Understanding CRRT means being able to trace what enters and leaves the circuit, explain how solute removal occurs and identify why delivered treatment may differ from the prescription. For FFICM, keep three questions separate: what the circuit is designed to do, what it actually delivered and what the patient currently needs.
This article uses original conceptual diagrams and a fictional downtime exercise created in September 2026. It is not a device-operating guide or a renal-replacement prescription. As checked on 27 September 2026, KDIGO's AKI and AKD page describes the 2026 update as being prepared for publication after public review. A public-review draft should not be presented as an already final guideline.
Trace blood and fluid through the circuit
Start with the blood pathway. Blood leaves the patient through the access limb, passes through the extracorporeal circuit and filter, and returns through the return limb. The filter membrane separates blood from fluid on the other side; it does not normally mix dialysate directly into the bloodstream.
The simplified schematic shows relationships rather than every sensor, clamp or safety feature. Actual component order and terminology vary by machine.
PATIENT access == blood pathway == [FILTER] == return == PATIENT
| |
replacement may enter | fluid removed across membrane
before or after filter v
EFFLUENT
^
|
spent dialysate when dialysis is used
Identify replacement fluid and dialysate separately. Replacement fluid substitutes for some of the fluid removed in a convective treatment. Dialysate establishes a solute gradient across the membrane. Net fluid removal concerns the patient's overall balance, not simply the volume collected in an effluent bag.
A useful oral explanation names the direction of blood, the role of the membrane and where each fluid acts before mentioning a modality abbreviation.
Compare CVVH, CVVHD and CVVHDF
Continuous venovenous haemofiltration, CVVH, primarily uses convection: solutes are carried with water moving across the membrane. Continuous venovenous haemodialysis, CVVHD, primarily uses diffusion: solutes move down a concentration gradient. Continuous venovenous haemodiafiltration, CVVHDF, combines the two mechanisms.
For the established definitions and renal-replacement principles, use the KDIGO 2012 AKI guideline and the machine-specific clinical documentation alongside local renal guidance. An abbreviation does not tell you every relevant parameter, anticoagulation arrangement or current patient requirement.
Pre-dilution introduces replacement before the filter and changes the concentration presented to it. Post-dilution places replacement after the filter. This affects the relationship between nominal effluent flow and solute clearance, as well as the operating conditions within the filter. It is one reason why a displayed effluent figure should not automatically be treated as exact clearance for every substance.
| Modality | Main transport mechanism | Question to explain |
|---|---|---|
| CVVH | Convection | What determines solute passage with ultrafiltration? |
| CVVHD | Diffusion | What concentration gradient and flows are being used? |
| CVVHDF | Both | How do the combined components contribute? |
Calculate delivery without confusing it with a prescription
The following calculation concerns treatment availability, not a recommended clinical dose. In a fictional 24-hour observation period, a circuit is unavailable for six hours because of imaging, access review and filter replacement. It therefore runs for 18 hours, or 75% of the observed period.
If the instantaneous treatment effect were otherwise constant, the time-averaged delivery would be three-quarters of that achieved during uninterrupted operation. That is a simplifying assumption. Real delivery may also change with filter performance, dilution, residual kidney function and the solute being considered.
The calculation is:
Observed operating fraction = running time / observation time
= 18 hours / 24 hours
= 0.75
Before applying a weight-normalised metric, establish the body-weight convention used by the prescription and local protocol. Mixing admission weight, current fluid-loaded weight and an estimated weight can create a misleading comparison. Record the convention rather than silently switching denominators.
The clinical lesson is not "increase treatment whenever downtime occurs". It is to reconcile the prescription, actual delivery, laboratory trajectory and clinical need with the renal and critical-care team. A machine that is prescribed continuously is not necessarily delivering continuously.
Separate solute clearance from net fluid removal
Effluent includes components other than the fluid removed from the patient overall. Replacement and dialysate flows influence the effluent volume, while net balance also depends on all other inputs and outputs. Reading one bag volume as the patient's fluid loss is therefore a category error.
Construct a balance record with measured inputs, urine and other outputs, circuit-related net removal and periods when treatment was interrupted. Include feeds, flushes and medication-associated fluids where relevant. A renal-replacement record that ignores the rest of the ICU chart cannot explain the whole fluid trajectory.
Ask what the patient tolerated during the observed interval. Worsening haemodynamics may change the net-removal plan without meaning that solute management is no longer needed. Conversely, an acceptable pressure does not settle whether congestion is improving.
Investigate pressure changes and a failing filter
Interpret the pattern, its timing and the machine's actual labels. A pressure alarm may relate to access, return, filter resistance, a connection problem or the measurement itself. The sign and meaning of a displayed pressure depend on where and how the device measures it.
| Pattern in an educational scenario | Questions to investigate |
|---|---|
| Access problem after repositioning | Has catheter position, patient position or a line kink changed? |
| Progressive filter-pressure concern | Is clotting or membrane performance changing, and what do the other pressures show? |
| Abrupt change after reconnecting | Are connections, clamps and the circuit arrangement correct? |
| Repeated interruptions without a clear record | What was delivered, what was lost and who reviewed the recurring cause? |
These are diagnostic questions, not instructions to bypass an alarm. Follow device guidance and local safety procedures, and seek trained assistance. Never silence a recurring alarm simply to increase the recorded running time.
In a fictional case, access alarms begin when the patient is turned. After appropriate assessment, the access problem improves, but the filter pressure trend continues to worsen. The two findings need separate explanations: resolving positional access limitation does not establish that the filter remains effective.
Explain anticoagulation and metabolic surveillance
Anticoagulation aims to preserve circuit function while accounting for patient-specific bleeding and metabolic risks. Regional citrate anticoagulation works by reducing available calcium within the circuit; its safe use requires the appropriate monitoring and replacement arrangements specified by the protocol. It is not simply a substitute label for systemic heparin.
Explain why circuit and systemic measurements may answer different questions. Acid-base status, electrolytes and the relationship between relevant calcium measurements can matter to interpretation. A concerning trend requires review of the whole clinical situation and protocol, not a single memorised response.
No anticoagulant amounts, replacement instructions or universal laboratory thresholds are provided here. Those decisions depend on the patient, technique, product information and current specialist guidance.
Present a coherent CRRT case summary
A complete summary identifies the reason for support, modality and transport mechanism, access, prescription convention, observed running time, delivered-treatment limitations, fluid balance, laboratory trend and current problem. It then states the next check and the clinician responsible for reviewing the plan.
For revision, ask a partner to change one detail: the patient begins passing more urine, the circuit stops repeatedly, or a calcium result changes unexpectedly. Explain why that detail changes the interpretation without pretending that the rest of the prescription can be reconstructed from memory.
Per iatroX product information for September 2026, its question banks and Socratic Tutor support mechanism-based practice. Use them to identify a conceptual gap, not to generate unsupervised machine instructions or replace renal-service advice.
Frequently asked questions
Is effluent volume the same as net fluid removed from the patient?
No. Effluent includes treatment-fluid components, so the patient's net balance requires a complete accounting of circuit and non-circuit inputs and outputs.
Does a continuous prescription prove continuous delivery?
No. Interruptions, filter performance and other factors can change delivery, making an actual running-time and treatment record essential.
Is the 2026 KDIGO AKI update already final?
The KDIGO page checked on 27 September 2026 describes publication preparation following public review. Do not cite the review draft as final guidance without a later publication confirming that status.