purple striped tubing is the replacement fluid line, it moves replacement fluid into the access (red striped) or return (blue striped) line.
- 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.
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.