In medicine

Medicine

Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....

, hemofiltration, also haemofiltration, is a renal replacement therapy

Renal replacement therapy

Renal replacement therapy is a term used to encompass life-supporting treatments for renal failure.It includes:*hemodialysis,*peritoneal dialysis,*hemofiltration and*renal transplantation.These treatments will not cure chronic kidney disease...

 similar to hemodialysis

Hemodialysis

In medicine, hemodialysis is a method for removing waste products such as creatinine and urea, as well as free water from the blood when the kidneys are in renal failure. Hemodialysis is one of three renal replacement therapies .Hemodialysis can be an outpatient or inpatient therapy...

 which is used almost exclusively in the intensive care setting. Thus, it is almost always used for acute renal failure

Acute renal failure

Acute kidney injury , previously called acute renal failure , is a rapid loss of kidney function. Its causes are numerous and include low blood volume from any cause, exposure to substances harmful to the kidney, and obstruction of the urinary tract...

.
It is a slow continuous therapy in which sessions usually last between 12 to 24 hours and are usually performed daily.
During hemofiltration, a patient's blood

Blood

Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 is passed through a set of tubing (a filtration circuit) via a machine to a semipermeable membrane

Semipermeable membrane

A semipermeable membrane, also termed a selectively permeable membrane, a partially permeable membrane or a differentially permeable membrane, is a membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally specialized "facilitated diffusion".The rate of...

 (the filter) where waste products and water are removed. Replacement fluid is added and the blood is returned to the patient.

The Principle of Hemofiltration

As in dialysis

Dialysis

In medicine, dialysis is a process for removing waste and excess water from the blood, and is primarily used to provide an artificial replacement for lost kidney function in people with renal failure...

, in hemofiltration one achieves movement of solutes across a semi-permeable membrane. However, solute movement with hemofiltration is governed by convection rather than by diffusion. With hemofiltration, dialysate is not used. Instead, a positive hydrostatic pressure drives water and solutes across the filter membrane from the blood compartment to the filtrate compartment, from which it is drained. Solutes, both small and large, get dragged through the membrane at a similar rate by the flow of water that has been engendered by the hydrostatic pressure. Thus convection overcomes the reduced removal rate of larger solutes (due to their slow speed of diffusion) seen in hemodialysis.

Replacement fluid composition

An isotonic

Isotonic

The term isotonic may refer to;*Isotonic for the term associated with muscle contraction*An isotone in nuclear physics*Sports drinks are sometimes designed in an isotonic way to assist athletes rehydrate while balancing electrolytes...

 replacement fluid is added to the blood to replace fluid volume and electrolytes. The replacement fluid must be of high purity, because it is infused directly into the blood line of the extracorporeal circuit. The replacement hemofiltration fluid usually contains lactate

Lactic acid

Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...

 or acetate as a bicarbonate-generating base, or bicarbonate itself. Use of lactate can occasionally be problematic in patients with lactic acidosis

Lactic acidosis

Lactic acidosis is a physiological condition characterized by low pH in body tissues and blood accompanied by the buildup of lactate especially D-lactate, and is considered a distinct form of metabolic acidosis. The condition typically occurs when cells receive too little oxygen , for example...

 or with severe liver disease, because in such cases the conversion of lactate to bicarbonate can be impaired. In such patients use of bicarbonate

Bicarbonate

In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

 as a base is preferred.

Hemodiafiltration

Hemofiltration is sometimes used in combination with hemodialysis, when it is termed hemodiafiltration. Blood is pumped through the blood compartment of a high flux dialyzer, and a high rate of ultrafiltration is used, so there is a high rate of movement of water and solutes from blood to dialysate that must be replaced by substitution fluid that is infused directly into the blood line. However, dialysis solution is also run through the dialysate compartment of the dialyzer. The combination is theoretically useful because it results in good removal of both large and small molecular weight solutes.

Intermittent vs. continuous modes of therapy

These treatments can be given intermittently, or continuously. The latter is usually done in an intensive care unit setting.

On-line intermittent hemofiltration (IHF) or hemodiafiltration (IHDF)

Either of these treatments can be given in outpatient dialysis units, three or more times a week, usually 3-5 hours per treatment. IHDF is used almost exclusively, with only a few centers using IHF. With both IHF or IHDF, the substitution fluid is prepared on-line from dialysis solution by running dialysis solution through a set of two membranes to purify it before infusing it directly into the blood line. In the United States, regulatory agencies have not yet approved on-line creation of substitution fluid because of concerns about its purity. For this reason, hemodiafiltration is almost never used in an outpatient setting in the United States as of 2007. Use of sterile, pre-packaged substitution fluid would be cost-prohibitive in the current economic environment.

Continuous hemofiltration (CHF) or hemodiafiltration (CHDF)

Hemofiltration is most commonly used in an intensive care unit setting, where it is either given as 8-12 hours treatments, so called SLEF (slow extended hemofiltration), or as CHF (continuous hemofiltration also sometimes called continuous veno-venous hemofiltration (CVVH)) or Continuous Renal Replacement Therapy (CRRT). Hemodiafiltration (SLED-F or CHDF or CVVHDF) also is widely used in this fashion. In the United States, the substitution fluid used in CHF or CHDF is commercially prepared, prepackaged, and sterile (or sometimes is prepared in the local hospital pharmacy), avoiding regulatory issues of on-line creation of replacement fluid from dialysis solution.

With slow continuous therapies, the blood flow rates are usually in the range of 100-200 ml/min, and access is usually achieved through a central venous catheter

Central venous catheter

In medicine, a central venous catheter is a catheter placed into a large vein in the neck , chest or groin...

 placed in one of the large central veins. In such cases a blood pump is used to drive blood flow through the filter. Native access for hemodialysis (eg AV fistulas

Arteriovenous fistula

An arteriovenous fistula is an abnormal connection or passageway between an artery and a vein. It may be congenital, surgically created for hemodialysis treatments, or acquired due to pathologic process, such as trauma or erosion of an arterial aneurysm....

or grafts) are unsuitable for CHF because the prolonged residence of the access needles required might damage such accesses.

Is on-line intermittent hemodiafiltration (IHDF) better than regular hemodialysis?

There is controversy about whether intermittent on-line hemodiafiltration (IHDF) gives better results than hemodialysis in an outpatient setting. In Europe, several observational studies have compared outcomes in patients getting dialysis with those getting IHDF. These have suggested a lower mortality rate and other favorable outcomes in patients getting IHDF vs. those getting ordinary hemodialysis. However, the issue is not settled at this time, because the required randomized controlled clinical trials have not been done. Another problem has been that in several of the trials done, IHDF was compared to dialysis using low-flux (small pore) membranes, and the benefit found may have been due more to the use of a high-flux membrane than to the addition of convective transport (filtration) to dialysis. A recent Cochrane database review of available trials could not find a definite benefit of either IHF or IHDF vs. hemodialysis in terms of outcomes.

External links

• http://vam.anest.ufl.edu/dialysis/ A free interactive simulation of a CVVH machine