Updated July 2026 · 10 min read
Part of the ICU Devices Hub — ECMO, MCS, CRRT, ventilators, and lines in one map.
The first time you take a CRRT patient, the machine looks like a wall of alarms and mysterious bags. It isn't. CRRT is just slow, continuous dialysis for a patient too unstable to tolerate the fast fluid shifts of intermittent hemodialysis, and once you understand what each circuit does, the alarms stop being scary and start being information.
Intermittent hemodialysis (iHD) removes fluid and solutes over three to four hours. That speed is fine for a stable outpatient, but in a septic, pressor-dependent ICU patient the rapid fluid removal drops preload and blood pressure fast, and the quick solute shifts can worsen cerebral edema. CRRT does the same work spread over the whole day, so removal is measured in a few hundred milliliters per hour instead of liters in an hour. The trade-off is that the patient is tethered to a circuit continuously, which becomes a nursing workload and an anticoagulation problem.
Common indications are the classic AEIOU of dialysis in a patient too unstable for iHD: severe Acidosis, Electrolyte derangement (refractory hyperkalemia), Intoxications, Overload (diuretic-resistant fluid overload), and Uremia. Add hemodynamic instability and CRRT becomes the modality of choice.
All CRRT in the ICU is "continuous veno-venous," meaning blood is pulled from and returned to a vein through a dialysis catheter — there is no arterial cannulation. What changes is how solutes are cleared.
| Mode | Clearance mechanism | Best at |
|---|---|---|
| CVVH (hemofiltration) | Convection — solutes dragged across the filter with fluid ("solvent drag"), then replaced | Middle-sized molecules |
| CVVHD (hemodialysis) | Diffusion — dialysate runs countercurrent, solutes cross by concentration gradient | Small molecules |
| CVVHDF (hemodiafiltration) | Both convection and diffusion combined | Broad clearance |
Practically, the intensivist and nephrologist choose the mode and the prescription; you run it. What you must understand is the difference between replacement fluid (added to replace ultrafiltrate in convective modes), dialysate (runs across the filter in diffusive modes), and the net ultrafiltration rate — the actual fluid you are pulling off the patient per hour, which is the number that changes their weight and blood pressure.
Blood clots when it meets a foreign surface, so the circuit needs anticoagulation or it will clot the filter within hours. Two strategies dominate.
Regional citrate anticoagulation (RCA) is now first-line in most units. Citrate is infused into the blood as it leaves the patient; it binds ionized calcium, and without calcium the clotting cascade stalls — but only inside the circuit. Calcium is then infused back into the patient to restore normal clotting systemically. The elegance is that the patient is never systemically anticoagulated, which matters in surgical and bleeding patients.
Systemic heparin is the alternative when citrate is contraindicated, but it anticoagulates the whole patient and carries bleeding and HIT risk. Some fragile patients run with no anticoagulation at all, relying on high blood-flow rates and frequent saline flushes to keep the filter open — at the cost of shorter filter life.
CRRT moves enormous volumes. A single filter run cycles many liters of dialysate and replacement fluid, and small errors compound over 24 hours. Your flowsheet math must be exact: hourly net fluid removal, running fluid balance, and reconciliation against the machine's own totals. A machine that thinks it removed 100 mL/hr but is actually pulling 300 will quietly render a patient hypovolemic by shift's end. Weigh the effluent bag mentality: trust but verify the numbers, and escalate any discrepancy.
CRRT clears more than urea. It removes phosphate, potassium, and magnesium, and it clears water-soluble drugs and micronutrients. Expect to replace phosphate and magnesium frequently, and expect antibiotic and other drug doses to be adjusted for CRRT clearance — always confirm renally dosed and CRRT-dosed medications with pharmacy, because underdosing antibiotics in a septic patient on CRRT is a real and dangerous error.
| Alarm | Usually means | First move |
|---|---|---|
| Access pressure very negative | Catheter not delivering blood (kink, clot, wall suck, low volume) | Check lines/patient position; assess volume; may need to reverse ports |
| Return (venous) pressure high | Return limb obstructed or clotting | Check for kinks/clot; inspect filter and bubble trap |
| Transmembrane pressure (TMP) rising | Filter clotting | Anticipate a filter/circuit change; check anticoagulation |
| Air detected | Air in the venous chamber | Do not ignore — follow machine prompt, never bypass the air detector |
| Blood leak | Filter membrane rupture | Stop and change the circuit — effluent should never be bloody |
A steadily climbing TMP with a darkening filter tells you the circuit is dying; changing it electively on your terms is far better than a clotted emergency change that returns none of the patient's blood volume. Losing a full circuit to a clot means losing the blood in it — for an anemic patient that is a meaningful hit.
Keep the access working and unkinked, honor the anticoagulation protocol and its calcium checks, reconcile fluid balance every hour, replace phosphate and magnesium, confirm CRRT drug dosing, and treat rising TMP as an early warning rather than a crisis. Watch the patient for the two ways CRRT hurts them — hypothermia from the extracorporeal circuit (many patients need a blood warmer) and hypotension from over-aggressive fluid removal.
This article is general educational information for licensed clinicians and students, not medical advice or a substitute for your institution's protocols, pharmacy guidance, or a provider's orders. Always follow facility policy and verify every setting independently.
Get The ICU Notebook Newsletter
Clinical tools and career insights for ICU nurses. One email per week, no fluff.
Yes, send it freeNo spam. Unsubscribe any time.