Hemofiltration

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In medicine, hemofiltration, also haemofiltration, is a renal replacement therapy similar to hemodialysis which is used almost exclusivley in the intensive care setting. Thus, it is almost always used for acute renal failure. 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 is passed through a set of tubing (a filtration circuit) via a machine to a semipermeable membrane (the filter) where waste products and water are removed. Replacement fluid is added and the blood is returned to the patient.

1 The Principle of Hemofiltration
2 Access for Hemofiltration
3 Different Types of Hemofiltration
4 Hemodiafiltration as a Chronic Renal Replacement Therapy
5 See also
6 External links

[edit] The Principle of Hemofiltration

In a similar fashion to dialysis, hemofiltration involves the movement of solutes across a semi-permeable membrane. However, the membrane used in hemofiltration is far more porous than that used in hemodialysis, and no dialysate is used-instead a positive hydrostatic pressure drives water and solutes across the filter membrane where they are drained away as filtrate. An isotonic replacement fluid is added to the resultant filtered blood to replace fluid volume and valuable electrolytes. This is then returned to the patient. The process is slower and the fluid removal is gradual, when compared to hemodialysis, and less likely to result in hemodynamic compromise (hypotension). This makes it well suited to critical care patients who often have hypotension and are usually bedbound and frequently unconscious. A common scenario in which it is used is acute renal failure precipitated by septic shock.

[edit] Access for Hemofiltration

Much like hemodialysis access is required to process large volumes of the patient's blood. This is always achieved with a central venous catheter placed in one of the large central veins. As the hemofiltration process is slow, a central catheter must be used, even if the patient has alternative vascular access (eg. an arteriovenous fistula), as the slow flows and long filtration times are not viable in fistulas or grafts.

[edit] Different Types of Hemofiltration

Historically, some older hemofilters used an arterial catheter as primary access and utilised the arterial pressure to drive blood through the circuit. These types of filtration have been largely superseded due to complications and the labile nature of arterial blood pressure in critical care patients. Modern hemofilters are usually Continuous Veno-Venous Hemofilters (CVVHF) meaning that blood removed is from a vein, pressure is generated via a pump on the machine and the blood is returned to a vein.

The replacement hemofiltration fluid usually utilises lactate as a blood buffer. This can occasionally be problematic in patients with severe lactic acidosis. This can be circumvented by the use of bicarbonate as an alternative buffer. This is known as Bicarbonate Hemofiltration.

[edit] Hemodiafiltration as a Chronic Renal Replacement Therapy

Hemofiltration is sometimes used in combination with hemodialysis, when it is termed hemodiafiltration, as a chronic treatment in some centres. By using large quantities of substitution fluids (60-90 liters per treatment) toxins and solutes can be removed from the patient, such as beta-2-microglobulin which is much more efficiently removed with hemofiltration than hemodialysis.

Hemodialysis is very efficient at removing low molecular weight toxins and hemofiltration is more efficient at the removal of higher molecular weight toxins; hemodiafiltration describes the combination of a hemofilter and a hemodialyser in the same circuit. This has been done by the addition of a dialysis membrane to a standard hemofiltration circuit in the ITU setting, and also by the addition of a hemofilter to a modified hemodialysis machine where the replacement fluid is generated by the machine during the dialysis session (on-line hemodiafiltration). More efficient removal of middle weight toxins by the additional hemofiltration might be expected to translate into better outcomes for dialysis patients receiving this treatment, however despite ongoing trials, no evidence of such a benefit has been shown.