Continuous Veno-venous Hemofiltration
Continuous veno-venous hemofiltration (CVVHF) - CVVHF is similar to standard hemofiltration, however, instead of the short intense treatment received under standard hemodialysis, CVVHF is performed continuously over a longer period of time, up to days. CVVHF is generally given in an intensive care setting over an 8+ hour time period. Standard hemofiltration is usually done in a clinic.
In hemofiltration solutes are moved across the dialyzer (hemofilter) membrane through convection rather than by diffusion, unlike hemodialysis or CVVHD. With hemofiltration or CVVHF, dialysate is not used. Instead, a positive pressure drives water and solutes across the filter membrane from the blood compartment to the filtrate compartment of the dialyzer, from which it is removed through the effluent line and disposed of. Solutes, both small and large, get passed through the membrane at a similar rate by the flow of water due to the pressure difference. Convection overcomes the reduced removal rate of larger solutes (due to their slower speed of diffusion) seen in hemodialysis.
During hemofiltration, replacement fluid is added to the blood to replace fluid volume and electrolytes. The replacement fluid is of high purity, because it is infused directly into the blood line of the extracorporeal circuit. The replacement fluid solutions in CRRT generally come from a pre-mixed bag that hospital staff hangs onto the CRRT machine. The replacement fluid usually contains lactate or acetate as a bicarbonate-generating base, or bicarbonate itself. Use of lactate can occasionally be problematic in patients with lactic acidosis or with severe liver disease, because in such cases the conversion of lactate to bicarbonate can be impaired. In such patients use of bicarbonate as a base is preferred. Figure 1 shows a typical hemodialysis set up for comparison.
Figure 1 - Typical hemodialysis set up.
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 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
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 CVVHF (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.
- 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.