When should continuous renal replacement therapy be initiated in a hemodynamically unstable critically ill patient with acute kidney injury, and what are the recommended initial settings, modality selection, anticoagulation, and monitoring?

Continuous Renal Replacement Therapy in Hemodynamically Unstable Critically Ill Patients with AKI

For hemodynamically unstable critically ill patients with acute kidney injury, initiate CRRT immediately rather than intermittent hemodialysis, using an effluent dose of 20-25 mL/kg/h, regional citrate anticoagulation as first-line, and bicarbonate-based replacement fluids. 1, 2

When to Initiate CRRT

Initiate CRRT emergently when any of these life-threatening conditions exist 1, 2:

• Severe hyperkalemia with ECG changes (peaked T waves, widened QRS, bradycardia) 2, 3
• Pulmonary edema unresponsive to diuretics causing respiratory compromise 1, 2
• Severe metabolic acidosis with impaired respiratory compensation 2, 3
• Uremic complications (encephalopathy, pericarditis, bleeding) 1, 2
• Severe symptomatic dysnatremia resistant to medical management 2, 3

Do not wait for specific BUN or creatinine thresholds in critically ill patients, as metabolic derangements develop rapidly and absolute laboratory values are poor triggers for initiation 2. The decision should be based on clinical status, not arbitrary numbers 1.

Modality Selection for Hemodynamically Unstable Patients

CRRT is strongly preferred over intermittent hemodialysis in hemodynamically unstable patients requiring vasopressor support 1. The guideline evidence is clear: CRRT provides superior hemodynamic stability, slower solute shifts, and better fluid removal tolerance 1.

Specific modality choice (CVVHDF vs CVVH vs CVVHD) 2, 4:

• CVVHDF or CVVH are preferred for hemodynamically unstable patients 2
• No clear superiority exists between CRRT and prolonged intermittent KRT (PIKRT), so choose based on institutional expertise 1
• Both continuous and intermittent modalities should be viewed as complementary therapies 1

Vascular Access Strategy

Insert an uncuffed non-tunneled dialysis catheter for acute CRRT initiation 1, 2. Use a cuffed catheter only if prolonged RRT (>2-3 weeks) is anticipated 1, 2.

Site selection hierarchy 1, 2:

1. First choice: Right internal jugular vein
2. Second choice: Femoral vein (avoid in high BMI patients)
3. Third choice: Left internal jugular vein
4. Last choice: Subclavian vein (dominant side preferred if necessary, but carries highest stenosis/thrombosis risk)

Always use ultrasound guidance for catheter insertion 1. Obtain a chest radiograph immediately after internal jugular or subclavian placement before first use 1.

Initial CRRT Settings and Dosing

Effluent dose prescription 1, 2:

• Deliver 20-25 mL/kg/h of effluent volume (this is the delivered dose, not prescribed) 1, 2
• Prescribe higher than 25 mL/kg/h to account for downtime and interruptions, as delivered dose is typically lower than prescribed 2, 4
• This recommendation is based on high-quality evidence (1A) showing no benefit from higher doses 1

Fluid composition 1, 2:

• Use bicarbonate-based replacement fluids rather than lactate-based solutions 1, 2
• Bicarbonate is mandatory (1B recommendation) in patients with circulatory shock 1
• Bicarbonate is strongly recommended (2B) in patients with liver failure or lactic acidemia 1
• Lactate-based solutions should only be used in hemodynamically stable patients without liver dysfunction 2

Membrane selection 1:

• Use biocompatible membranes for all CRRT (2C recommendation) 1

Anticoagulation Strategy

Regional citrate anticoagulation is first-line for patients without contraindications (2B recommendation) 1, 2. This provides longer filter life and lower bleeding risk compared to heparin 2, 4.

Anticoagulation algorithm 1:

For patients WITHOUT increased bleeding risk:

• Use regional citrate anticoagulation if no contraindications (liver failure, shock with hypoperfusion, citrate accumulation risk) 1
• If citrate contraindicated: use unfractionated or low-molecular-weight heparin 1

For patients WITH increased bleeding risk:

• Use regional citrate anticoagulation rather than no anticoagulation (2C) 1
• Avoid regional heparinization in bleeding-risk patients (2C) 1

For heparin-induced thrombocytopenia (HIT):

• Stop all heparin immediately and use direct thrombin inhibitors (argatroban) or Factor Xa inhibitors (1A recommendation) 1
• Argatroban is preferred in HIT patients without severe liver failure (2C) 1

Citrate monitoring considerations 2, 4:

• Monitor ionized calcium and total calcium/ionized calcium ratio to detect citrate accumulation 2
• Citrate metabolism generates bicarbonate, which can mask ongoing metabolic acidosis 2

Fluid Management and Hemodynamic Considerations

Avoid excessive fluid removal that causes hypotension, as this impedes renal recovery and worsens outcomes 1, 2. CRRT provides superior hemodynamic tolerance, but overly aggressive ultrafiltration is harmful 2, 5.

Fluid balance targets 2:

• Maintain negative fluid balance in acute lung injury patients (shortens ICU stay) 2
• Volume overload is associated with adverse outcomes 2
• Coordinate fluid administration with CRRT prescription to avoid volume overload 2

Use integrated fluid-balancing systems specifically designed for CRRT rather than adapted IV pumps to reduce delivery errors 2.

Monitoring Requirements

Regular assessment is essential 1, 2:

• Electrolytes and acid-base status: Monitor frequently, especially for CRRT-induced hypokalemia and hypophosphatemia with high-dose or prolonged therapy 2, 6
• Fluid balance: Continuous monitoring with hourly vital signs 3
• Filter performance: Assess circuit pressures and blood flow 4, 7
• Delivered dose: Frequently assess actual delivered dose and adjust prescription accordingly (1B recommendation) 1

Common pitfall: Hypokalemia and hypophosphatemia can develop paradoxically during CRRT, especially with high-dose or prolonged treatment 6, 4. Prevent this with close monitoring and appropriate CRRT fluid adjustments 6.

Transitioning from CRRT

Consider transition to intermittent hemodialysis when all of the following criteria are met 1, 2:

• Vasopressor support has been discontinued 1, 2
• Hemodynamic stability achieved 1, 2
• Intracranial hypertension resolved (if present) 1, 2
• Fluid balance can be controlled by intermittent hemodialysis 1, 2

Discontinue CRRT when kidney function has recovered or when RRT no longer aligns with patient care goals 1, 2. Renal recovery is defined as sustained independence from RRT for at least 14 days 2, 3.

Special Populations

Patients with acute brain injury or increased intracranial pressure 1:

• Use CRRT rather than intermittent RRT (2B recommendation) 1
• Both modalities can cause intracranial pressure changes, but risk is higher with intermittent RRT 1

Patients on extracorporeal life support (ECMO/VAD) 1, 2:

• Integrate CRRT with ECMO to prevent fluid overload that impairs ECMO performance 2
• Implementation should be based on institutional expertise and available technology 2

Patients with severe fluid overload 2:

• CRRT is specifically indicated when fluid overload is unresponsive to diuretics 2
• Better tolerance of fluid removal compared to intermittent modalities 1, 5

Key Technical Considerations

Avoid topical antibiotics at the catheter insertion site in ICU patients (2C recommendation) 1. Avoid antibiotic locks for prevention of catheter-related infections in nontunneled catheters (2C recommendation) 1.

Dialysis fluids must comply with AAMI standards for bacterial and endotoxin contamination (1B recommendation) 1.

Pre-dilution of replacement fluid can enhance ultrafiltration rates and reduce filter clotting 2.