Hemofiltration is a therapy similar to hemodialysis, used to replace the function of the kidneys in the case of renal failure. Unlike hemodialysis, hemofiltration is nearly always used in intensive care settings in cases of acute renal failure. The therapy works by passing the patient's blood through a machine that filters out waste products and water, and then adds replacement fluid before returning the blood to the body. The replacement fluid maintains fluid volume in the blood and provides electrolytes.
Hemofiltration is a lengthy, continuous procedure, usually requiring 12 to 24 hours per session. It is usually performed daily for as long as necessary. Acute renal failure can be reversed if treatment begins early enough.
Hemofiltration mainly differs from dialysis in the method of filtration. Both procedures use a semi-permeable membrane to filter the blood, but hemofiltration uses convection along with diffusion, while dialysis uses diffusion alone. Convection allows positive fluid pressure to drive water and solutes through the filtering membrane. Diffusion is simply the random motion of particles in the blood, so larger solutes are not filtered as quickly as smaller ones because they move more slowly.
The convection method solves the problem inherent in diffusion alone, allowing solutes of all sizes to be filtered at a similar rate. The rate of filtration of both large and small solutes can be even more balanced by the use of hemofiltration and dialysis in tandem. This procedure is known as hemodiafiltration.
The replacement fluid used in hemofiltration contains either lactate or acetate to generate the electrolyte bicarbonate, or bicarbonate on its own. Lactate can cause problems for certain patients, such as those with liver disease or lactate acidosis, a condition in which the buildup of lactic acid in the bloodstream causes the blood to become acidic. Bicarbonate is commonly used for such patients.
Hemofiltration and hemodiafiltration may be performed either continuously or intermittently. The on-line, intermittent method can be used for outpatient treatment, while the continuous method is normally used in intensive care. The intermittent method prepares replacement fluid on-line by filtering the dialysis fluid, while the continuous method uses pre-packaged, sterile, commercially prepared replacement fluid.
Intermittent filtration therapies are not currently approved by the United States regulatory agencies, and hemodiafiltration is rarely used in outpatient care in the United States. In Europe, however, the practice is more common, though still somewhat controversial. Recent studies have suggested that intermittent hemodiafiltration may be a more effective therapy than dialysis, but more extensive studies, including randomized controlled clinical trials, are required before the issue can be settled.