Haemofiltration

Haemofiltration is a medical procedure used to clean the blood when the kidneys cannot work properly, particularly in intensive care units where patients need continuous support for acute kidney problems.

Table of contents

• What is Haemofiltration
• How Haemofiltration Works
• When Haemofiltration is Used
• Continuous Venovenous Haemofiltration
• Replacement Fluid
• Hemodiafiltration
• Comparison with Hemodialysis
• Preventing Filter Clotting

haemofiltration, hemofiltration

What is Haemofiltration

Haemofiltration is a renal replacement therapy (a treatment that takes over the work of the kidneys) used almost exclusively in intensive care units. It is a form of treatment for acute kidney injury, which is a sudden loss of kidney function that happens over hours or days rather than months or years^[1][3].

During haemofiltration, a patient’s blood is passed through a set of tubing via a machine to a semipermeable membrane (a filter that allows some substances to pass through while blocking others). At this filter, waste products and water are removed from the blood and collected as ultrafiltrate. A specially prepared replacement fluid is then added to the blood, which is returned to the patient^[3][7].

How Haemofiltration Works

Haemofiltration works differently from regular dialysis. The main difference is that haemofiltration uses convection rather than diffusion to remove waste products from the blood. In convection, positive pressure pushes water and dissolved substances through the membrane together. This means that small molecules like urea and larger molecules like certain proteins move across the filter at a similar rate, carried along by the flow of water^[3][7].

Unlike hemodialysis, haemofiltration does not use dialysate (a special cleaning fluid on the other side of the membrane). Instead, it relies on high rates of fluid removal across the filter membrane, driven by hydrostatic pressure^[3].

Because convection removes both small and large molecules effectively, haemofiltration can eliminate larger compounds that diffusion-based treatments struggle with, such as myoglobin (a protein released from damaged muscle) or cytokines (molecules involved in inflammation)^[1][3].

When Haemofiltration is Used

Haemofiltration is primarily used to treat patients with acute kidney injury in the intensive care setting. It may be beneficial in patients who also have multiple organ dysfunction syndrome or sepsis^[7].

The procedure is particularly useful when patients are hemodynamically unstable, meaning their blood pressure and circulation are not stable. Haemofiltration removes water and waste products slowly and continuously, which is much gentler on the body than rapid removal. This makes it better tolerated by critically ill patients compared to intermittent hemodialysis^[2][4].

Haemofiltration can also be used to correct fluid overload, remove excess electrolytes, manage acid-base balance problems, and remove toxins or drugs in cases of poisoning^[2].

Continuous Venovenous Haemofiltration

In modern intensive care units, haemofiltration is most commonly performed as continuous venovenous haemofiltration, abbreviated as CVVH or CVVHF. This is also sometimes called continuous renal replacement therapy (CRRT)^[6][7].

The term “venovenous” means that blood is taken from a vein and returned to a vein. Early versions of this therapy used arterial access (taking blood from an artery), but this approach caused complications. Modern systems use a blood pump to control blood flow through the circuit, eliminating the need for arterial access^[2][6].

Continuous haemofiltration typically runs slowly but steadily over 24 hours, removing water and wastes at a rate more consistent with normal kidney function^[2][7]. Sessions usually last between 12 to 24 hours and are performed daily^[3].

Replacement Fluid

Because haemofiltration removes large volumes of fluid from the blood, this fluid must be replaced with a specially prepared solution called replacement fluid. The replacement fluid is of high purity and contains the right balance of salts and other substances the body needs^[3].

The composition of replacement fluid varies depending on the patient’s needs. A typical non-citrate replacement fluid contains sodium, calcium, magnesium, chloride, bicarbonate, and lactate in carefully controlled amounts^[15].

The replacement fluid can be added either before the blood enters the filter (pre-dilution) or after it leaves the filter (post-dilution). Pre-dilution reduces the risk of the filter clotting but makes the treatment slightly less efficient. Post-dilution is more efficient but can lead to blood becoming too concentrated in the filter, increasing the risk of clotting^[1][15].

Hemodiafiltration

Haemofiltration is sometimes combined with hemodialysis in a technique called hemodiafiltration. In this approach, dialysate is run through the dialysate compartment of the filter while replacement fluid is also given. This combination is designed to provide good removal of both small and large molecular weight substances^[3][7].

Continuous venovenous hemodiafiltration (CVVHDF) uses both diffusion and convection principles. The addition of dialysate flow enhances the clearance of small and middle-sized molecules^[2][15].

Comparison with Hemodialysis

The main difference between haemofiltration and hemodialysis lies in how they remove waste products. Hemodialysis relies primarily on diffusion, where substances move from areas of high concentration to low concentration across a membrane. This makes hemodialysis very efficient at removing small molecules but less effective at eliminating larger compounds^[1][3].

Haemofiltration uses convection, where substances are carried across the membrane by water flow. This allows more effective removal of larger molecules that are difficult to eliminate by diffusion alone^[1][3].

Despite these technical differences, clinical studies have not found clear evidence that one method produces better patient outcomes than the other. A systematic review found no difference in mortality or other clinical outcomes such as recovery of kidney function between haemofiltration and hemodialysis^[1]. Current medical guidelines state that the choice between these modalities should be based on the individual patient’s clinical condition, the expertise of medical and nursing staff, and local availability^[1].

Preventing Filter Clotting

A major challenge with haemofiltration is preventing blood from clotting in the filter and tubing. Without preventive measures, the filter can clot, causing treatment downtime and potentially undertreating the patient^[14].

Two main approaches are used to prevent clotting. The first is regional citrate anticoagulation, where citrate is added to the blood before it enters the filter and then removed afterward. The second is systemic heparin, a blood-thinning medication given to the patient^[1][15].

Studies have shown that regional citrate anticoagulation can extend filter life by about 11 hours compared to systemic heparin^[14]. However, in some critically ill patients who are at high risk of bleeding, doctors may choose to run haemofiltration without any anticoagulation at all^[14].

Other factors that affect how long a filter lasts include the rate of blood flow through the circuit, the type of filter used, and the location and type of catheter used to access the blood vessels^[14].