Haemofiltration is a medical treatment used primarily in intensive care settings to support patients whose kidneys are not functioning properly. This therapy removes waste products and excess fluid from the blood when the kidneys cannot do so on their own, offering a gentler alternative to traditional dialysis for critically ill patients.

Understanding Kidney Support in Critical Illness

When the kidneys fail suddenly due to injury or severe illness, the body cannot remove toxic waste products and excess water. This condition, known as acute kidney injury, is common in intensive care units, affecting roughly 40% of critically ill patients. Without treatment, these waste products accumulate to dangerous levels, threatening other organs and potentially leading to death.^[1]

Haemofiltration emerged as a way to replace kidney function temporarily while doctors address the underlying cause of kidney failure. Unlike some treatments that work quickly but can strain an already weakened body, haemofiltration works slowly and continuously, mimicking the natural pace at which healthy kidneys clean the blood. This gentler approach makes it particularly suitable for patients whose bodies cannot tolerate rapid changes in fluid or chemical balance.^[2]

The treatment goals focus on removing surplus water to prevent fluid overload in the lungs or tissues, eliminating toxic metabolic waste products like urea, correcting dangerous imbalances in blood chemistry such as too much potassium or severe acidity, and supporting the body while it heals. Treatment continues until the kidneys recover function or, in some cases, until a patient receives a kidney transplant.^[3]

How Haemofiltration Works

The basic principle of haemofiltration relies on convection rather than diffusion, which distinguishes it from traditional hemodialysis. During haemofiltration, blood is removed from the patient through a large tube called a catheter, usually placed in a large vein in the neck, groin, or upper chest. A pump then pushes this blood through a special filter containing a semi-permeable membrane with microscopic pores.^[3]

In the filter, pressure forces water and dissolved substances through the membrane pores. Small and medium-sized molecules—including waste products, excess salts, and even some larger compounds like inflammatory proteins called cytokines—get carried along with the water flow. This process, called convection, works somewhat like pouring water through a coffee filter: the liquid passes through, carrying dissolved substances with it. The filtered waste liquid, called ultrafiltrate, is then discarded.^[6]

Because haemofiltration removes large volumes of fluid containing essential substances the body needs, clean replacement fluid must be added back. This sterile fluid contains the right balance of salts and minerals to keep blood chemistry stable. It can be added either before the blood enters the filter (called pre-dilution) or after blood exits the filter (post-dilution). Each approach has advantages: pre-dilution reduces the risk of filter clotting but is slightly less efficient at removing waste, while post-dilution provides better waste removal but may increase clotting risk.^[3]

Standard Treatment Approaches

Haemofiltration is most commonly delivered as continuous venovenous haemofiltration (CVVH), meaning blood flows from a vein through the machine and returns to a vein, running continuously around the clock. This differs from intermittent hemodialysis, which typically runs for only three to four hours at a time, two to three times per week, and is more common for patients with long-term kidney failure.^[7]

In intensive care settings, specialized machines such as the Baxter PrisMax or PrismaFlex control the treatment. These devices regulate blood flow rate, typically aiming for 3 to 5 milliliters per kilogram of body weight per minute, and monitor pressures throughout the system to detect problems. The machines use sterile, pre-packaged replacement fluids that meet strict purity standards since they enter the bloodstream directly.^[2]

The choice of catheter size depends on the patient’s size, ranging from small 5-French catheters for infants weighing less than 3 kilograms to 14-French catheters for adults. Blood flow rates are carefully matched to catheter size and patient weight to optimize treatment effectiveness while minimizing complications. The filter itself varies in size, with larger patients requiring filters capable of handling higher blood flow and producing more filtrate.^[15]

Preventing blood from clotting inside the filter tubing represents a major challenge during haemofiltration. Blood naturally clots when it contacts foreign surfaces like plastic tubing, so anticoagulation medication is usually necessary. The most common approaches include systemic heparin, which thins blood throughout the body but increases bleeding risk, or regional citrate anticoagulation, which prevents clotting only within the filter circuit. With citrate, the medication is added before blood enters the filter, then calcium is added after the filter to reverse the anticoagulation effect before blood returns to the patient.^[14]

Replacement fluid composition is carefully controlled to match the patient’s needs. Standard solutions contain sodium, calcium, magnesium, chloride, and bicarbonate in concentrations similar to healthy blood. However, these concentrations may be adjusted for individual patients, particularly those with severe abnormalities in sodium levels. For example, patients with dangerously low sodium require replacement fluid with reduced sodium to prevent rapid correction, which could damage the brain. Conversely, those with very high sodium need fluid with increased sodium concentration.^[9]

The duration of haemofiltration varies widely depending on the underlying cause of kidney failure and how quickly the kidneys recover. Some patients need treatment for just a few days, while others require weeks of continuous therapy. Medical teams regularly assess kidney function through urine output and blood tests to determine when treatment can be stopped. Throughout treatment, patients typically remain sedated or receive pain medication to tolerate the necessary tubes and restricted movement.^[6]

Possible Side Effects and Complications

Several complications can occur during haemofiltration, though careful monitoring helps minimize risks. Bleeding represents the most significant concern when anticoagulation medications are used. Critically ill patients often already have abnormal blood clotting due to their underlying conditions, and adding anticoagulants increases this risk. Bleeding may occur from catheter insertion sites, surgical wounds, or internally in organs like the brain or digestive tract. For this reason, up to 24% of patients receive haemofiltration without any anticoagulation medication, accepting more frequent filter clotting as preferable to bleeding complications.^[1]

Filter clotting causes treatment interruptions, requiring replacement of the entire circuit including tubes and filter. Each replacement means wasted time when the patient receives no kidney support, potentially allowing toxins to accumulate. Clotted filters also waste blood remaining in the tubing, which can be significant for small children. Research shows that regional citrate anticoagulation extends average filter life by approximately 11 hours compared to heparin, reducing interruptions and improving treatment efficiency.^[14]

Catheter-related problems include infection at the insertion site or in the bloodstream, bleeding around the catheter, blood clots forming in the vein where the catheter sits, and mechanical issues like kinking or accidental removal. Larger catheters carry higher risks but are necessary for adequate blood flow in larger patients. Healthcare teams use strict sterile techniques during catheter insertion and maintenance to prevent infection.^[2]

Electrolyte imbalances can develop if replacement fluid composition does not match patient needs or if adjustments are not made as blood chemistry changes. Too much or too little sodium, potassium, calcium, magnesium, or phosphate can cause serious complications affecting the heart, nerves, and muscles. When citrate anticoagulation is used, careful monitoring prevents citrate accumulation, which could cause dangerous acidity, or inadequate calcium replacement, which could affect heart rhythm.^[9]

Low blood pressure sometimes occurs during haemofiltration, particularly when fluid is removed too rapidly. Unlike intermittent hemodialysis, which removes large fluid volumes quickly and often causes blood pressure drops, continuous haemofiltration removes fluid gradually, making it better tolerated by unstable patients. However, excessive fluid removal or poor cardiac function can still cause problems requiring medication adjustments or reduced filtration rates.^[4]

Haemofiltration Compared to Hemodialysis

Understanding the difference between haemofiltration and hemodialysis helps clarify when each treatment is most appropriate. Traditional hemodialysis primarily uses diffusion: blood flows on one side of a membrane while a cleaning fluid called dialysate flows on the other side. Dissolved substances move from areas of high concentration to low concentration, crossing the membrane until concentrations equalize. This process efficiently removes small molecules like urea and creatinine but is less effective for larger compounds.^[1]

Haemofiltration relies on convection, where pressure pushes water and all dissolved substances through membrane pores together. This removes both small molecules and larger ones—including middle-sized molecules that may contribute to long-term complications, potentially toxic proteins like myoglobin that accumulate when muscles break down, and inflammatory substances like cytokines produced during severe infections. The ability to remove these larger molecules represents a theoretical advantage, though clinical trials have not consistently shown this translates into better patient survival.^[1]

A systematic review of clinical trials comparing the two approaches found no significant difference in mortality or other important outcomes such as kidney function recovery, organ dysfunction, or need for blood pressure medications. Haemofiltration did show better removal of larger molecules, as expected, but also shorter filter life, meaning more frequent interruptions. Current guidelines from major kidney organizations therefore do not recommend one method over the other, leaving the choice to individual patient circumstances and local expertise.^[1]

Many modern intensive care units use haemodiafiltration, which combines both diffusion and convection. Blood passes through a filter while dialysate flows on the opposite side, providing both diffusive removal of small molecules and convective removal of larger ones. This hybrid approach theoretically offers benefits of both methods. Large volumes of replacement fluid are added to maximize convection, while dialysate provides additional diffusive clearance. Clinical uptake varies by region, with some countries like Belgium using haemodiafiltration for nearly 30% of dialysis patients, while others rarely employ it.^[13]

Emerging Research and Clinical Trials

Ongoing research continues exploring ways to optimize haemofiltration and expand its applications beyond simple kidney support. Scientists are particularly interested in whether removing inflammatory substances during severe infections or sepsis might improve outcomes, based on the theory that excessive inflammation contributes to organ damage and death in critically ill patients.^[6]

Experimental studies suggest that haemofiltration might reduce levels of cytokines and other inflammatory mediators circulating in the blood during septic shock. Researchers have investigated modifications to standard circuits that might enhance removal of these substances, including special filters with enhanced adsorption properties that bind and remove specific molecules. Some centers have tested higher filtration volumes than traditionally used, reasoning that greater fluid exchange might clear inflammatory substances more effectively.^[6]

However, clinical trials examining these approaches have yielded mixed results. While laboratory measurements often show reduced inflammatory marker levels, this has not consistently translated into improved survival or faster organ function recovery. The relationship between blood levels of inflammatory substances and clinical outcomes appears more complex than initially thought, with some inflammation possibly necessary for healing.^[1]

Research continues examining optimal treatment intensity, meaning how much blood to filter and how much filtrate to produce per hour. Early trials suggested higher intensities might improve outcomes, but a large international study found no benefit from more aggressive treatment beyond standard levels. Current guidelines recommend filtration rates of 20 to 25 milliliters per kilogram per hour as adequate for most patients, with adjustments based on individual response.^[14]

Advances in filter membrane technology aim to improve performance and reduce clotting. Newer membranes with modified surface properties may interact less with blood components, potentially extending filter life without anticoagulation or reducing the amount of anticoagulant needed. Manufacturers continue developing filters with different pore sizes and materials to optimize removal of specific substances while preserving beneficial blood components.^[12]

Clinical trials are also investigating timing—when to start haemofiltration after kidney injury develops. Some researchers propose early initiation might prevent complications and improve recovery, while others argue that waiting allows kidneys more time to recover spontaneously, avoiding unnecessary treatment risks. Recent large trials comparing early versus delayed initiation have not shown clear benefits from earlier treatment, though debate continues regarding optimal timing criteria.^[1]

Studies examining citrate anticoagulation protocols continue refining this approach to maximize filter life while minimizing complications. Different citrate concentrations, infusion rates, and calcium replacement strategies are being compared to identify the safest and most effective protocols. Evidence increasingly favors citrate over heparin for most patients, though citrate requires more complex monitoring and may not be suitable for all patients, particularly those with severe liver disease who cannot metabolize citrate properly.^[14]

Most Common Treatment Methods

• Continuous Venovenous Haemofiltration (CVVH)
  • Blood is continuously removed from a vein, filtered, and returned to a vein using a pump-driven system
  • Runs 24 hours per day until kidney function recovers or patient no longer needs intensive care
  • Uses convection to remove waste products and excess fluid
  • Requires large volumes of sterile replacement fluid to be infused
  • Better tolerated than intermittent treatments by hemodynamically unstable patients
• Continuous Venovenous Haemodiafiltration (CVVHDF)
  • Combines convection and diffusion for enhanced removal of both small and large molecules
  • Uses dialysate fluid flowing opposite to blood flow plus replacement fluid
  • Theoretically provides benefits of both haemofiltration and hemodialysis
  • Increasingly used in modern intensive care units with appropriate equipment
• Anticoagulation Strategies
  • Regional citrate anticoagulation prevents clotting within the filter circuit while minimizing bleeding risk throughout the body
  • Systemic heparin administration thins blood throughout circulation, effective but increases bleeding complications
  • Some patients receive no anticoagulation when bleeding risks are extremely high, accepting more frequent filter changes
  • Choice depends on patient bleeding risk, liver function, and local expertise
• Catheter Access
  • Large-bore double-lumen catheters placed in major veins provide blood access
  • Common sites include internal jugular vein in the neck, femoral vein in the groin, or subclavian vein near the collarbone
  • Catheter size selected based on patient size and required blood flow rates
  • Strict sterile technique during insertion and maintenance reduces infection risk
• Replacement Fluid Management
  • Pre-packaged sterile solutions containing appropriate electrolyte concentrations
  • Composition adjusted for individual patient needs, particularly sodium levels
  • Can be administered pre-filter or post-filter depending on clinical circumstances
  • Continuous monitoring ensures appropriate replacement volume and composition