Haemofiltration is a specialized medical treatment used in intensive care units to support patients whose kidneys are not working properly. This therapy helps remove excess fluid and waste products from the blood when the body cannot do so on its own, providing vital support during critical illness.

Understanding Haemofiltration

Haemofiltration, also spelled hemofiltration, is a type of renal replacement therapy that substitutes for the normal filtering function of the kidneys. This treatment is almost exclusively used in intensive care settings for patients with acute kidney injury, which is a sudden loss of kidney function that occurs over hours or days rather than the slow decline seen in chronic kidney disease.^[1][3]

During haemofiltration, blood is removed from the patient’s body through a tube, passed through a special filtering device called a hemofilter, and then returned to the patient after waste products and excess water have been removed. The filter acts as an artificial kidney, performing many of the functions that damaged kidneys cannot accomplish on their own.^[2][3]

Unlike regular hemodialysis, which relies primarily on a process called diffusion to remove waste, haemofiltration uses a different principle called convection. In convection, positive pressure forces water and dissolved substances through the filter membrane together, similar to how water carries dirt through a strainer. This means that both small molecules and larger molecules are removed at similar rates, as they are dragged along by the flow of water created by the pressure.^[3][7]

How Haemofiltration Works

The technical process of haemofiltration involves several key components working together. Blood flow is controlled by a pump that moves blood from the patient through the circuit at a carefully regulated speed. For most patients, the blood flow rate is targeted at about three to five milliliters per kilogram of body weight per minute.^[6][15]

The heart of the system is the semipermeable membrane inside the filter. This membrane has tiny pores that allow water and small to medium-sized molecules to pass through while keeping blood cells and large proteins like albumin inside the blood compartment. The size of these pores determines what can and cannot be filtered out.^[3]

As blood passes through the filter, positive pressure pushes fluid and dissolved waste products across the membrane into a collection compartment. This filtered fluid, called ultrafiltrate, contains water, electrolytes, urea, creatinine, and other waste substances that need to be removed from the body.^[7]

Because haemofiltration removes large volumes of fluid from the blood, this fluid must be replaced to prevent dehydration and maintain proper blood volume. A specially prepared sterile solution called replacement fluid is infused directly into the bloodstream. This fluid contains the right balance of salts and minerals that the body needs, but without the waste products that were removed.^[3][4]

The replacement fluid can be added at two different points in the circuit. When added before the blood enters the filter, it is called pre-filter dilution or pre-dilution. When added after the blood exits the filter, it is called post-filter dilution or post-dilution. Pre-filter dilution helps prevent the filter from clotting but reduces treatment efficiency because the blood is diluted before filtering. Post-filter dilution is more efficient but may increase the risk of the filter becoming clogged.^[3][10]

Access to the Bloodstream

To perform haemofiltration, doctors need a way to remove blood from the body and return it after filtering. This requires placing a special tube called a central venous catheter into a large vein, typically in the neck, chest, or groin area. Modern haemofiltration uses what are called venovenous circuits, meaning blood is taken from a vein and returned to a vein.^[6][10]

In the past, blood was taken from an artery and returned to a vein, using the natural pressure difference between arteries and veins to move blood through the circuit. However, this approach caused significant complications related to arterial puncture and has been largely abandoned. The introduction of blood pumps eliminated the need for arterial access, making the treatment much safer for patients.^[6][10]

The size of the catheter used depends on the patient’s size and weight. For infants weighing less than three kilograms, small five-French catheters are used, while adults typically require fourteen-French catheters or larger. The catheter size influences how much blood can flow through the circuit each minute.^[15]

Continuous Versus Intermittent Treatment

Haemofiltration can be delivered in different time patterns depending on the patient’s needs and stability. Continuous hemofiltration, often abbreviated as CHF or CVVH (continuous venovenous hemofiltration), runs twenty-four hours a day and is most commonly used in intensive care units. This approach removes fluid and waste slowly and steadily, similar to how natural kidneys work throughout the day.^[6][7]

The slow, continuous nature of this treatment is especially important for critically ill patients who are hemodynamically unstable, meaning their blood pressure is fragile or they require medications to support their circulation. Rapid removal of fluid, as occurs with intermittent treatments, can cause dangerous drops in blood pressure. Continuous therapy avoids this problem by working gradually over many hours.^[2][10]

Some centers also use intermittent hemofiltration, which runs for eight to twelve hours at a time rather than continuously. This approach, sometimes called slow extended hemofiltration or SLEF, offers some of the gentleness of continuous therapy while requiring less total treatment time.^[7]

In countries like Australia and many parts of Europe, continuous renal replacement therapy using haemofiltration or related techniques has become the dominant or exclusive form of kidney support in intensive care units. In some institutions, it has almost completely replaced traditional intermittent hemodialysis for critically ill patients.^[6]

Preventing Filter Clotting

One of the major challenges with haemofiltration is preventing the blood from clotting inside the filter and tubing. When blood leaves the body and contacts artificial surfaces, it naturally tends to form clots as part of the body’s normal protective mechanisms. If the circuit clots, the treatment must be stopped and the entire system replaced, resulting in downtime that reduces the effectiveness of therapy and increases costs.^[1][14]

To prevent clotting, most patients receive medications called anticoagulants during haemofiltration. The two most commonly used approaches are systemic heparin and regional citrate anticoagulation.^[14]

Systemic heparin is given into the blood before it enters the filter, preventing clotting throughout the entire circuit and in the patient’s body. The main concern with this approach is that it increases the risk of bleeding, which can be particularly dangerous in critically ill patients who may already have clotting abnormalities.^[14][15]

Regional citrate anticoagulation works differently by adding citrate to the blood just before it enters the filter. Citrate prevents clotting by binding to calcium, which is necessary for blood to clot. After the blood exits the filter, calcium is added back to restore normal clotting ability before the blood returns to the patient. This approach keeps anticoagulation limited to the circuit itself rather than affecting the whole body, reducing bleeding risk. Studies have shown that regional citrate can extend filter life by approximately eleven hours compared to systemic heparin.^[1][14][15]

However, about one-quarter of critically ill patients receiving haemofiltration do not receive any anticoagulation at all. This may occur when patients are considered at very high risk for bleeding due to recent surgery, trauma, or severe clotting abnormalities. While avoiding anticoagulation eliminates bleeding risk from these medications, filters tend to clot more quickly, requiring more frequent replacement.^[14]

Indications for Haemofiltration

Haemofiltration is used to treat several urgent medical problems related to kidney failure or severe metabolic disturbances. The most common reason is acute kidney injury in critically ill patients. Acute kidney injury occurs frequently in intensive care units, affecting about forty percent of critically ill patients. Of these, roughly seventeen to twenty-four percent require some form of renal replacement therapy during their intensive care stay.^[6][14]

Haemofiltration helps correct water overload when the body retains too much fluid and the kidneys cannot eliminate it through normal urine production. This excess fluid can accumulate in the lungs, causing breathing difficulties, or throughout the body, causing swelling. Removing this fluid is often necessary to allow administration of essential medications, nutrition, and blood products that patients need during critical illness.^[2][15]

The treatment also corrects dangerous imbalances in blood chemistry, including removal of excess potassium, which can cause fatal heart rhythm problems, and treatment of severe metabolic acidosis, where the blood becomes too acidic. Haemofiltration removes waste products like urea that accumulate when kidneys fail, and can even help eliminate certain poisons or drug overdoses from the bloodstream.^[15]

Some researchers are investigating whether haemofiltration might help treat sepsis and septic shock, life-threatening conditions where the body’s response to infection causes widespread inflammation. The theory is that haemofiltration might remove inflammatory molecules called cytokines that contribute to organ damage during sepsis. However, this remains an area of active research, and standard treatment for sepsis does not routinely include haemofiltration unless kidney failure develops.^[6][7]

Hemodiafiltration: A Combined Approach

Haemofiltration is sometimes combined with hemodialysis in a hybrid treatment called hemodiafiltration. This approach uses both convection (the principle of haemofiltration) and diffusion (the principle of hemodialysis) simultaneously to remove waste products.^[3][7]

In hemodiafiltration, blood flows through one side of the filter membrane while a dialysate solution flows along the other side in the opposite direction. This allows small molecules to move across the membrane by diffusion while convection removes larger molecules. The combination theoretically provides good removal of both small and large molecular weight substances.^[3][7]

This versatility makes hemodiafiltration popular in some intensive care units, where it can be adjusted based on the specific needs of individual patients. Modern continuous renal replacement therapy machines can easily switch between pure haemofiltration, pure hemodialysis, or hemodiafiltration with simple adjustments to the setup.^[1]

Advantages and Limitations

One significant advantage of haemofiltration over regular hemodialysis is its ability to remove larger molecules more effectively. Diffusive therapy removes small solutes very efficiently but struggles with larger compounds. In contrast, convective therapy removes both small and large molecules that can pass through the membrane pores at similar rates because they are carried along with water flow.^[1][3]

This characteristic is particularly valuable for removing substances like myoglobin, a muscle protein that can accumulate and damage kidneys in conditions like crush injuries, or potentially inflammatory cytokines during sepsis. The improved removal of these middle-sized and larger molecules represents a theoretical advantage of haemofiltration.^[1][7]

However, haemofiltration requires very high rates of fluid flow across the membrane to be effective. This creates the risk of hemoconcentration inside the filter, where the blood becomes thick as fluid is removed, predisposing it to clotting. This is one reason why anticoagulation strategies are so important and why the choice between pre- and post-dilution matters.^[1]

Despite these technical differences and theoretical benefits, large clinical studies have not conclusively demonstrated that haemofiltration produces better patient outcomes than hemodialysis. A systematic review and analysis of clinical trials comparing the two approaches found no significant differences in mortality, recovery of kidney function, organ dysfunction, or need for blood pressure support medications.^[1]

Current clinical guidelines from organizations like KDIGO (Kidney Disease Improving Global Outcomes) and the UK Renal Association acknowledge that there is insufficient evidence to recommend one approach over the other. The choice between haemofiltration and hemodialysis therefore depends on the individual patient’s condition, the expertise of the medical and nursing staff, and what equipment and resources are available at each hospital.^[1]

Patient Monitoring During Treatment

Patients receiving haemofiltration require close monitoring to ensure the treatment is safe and effective. Blood tests to check levels of sodium, potassium, chloride, bicarbonate, calcium, and glucose are typically performed every four hours during treatment. If these values are abnormal at the start, they may need to be checked every hour initially.^[15]

Other important minerals like magnesium and phosphate are usually checked twice daily. These substances can be lost during haemofiltration and may need to be supplemented to prevent deficiencies.^[15]

Managing sodium levels requires special attention, particularly in patients who start treatment with very low sodium (hyponatraemia) or very high sodium (hypernatraemia). Correcting these imbalances too quickly can cause serious brain complications, including pontine myelinosis or cerebral swelling. To prevent this, the sodium concentration in the replacement fluid can be adjusted by adding concentrated salt solution or sterile water, allowing gradual correction over twenty-four hours rather than rapid changes.^[9]

Drug Dosing During Haemofiltration

Medications given to patients receiving haemofiltration may need dose adjustments because the treatment can remove drugs from the bloodstream. However, no form of renal replacement therapy is as effective as healthy kidneys at removing substances, so medication doses used during haemofiltration will never exceed those used in people with normal kidney function.^[4]

Several factors determine whether a drug will be removed by haemofiltration. Medications that are highly bound to proteins in the blood are generally not removed because the protein-drug complex is too large to pass through the filter membrane. Very large drug molecules are also less likely to cross the membrane. Water-soluble drugs enter the dialysate solution more readily than fat-soluble drugs, which tend to distribute widely throughout body tissues rather than remaining in the bloodstream.^[4]

Interestingly, drugs that are normally removed by the kidneys are usually also removed by haemofiltration. In the absence of specific dosing guidelines for a particular drug during haemofiltration, physicians typically dose medications as if the patient has moderate kidney impairment, equivalent to a glomerular filtration rate of about fifteen to twenty-five milliliters per minute.^[4]

For continuous treatments that run around the clock, there is no need to time medication doses around treatment sessions. However, for intermittent hemodialysis, medications should ideally be given after the dialysis session ends to prevent the drug from being removed before it has time to work.^[4]

Current Use and Future Directions

The adoption of haemofiltration and continuous renal replacement therapy varies considerably around the world. Usage ranges from minimal in the United States to about three percent in the United Kingdom, six percent in Australia, and nearly twenty-nine percent in Belgium.^[1]

This wide variation reflects ongoing uncertainty about the optimal way to provide kidney support in intensive care. Research continues to explore fundamental questions about when to start treatment, how intensive it should be, and whether particular approaches offer advantages for specific patient groups.^[1][6]

The evolution of haemofiltration technology continues as researchers develop specialized filters and modified circuits that might enhance the removal of inflammatory molecules during sepsis or multiple organ failure. These experimental approaches represent the concept of blood purification beyond simple kidney replacement, potentially offering new ways to support critically ill patients.^[6]

Since continuous haemofiltration was first described in 1977 for treating fluid overload unresponsive to medications, the field has undergone remarkable technical and conceptual advancement. Specially designed machines are now widely available, and the therapy has moved from simple experimental circuits to sophisticated systems that allow precise control of treatment intensity and composition.^[6][7]

As research continues and technology advances, haemofiltration and related techniques will likely play an increasingly important role in supporting critically ill patients, though many questions about optimal use remain to be answered through well-designed clinical studies.