Continuous renal replacement therapy (CRRT) is a slow, around-the-clock form of dialysis used in intensive care units when a patient’s kidneys can no longer filter blood effectively. Unlike standard hemodialysis, which cleans the blood in a few hours at high speed, CRRT runs 24 hours a day at gentle flow rates of 17 to 34 mL per minute. That slow pace is the entire point: it keeps blood pressure stable in critically ill patients who couldn’t tolerate the rapid fluid shifts of conventional dialysis.
The Basic Circuit
CRRT works by drawing blood out of the body through a large catheter, usually placed in a major vein in the neck, groin, or chest. A pump on the CRRT machine moves blood through tubing and into a hemofilter, which is the core of the system. The hemofilter contains a bundle of hollow fibers made from a semipermeable membrane, with thousands of microscopic pores. Blood flows through the inside of these fibers while fluid flows on the outside. Waste products and excess water cross from the blood side to the outer side, then drain away as waste fluid called effluent. The cleaned blood returns to the patient through the same catheter or a separate line.
Several pumps work in coordination. One controls blood flow, others regulate the delivery of fresh replacement fluid and dialysate solution, and another manages how much fluid is removed overall. Sensors throughout the circuit monitor pressures, detect air bubbles, and track how well the filter is performing.
How Waste Gets Removed
CRRT clears toxins from the blood through two physical processes: diffusion and convection. Most machines use one or both, depending on the treatment mode selected.
Diffusion works the same way a tea bag steeps in hot water. Waste molecules in the blood move across the filter membrane toward a cleaner solution (called dialysate) flowing on the other side. This happens naturally because molecules drift from areas of high concentration to low concentration. Diffusion is best at removing small molecules, those with a molecular weight under about 1,000 daltons. Common waste products like urea and creatinine fall into this category. How much gets removed depends on the concentration difference between the blood and the dialysate, the membrane’s pore size and surface area, and how fast each fluid is flowing.
Convection works differently. Instead of relying on concentration differences, it uses water pressure to push fluid through the membrane, and dissolved waste molecules get swept along with it. Think of it like water carrying sand through a screen. Convection can clear larger molecules that diffusion misses, up to roughly 15,000 daltons. This makes it useful for removing inflammatory substances and mid-sized toxins that accumulate during critical illness.
Three Treatment Modes
CRRT machines can operate in three main modes, each using a different combination of diffusion and convection.
- CVVH (continuous venovenous hemofiltration) relies entirely on convection. The machine generates a large volume of ultrafiltrate by pushing plasma water through the membrane under pressure, then replaces most of that volume with clean replacement fluid. This mode is effective at clearing both small and mid-sized molecules.
- CVVHD (continuous venovenous hemodialysis) relies on diffusion. Dialysate fluid flows on the outside of the filter membrane in the opposite direction of the blood, creating a steady concentration gradient that pulls small waste molecules across. No replacement fluid is needed because the blood volume stays relatively stable.
- CVVHDF (continuous venovenous hemodiafiltration) combines both methods. It uses dialysate and replacement fluid simultaneously, clearing small molecules through diffusion and mid-sized molecules through convection. This is the most thorough mode and is widely used in ICUs.
Where Replacement Fluid Goes In
In modes that use convection (CVVH and CVVHDF), clean replacement fluid must be infused back into the blood to compensate for the large volume of plasma water removed. This fluid can be added before the filter (pre-dilution) or after the filter (post-dilution), and the choice matters.
Post-dilution delivers better waste clearance. Studies show urea clearance with post-dilution is about 15% higher than with pre-dilution, because the blood entering the filter is still concentrated, so more waste crosses the membrane per unit of fluid removed. The tradeoff is that concentrated blood inside the filter is more likely to clot, which can shorten the filter’s lifespan. Pre-dilution thins the blood before it enters the filter, which protects the circuit but reduces how efficiently waste is removed. In practice, many centers use a blend of both or adjust based on how quickly filters are clotting.
Keeping the Circuit From Clotting
Blood naturally starts to clot when it contacts the plastic tubing and membrane fibers inside the circuit. Without anticoagulation, the filter can clog within hours, forcing the treatment to stop. The two most common strategies are heparin (a blood thinner given systemically) and regional citrate anticoagulation.
Citrate has become the preferred method in many ICUs because it only thins the blood inside the circuit, not throughout the patient’s body. Here’s how it works: citrate is infused into the blood as it leaves the patient, where it binds to calcium ions. Since calcium is essential for blood clotting, removing it from circulation effectively shuts down the clotting process within the tubing and filter. The citrate-calcium complexes are small enough to pass through the membrane and get washed away in the effluent. Whatever citrate-calcium remains in the blood returning to the patient gets broken down by the liver into bicarbonate, releasing the calcium back into circulation. Because so much calcium is lost in the effluent, a separate calcium infusion runs continuously into another IV line to keep the patient’s blood calcium at safe levels.
The results are significant. In one study of over 490 CRRT sessions, filters anticoagulated with citrate lasted a median of 31 hours, compared to just 14 hours with heparin. Citrate also carried a lower risk of unplanned treatment interruptions.
How Treatment Intensity Is Measured
CRRT dosing is measured by the effluent flow rate, essentially how much waste fluid the machine produces per kilogram of body weight per hour. Major guidelines from the Kidney Disease: Improving Global Outcomes (KDIGO) group recommend a target of 20 to 25 mL/kg/hr. For a 70 kg patient, that translates to roughly 1.4 to 1.75 liters of effluent every hour.
This target accounts for the reality that prescribed doses often don’t match what’s actually delivered. Treatment interruptions for filter changes, imaging studies, or procedures reduce the effective dose. ICU teams typically prescribe slightly above the target to ensure adequate clearance over a 24-hour period.
Fluid Removal and Blood Pressure Stability
Beyond clearing waste, CRRT removes excess fluid from patients who are overloaded, often from the large volumes of IV medications and fluids given during critical illness. The machine’s net fluid removal rate can be dialed to as little as 50 to 100 mL per hour, which is gentle enough that the body’s circulatory system has time to refill the blood vessels from surrounding tissues. Standard hemodialysis, by comparison, removes fluid at rates exceeding 500 mL per minute of blood flow, which can cause dangerous drops in blood pressure.
CRRT’s slow, continuous nature also produces a mild cooling effect on the blood as it circulates through the external tubing. This slight temperature drop appears to improve blood vessel tone and support blood pressure, adding another layer of stability for patients who are already on medications to maintain their circulation.
Electrolyte Shifts During Treatment
Because CRRT runs continuously for days or even weeks, it strips out more than just waste products. Essential minerals like potassium, phosphate, magnesium, and calcium get pulled across the membrane along with toxins. If not carefully monitored and replaced, these losses create dangerous deficiencies.
Phosphate depletion (hypophosphatemia) is one of the most common complications, especially with prolonged or high-dose treatment. Low phosphate levels can cause muscle weakness, difficulty weaning off a ventilator, and heart rhythm problems. ICU teams manage this by using CRRT solutions that already contain phosphate or by giving intravenous phosphate supplements during treatment.
Potassium levels require constant adjustment. The CRRT solutions can be customized with different potassium concentrations. When a patient’s potassium is low, solutions with higher potassium content are used. When potassium is elevated, as often happens with kidney failure, potassium-free solutions are selected to bring levels down. Magnesium losses are handled with continuous IV infusions, typically 2 to 4 grams per day of magnesium sulfate. Calcium management becomes especially critical when citrate anticoagulation is used, since citrate actively removes calcium from circulation as part of its clotting-prevention mechanism.
Blood draws to check these mineral levels happen every 4 to 6 hours during CRRT, making it one of the more labor-intensive treatments in the ICU. The tradeoff is precise, real-time control over a patient’s internal chemistry during a period when their own kidneys can’t do the job.