CRRT stands for continuous renal replacement therapy, a slow, around-the-clock form of dialysis used in intensive care units for patients whose kidneys have suddenly stopped working. Unlike standard dialysis, which filters blood in concentrated three- to four-hour sessions, CRRT runs 24 hours a day, gently removing waste and excess fluid at a pace that critically ill patients can tolerate without dangerous drops in blood pressure.
Why CRRT Instead of Standard Dialysis
Standard hemodialysis pulls large amounts of fluid and waste from the blood in a short window. For a stable outpatient with chronic kidney disease, that works well. But ICU patients often have failing hearts, severe infections, or plummeting blood pressure. Removing fluid quickly in these patients can cause cardiovascular collapse.
CRRT solves this by spreading the same work over an entire day. The slower pace keeps blood pressure more stable and gives the body time to adjust as fluid shifts between tissues and the bloodstream. It is the preferred option for patients who are hemodynamically unstable, meaning their circulation is already fragile. It also works well for patients with liver failure, massive fluid overload, or the extreme metabolic demands of sepsis.
How the Machine Cleans the Blood
A CRRT machine draws blood from a large catheter, passes it through a filter packed with thousands of tiny hollow fibers, and returns the cleaned blood to the body. The walls of those fibers act as a semipermeable membrane: small and medium-sized waste molecules can pass through the pores, but blood cells and essential proteins are too large to escape.
Three physical processes do the actual work:
- Diffusion moves waste molecules from the blood side of the membrane (where their concentration is high) to a clean fluid called dialysate on the other side (where the concentration is low). This is the same principle that makes a tea bag release flavor into hot water.
- Ultrafiltration uses pressure differences across the membrane to push excess water out of the blood. The machine’s pump creates higher pressure on the blood side and lower pressure on the dialysate side, and water follows that gradient.
- Convection happens when that moving water drags dissolved waste molecules along with it. This “solvent drag” is especially good at clearing medium-sized toxins that diffusion alone handles poorly.
The filter’s effectiveness depends on pore size, number of pores, and membrane thickness. Modern CRRT filters use high-flux membranes with larger pores and thinner walls, allowing more waste and water to cross with less resistance. The membranes must also be biocompatible, meaning they don’t trigger excessive immune reactions when blood touches them.
The Three Main CRRT Modes
Doctors choose a CRRT mode based on which cleaning mechanism best fits the patient’s needs. All three run continuously and use a catheter placed in a large central vein.
- CVVH (continuous venovenous hemofiltration) relies mainly on convection. A pump pushes blood through the filter under pressure, forcing large volumes of plasma water and dissolved waste across the membrane. Replacement fluid is then infused to restore the lost volume. This mode is effective at removing both small and mid-sized molecules.
- CVVHD (continuous venovenous hemodialysis) relies mainly on diffusion. Clean dialysate flows on one side of the membrane while blood flows on the other, and waste molecules migrate across based on concentration differences. It targets small molecules like potassium and urea efficiently.
- CVVHDF (continuous venovenous hemodiafiltration) combines both. It uses dialysate for diffusion and pressure-driven filtration for convection simultaneously. The collected waste fluid (effluent) contains both spent dialysate and filtered plasma water. This is the most versatile mode and is widely used in ICUs.
Dose and Delivery Targets
The “dose” of CRRT refers to how much blood cleaning the machine delivers per hour, adjusted for the patient’s body weight. Current guidelines from KDIGO (Kidney Disease: Improving Global Outcomes) recommend a delivered dose of 20 to 25 milliliters per kilogram per hour. For a 70-kilogram adult, that works out to roughly 1.4 to 1.75 liters of effluent every hour.
Hitting that target matters. A retrospective study found that patients who consistently received doses below the KDIGO recommendation had higher 90-day mortality. In practice, circuit downtime for filter changes, clotting, or procedures often reduces the actual delivered dose below what was prescribed, so ICU teams typically prescribe slightly above the target to compensate.
Keeping the Circuit From Clotting
Blood naturally begins to clot when it contacts the plastic tubing and filter membrane of the CRRT circuit. Without anticoagulation, the filter can clog within hours, interrupting treatment. Two main strategies prevent this.
Systemic heparin, a traditional blood thinner, is inexpensive and straightforward to monitor. The downside is that it thins the blood throughout the entire body, not just inside the machine. In a randomized study comparing the two approaches, 42% of patients receiving heparin experienced bleeding complications, compared to only 11.5% of those receiving the alternative.
That alternative is regional citrate anticoagulation. Citrate is infused into the blood as it enters the circuit, where it binds calcium (a key ingredient in clot formation) and prevents clotting locally. Before the blood returns to the patient, calcium is reinfused to restore normal clotting ability. Because the anticoagulation effect stays within the circuit, bleeding risk drops significantly. In the same study, filters treated with citrate lasted an average of 45 hours versus 26 hours with heparin, meaning fewer interruptions and more consistent therapy delivery. About 15% of citrate patients developed low calcium levels, but these were corrected by adjusting the infusion. Citrate is not suitable for everyone: patients with acute liver failure or severe lactic acidosis cannot metabolize citrate properly and risk dangerous accumulation.
Vascular Access
CRRT requires a large-bore, dual-lumen catheter placed in a central vein. The internal jugular vein in the neck is the preferred site because it offers a relatively straight path to the heart and carries a lower risk of long-term vein damage. The catheter tip sits near the right atrium to ensure adequate blood flow through the circuit.
The femoral vein in the groin is an alternative when neck access is not available. The subclavian vein (near the collarbone) is generally avoided because studies show it causes vein narrowing in over 80% of patients who have catheters placed there, which can complicate future vascular access. Catheter placement carries the same risks as any central line insertion: accidental arterial puncture, bleeding, and, for chest sites, a small risk of lung puncture.
Common Complications
Hypothermia
CRRT continuously circulates blood outside the body through tubing and a filter at room temperature. This steady heat loss causes body temperature to drop below 35°C in up to 44% of patients. Below 34°C, hypothermia can suppress heart function, trigger abnormal heart rhythms, and mask fevers, which delays recognition of new infections. The temperature drop is greatest when blood flow rates are slow and dialysate flow rates are high, with one study measuring a difference of 5.5°C between the blood leaving and returning to the patient under those conditions. Most modern CRRT machines include blood warmers to counteract this.
Electrolyte Imbalances
Because CRRT continuously removes solutes from the blood, it can strip out essential minerals alongside waste. Phosphorus depletion (hypophosphatemia) is one of the most common problems, driven by the combination of CRRT clearance, poor nutrition in critically ill patients, and metabolic shifts from sepsis or refeeding. Even with established replacement protocols, some patients still develop low phosphorus levels, which can cause muscle weakness and breathing difficulty. Potassium, magnesium, and calcium levels also require frequent monitoring and supplementation.
Catheter-Related Infections
The central venous catheter is a direct pathway for bacteria to enter the bloodstream. Infection typically begins with skin bacteria colonizing the catheter surface and forming a biofilm, eventually progressing to bloodstream infection. The most common culprits are Staphylococcus species, though Gram-negative bacteria and fungal organisms can also be responsible. ICU teams follow strict sterile protocols during catheter insertion and maintenance to reduce this risk.
Blood Clots
Catheter-related blood clots are a persistent concern. Critically ill patients are already at elevated risk for clotting due to their underlying illness, immobility, and inflammation. A clot forming around the catheter tip can block blood flow through the circuit, require catheter replacement, or, in rare cases, break free and travel to the lungs.
What It Feels Like for the Patient
Patients on CRRT are almost always sedated or critically ill in the ICU, so most are not awake to experience the treatment. The machine sits at the bedside and runs quietly. Nurses monitor it around the clock, adjusting fluid removal rates and checking for alarms that signal filter clotting, air in the tubing, or pressure changes. The treatment can run for days or even weeks, depending on how quickly kidney function recovers. During this time, blood draws are frequent to track electrolytes, clotting function, and acid-base balance. When kidney function begins to return, the care team gradually reduces CRRT support before discontinuing it entirely.