12.3 Renal Replacement Therapy: Hemodialysis, CRRT, SCUF and Ultrafiltration
Key Takeaways
- Urgent indications for renal replacement therapy follow AEIOU: refractory acidosis, electrolyte derangement (especially hyperkalemia above about 6.5 mEq/L or with ECG changes), intoxications, volume overload unresponsive to diuretics, and uremia with pericarditis, encephalopathy or bleeding.
- STARRT-AKI and AKIKI showed no survival benefit to routinely starting renal replacement therapy early in the absence of an urgent indication, and accelerated initiation left more patients dialysis-dependent at 90 days.
- The standard continuous renal replacement therapy dose is a delivered effluent of 20-25 mL/kg/h, usually prescribed at 25-30 mL/kg/h to allow for downtime; higher doses have not improved survival.
- Regional citrate anticoagulation is the KDIGO-preferred first-line circuit anticoagulant: target post-filter ionized calcium 0.25-0.35 mmol/L and systemic ionized calcium 1.0-1.2 mmol/L, and treat a total calcium to ionized calcium ratio above 2.5 with worsening metabolic acidosis as citrate accumulation.
- Continuous therapy is preferred in the hemodynamically unstable cardiac patient because slow, continuous fluid removal avoids the intradialytic hypotension and myocardial stunning caused by rapid intermittent hemodialysis; net ultrafiltration rates above about 1.75 mL/kg/h are associated with worse outcomes.
When to Start: AEIOU and the Timing Evidence
Acute kidney injury (AKI) complicates a third or more of cardiogenic shock, decompensated heart failure and post-arrest admissions, and cardiorenal interaction is bidirectional - low forward flow and high central venous pressure both injure the kidney, and the failing kidney worsens congestion, acidosis, and arrhythmia risk.
The urgent indications are remembered as AEIOU:
| Letter | Indication | Findings that make it urgent |
|---|---|---|
| A | Acidosis | Metabolic acidosis with pH below about 7.1-7.15 refractory to bicarbonate, especially when bicarbonate loading is limited by volume overload or hypernatremia |
| E | Electrolytes | Hyperkalemia above about 6.5 mEq/L, or any level with ECG changes (peaked T waves, widened QRS, sine wave), refractory to medical therapy; also severe hypercalcemia or tumor lysis |
| I | Intoxications | Dialyzable toxins - salicylates, lithium, toxic alcohols (methanol, ethylene glycol), metformin-associated lactic acidosis, theophylline, valproate |
| O | Overload | Pulmonary edema and hypoxemia unresponsive to escalating loop diuretics, particularly when it prevents delivery of nutrition, blood products, or antibiotics |
| U | Uremia | Uremic pericarditis (a pericardial friction rub - a hard indication and a tamponade risk), uremic encephalopathy or asterixis, uremic bleeding from platelet dysfunction |
Timing in the absence of an urgent indication has been settled by trial data. ELAIN favored early initiation but was single-center; AKIKI and IDEAL-ICU found no benefit; STARRT-AKI (2020) randomized more than 3,000 patients to accelerated versus standard initiation and found no difference in 90-day mortality, with more dialysis dependence at 90 days in the accelerated arm. Current practice is therefore to start for a clear indication or persistent, non-recovering severe AKI, not by creatinine number alone. The CMC-relevant version: a rising creatinine with adequate urine output and no AEIOU criterion is not by itself an indication to cannulate a shocked cardiac patient.
The Four Transport Principles
- Diffusion - solute moves down a concentration gradient across the membrane into countercurrent dialysate. Efficient for small molecules: potassium, urea, creatinine. This is what makes a therapy a "D" therapy (HD, CVVHD).
- Convection - solvent drag: a pressure gradient pushes plasma water across the membrane and solutes are carried with it. Clears middle molecules better than diffusion. This is the "H" in CVVH and requires replacement fluid.
- Ultrafiltration - the pressure-driven removal of plasma water itself. This is how volume comes off, and it is the only thing SCUF does.
- Adsorption - solute binding to the membrane surface. Contributes to cytokine and some drug removal and to progressive loss of filter performance over time.
| Modality | Principle | Duration | Fluid removal | Best fit |
|---|---|---|---|---|
| Intermittent hemodialysis (IHD) | Diffusion (plus UF) | 3-4 h, 3-6 times/week | Rapid, often 1-4 L per session | Hemodynamically stable patient, severe hyperkalemia needing fast correction, dialyzable intoxication |
| CVVH | Convection | Continuous | Titratable, gradual | Middle-molecule clearance, unstable patient |
| CVVHD | Diffusion | Continuous | Titratable, gradual | Small-solute clearance, unstable patient |
| CVVHDF | Both | Continuous | Titratable, gradual | Most common ICU continuous prescription |
| SCUF | Ultrafiltration only | Continuous | Pure volume removal, typically 100-300 mL/h | Diuretic-resistant volume overload without a clearance problem |
| SLED / PIRRT | Diffusion at lower flows | 6-12 h daily | Gradual within the session | Middle ground - hemodynamic tolerance with daytime staffing |
| Isolated venovenous ultrafiltration | Ultrafiltration only | Hours to days | Set rate, commonly 100-500 mL/h | Diuretic-refractory acute decompensated heart failure |
Why Continuous Therapy in the Unstable Cardiac Patient
Intermittent hemodialysis removes fluid over 3-4 hours. A patient with a fixed or barely responsive stroke volume - severe systolic dysfunction, aortic stenosis, restrictive physiology, right ventricular failure, or an LVAD with limited preload reserve - cannot increase cardiac output to compensate for a rapid drop in plasma volume, and plasma refill from the interstitium is too slow. The result is intradialytic hypotension, which occurs in up to a quarter of sessions and is not merely a nuisance: repeated intradialytic hypotension causes myocardial stunning, with new regional wall-motion abnormalities that appear during the session, resolve over hours, and over time contribute to progressive myocardial fibrosis and worse survival. Continuous therapy removes the same volume over 24 hours at a fraction of the hourly rate, preserving the ability of interstitial fluid to refill the vascular space. That hemodynamic tolerance - not superior survival, which trials have not demonstrated - is the reason continuous modalities dominate in cardiogenic shock and post-arrest care.
A patient on CVVHDF with regional citrate anticoagulation has been in cardiogenic shock with congestive hepatopathy and a lactate of 6.2 mmol/L. Laboratory results now show total serum calcium 11.4 mg/dL, systemic ionized calcium 0.94 mmol/L, and a widening base deficit with a worsening metabolic acidosis. What should the nurse recognize and anticipate?
The Prescription the Nurse Must Understand
| Element | Typical value | Why it matters |
|---|---|---|
| Blood flow rate (Qb) | 100-250 mL/min for CRRT (often 150-200) | Too low promotes clotting; too high is limited by access performance |
| Effluent dose | Deliver 20-25 mL/kg/h, prescribe 25-30 mL/kg/h | Higher doses (35-45 mL/kg/h in ATN and RENAL) did not improve survival and increase electrolyte and drug losses; downtime means delivered dose runs below prescribed |
| Replacement fluid position | Pre-filter or post-filter | Pre-dilution lowers hematocrit and filtration fraction in the filter, prolonging filter life, but dilutes solute and reduces clearance efficiency by roughly 15%; post-dilution is more efficient but raises filtration fraction and clotting risk |
| Filtration fraction | Keep below 20-25% | The fraction of plasma water removed in the filter; exceeding it hemoconcentrates blood in the fibers and clots the circuit |
| Net ultrafiltration rate | Individualized; commonly 0-200 mL/h in shock | Rates above about 1.75 mL/kg/h are associated with increased mortality; net rate = total ultrafiltrate minus replacement and other fluids |
| Dialysate/replacement potassium | 0-4 mEq/L | Matched to serum potassium; the arrhythmia-prone cardiac patient is easily driven to hypokalemia on a 0 or 2 bath |
Net ultrafiltration is the number the nurse owns. Machines report gross removal; the patient's actual balance is gross removal minus every infusion, flush, drug, and nutrition volume. Reconciling this hourly, against the ordered goal, is the single most consequential CRRT nursing task in a cardiac patient.
Anticoagulation
Regional citrate anticoagulation (RCA) is the KDIGO-suggested first-line choice for continuous therapy in patients without contraindications. Citrate infused pre-filter chelates ionized calcium inside the circuit, blocking the coagulation cascade only there; calcium is replaced systemically after the filter, so the patient is not systemically anticoagulated. This makes RCA the strategy of choice in a bleeding or post-operative patient and it prolongs filter life compared with heparin.
Monitoring:
- Post-filter ionized calcium 0.25-0.35 mmol/L - the measure of circuit anticoagulation; too high means clotting risk, too low wastes citrate
- Systemic ionized calcium 1.0-1.2 mmol/L - the measure of patient safety
- Total calcium to ionized calcium ratio above 2.5 - the flag for citrate accumulation, seen in liver failure, shock liver, and severe lactic acidosis; it presents with a rising total calcium, falling ionized calcium, high anion gap acidosis, and often refractory hypotension
- Metabolic alkalosis - the opposite problem, from excess conversion of citrate to bicarbonate; managed by reducing citrate or increasing dialysate flow
Systemic unfractionated heparin is the alternative when citrate is unavailable or contraindicated, monitored by aPTT or anti-Xa, at the cost of systemic bleeding risk. In the patient with heparin-induced thrombocytopenia (HIT) heparin is prohibited in the circuit, in flushes, and in catheter locks; argatroban (a direct thrombin inhibitor, hepatically cleared, typically 0.5-2 mcg/kg/min in critical illness with reduced starting doses in hepatic dysfunction) or bivalirudin is substituted, or citrate is used. Some patients with coagulopathy run safely with no anticoagulation and frequent saline flushes.
Access and Complications
A non-tunneled dual-lumen dialysis catheter is placed preferentially in the right internal jugular vein (the straightest path to the right atrium, best flows, longest filter life), then femoral, then left internal jugular; the subclavian route is avoided because of central venous stenosis that would compromise a future arteriovenous fistula. The catheter is a dedicated dialysis access - it is not used for routine blood draws or infusions unless institutional policy allows and no alternative exists. Reversing the lines to fix poor flow causes recirculation and lowers delivered clearance; document it if done.
Complications to monitor:
- Hypotension - from excessive net ultrafiltration, circuit blood volume shift at initiation, or bradykinin release
- Hypothermia - extracorporeal blood cooling masks fever, causes shivering and vasoconstriction, and raises oxygen consumption; use the blood warmer and monitor core temperature
- Electrolyte and micronutrient depletion - hypophosphatemia (a common and under-recognized cause of failure to wean from the ventilator), hypokalemia, hypomagnesemia, and losses of water-soluble vitamins and trace elements; amino acid losses require increased protein delivery
- Bleeding and circuit clotting - track filter life and transmembrane pressure trends
- Air embolism - a hard-stop machine alarm; clamp, position the patient left lateral head-down, give 100% oxygen, and call for help
- Dialysis disequilibrium syndrome - headache, nausea, confusion, seizures from rapid urea removal and cerebral edema; a risk of aggressive initial IHD in the severely uremic patient, and an argument for gentle first sessions
- Drug clearance - antibiotics (beta-lactams, vancomycin, aminoglycosides), levetiracetam, and many sedatives are removed and must be re-dosed and timed around therapy; vasoactive catecholamines are not meaningfully cleared, but circuit initiation still commonly requires a transient increase in the infusion rate
A patient in cardiogenic shock on norepinephrine 0.14 mcg/kg/min and dobutamine is receiving CVVHDF with a net ultrafiltration goal of 150 mL/h. Over 3 hours the mean arterial pressure has drifted from 71 to 58 mmHg, the norepinephrine has been titrated up twice, central venous pressure has fallen from 16 to 8 mmHg, and the central venous oxygen saturation has fallen from 64% to 52%. Which nursing action addresses the problem most directly?
Ultrafiltration Versus Escalating Diuretics in Acute Decompensated Heart Failure
This question shows up repeatedly because the evidence is genuinely nuanced.
- UNLOAD (2007) compared venovenous ultrafiltration with intravenous diuretics in acute decompensated heart failure and found greater weight and fluid loss at 48 hours and fewer heart failure rehospitalizations at 90 days with ultrafiltration.
- CARRESS-HF (2012) studied acute decompensated heart failure with worsening renal function (cardiorenal syndrome) and compared ultrafiltration with a stepped pharmacologic diuretic protocol. Ultrafiltration was inferior for the primary bivariate endpoint: creatinine rose significantly more with ultrafiltration, weight loss at 96 hours was not different, and adverse events including bleeding and catheter complications were more frequent.
- AVOID-HF (2016) was terminated early for slow enrollment and showed only a non-significant trend toward longer time to a heart failure event with ultrafiltration.
The 2022 AHA/ACC/HFSA heart failure guideline does not carry a stand-alone class of recommendation endorsing routine ultrafiltration; it discusses it with explicit caution about patient selection, vascular access complications, and the absence of demonstrated benefit over a properly executed diuretic strategy (the earlier 2013 guideline had listed it as a Class 2b option). The practical position: an aggressive, protocolized loop diuretic strategy - adequate intravenous bolus and infusion dosing, sequential nephron blockade with a thiazide such as metolazone or intravenous chlorothiazide, and objective assessment of diuretic response by spot urine sodium and hourly output - comes first. Ultrafiltration is reserved for genuinely diuretic-refractory congestion, and it should not be chosen for the purpose of protecting renal function, which CARRESS-HF specifically failed to show.
Nursing Responsibilities Checklist
- Hourly fluid balance reconciliation: machine effluent, replacement, dialysate, citrate and calcium infusions, drug volumes, nutrition, blood products, and urine output, compared against the prescribed net goal. Reset the machine totals per policy and hand off both machine numbers and true patient balance.
- Hemodynamic surveillance tied to the ultrafiltration rate: mean arterial pressure, vasopressor dose, central venous pressure trend, central or mixed venous oxygen saturation, lactate. Falling perfusion with a falling filling pressure means slow the fluid removal.
- Alarm interpretation: an increasingly negative access (arterial) pressure means an inflow problem - catheter position, kinking, hypovolemia; a rising return (venous) pressure means an outflow problem - clot in the return limb or catheter; a rising transmembrane pressure or filter pressure drop signals filter clotting and predicts imminent loss of the circuit, so escalate before it clots and the patient loses the blood volume in it.
- Filter-life optimization: minimize circuit interruptions for imaging and procedures, keep blood flow adequate, maintain pre-dilution where prescribed, keep filtration fraction below 25%, and troubleshoot access early.
- Medication management: re-dose antibiotics per pharmacy guidance for the modality and effluent dose, time levels around therapy, remember phosphate and thiamine, and never assume a drug level drawn during CRRT reflects a stable steady state.
- Skin, temperature, and infection prevention: dialysis catheter site care with a chlorhexidine-based regimen, no non-dialysis use of the lumens without an order, monitor for hypothermia and use the blood warmer.
- Documentation and hand-off: prescribed versus delivered dose, circuit downtime, filter change times, anticoagulation values, and cumulative balance since initiation.
A patient with an ejection fraction of 20%, severe aortic stenosis, and stage 4 chronic kidney disease requires renal replacement therapy for refractory volume overload and uremia. The team is deciding between intermittent hemodialysis and continuous venovenous hemodiafiltration. Which rationale best supports choosing the continuous modality for this patient?