How does Continuous Renal Replacement Therapy (CRRT) work and what are the key components of a Continuous Veno-Venous Hemodiafiltration (CVVHD) prescription?

How CRRT Works: A Detailed Explanation

Basic Mechanism

CRRT is an extracorporeal blood purification therapy that runs continuously (24 hours per day) to substitute for impaired kidney function by removing excess fluid and retained solutes through a combination of diffusion, convection, ultrafiltration, and adsorption. 1, 2

The Four Transport Mechanisms

• Diffusion: Solutes move across a semipermeable membrane from areas of high concentration (blood) to low concentration (dialysate) based on concentration gradients 2, 3
• Convection: Solutes are dragged along with water movement (ultrafiltrate) across the membrane, particularly effective for middle and large molecular weight molecules 4, 2
• Ultrafiltration: Pressure gradients force water and dissolved solutes across the membrane to achieve fluid removal 2
• Adsorption: Certain molecules bind directly to the membrane surface, though this plays a minor role 2

Circuit Components

• Vascular access: A large-bore dialysis catheter (typically venovenous) provides blood flow in and out of the patient 1, 5
• Blood pump: An external pump drives blood through the circuit at controlled rates, unlike older arteriovenous systems that relied on patient blood pressure 4, 1
• Hemofilter/dialyzer: A highly permeable membrane where blood purification occurs 4, 5
• Integrated fluid balancing system: Precisely controls ultrafiltrate removal and replacement fluid administration 1, 5

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CVVHD Prescription: Step-by-Step Guide

1. Vascular Access Selection

Place an uncuffed nontunneled dialysis catheter using this order of preference: right internal jugular vein first, then femoral vein, then left internal jugular vein, with subclavian vein as last resort only. 5

• Always use ultrasound guidance for catheter insertion 5
• Obtain chest radiograph promptly after internal jugular or subclavian placement before first use 5
• Avoid subclavian veins due to increased risk of thrombosis and late stenosis 5, 6

2. Anticoagulation Strategy

Use regional citrate anticoagulation as first-line therapy for patients without contraindications to citrate. 5, 6

Decision Algorithm:

• Low bleeding risk + no citrate contraindications: Regional citrate anticoagulation 5
• Low bleeding risk + citrate contraindicated: Unfractionated heparin or low-molecular-weight heparin 5
• High bleeding risk + no citrate contraindications: Regional citrate anticoagulation 5
• Heparin-induced thrombocytopenia (HIT): Direct thrombin inhibitors or Factor Xa inhibitors 5

Citrate Contraindications to Watch:

• Severe liver failure (impaired citrate metabolism leading to citrate accumulation) 7
• Severe lactic acidosis 5
• Shock states with impaired citrate clearance 7

3. CVVHD-Specific Parameters

CVVHD uses diffusion as the primary solute removal mechanism, with dialysate flowing countercurrent to blood flow at 1-2 L/hour, and does not routinely require replacement fluid administration. 4, 6

Blood Flow Rate:

• Target: 150-200 mL/min for most adults 8
• Adjust based on catheter function and patient tolerance 8

Dialysate Flow Rate:

• Standard: 1-2 L/hour (approximately 17-33 mL/min) 4, 6
• This slower rate compared to blood flow optimizes diffusive clearance 4

Effluent Dose:

• Prescribe 20-25 mL/kg/h of effluent volume 5, 6
• For a 70 kg patient: 1,400-1,750 mL/hour total effluent 5
• This equals dialysate flow rate plus net ultrafiltration rate 8

4. Fluid Composition

Use bicarbonate-buffered (not lactate-buffered) dialysate solution for all patients, especially those with circulatory shock, liver failure, or lactic acidemia. 5

• Ensure all fluids comply with AAMI standards for bacterial and endotoxin contamination 5
• Avoid supra-physiologic glucose concentrations to prevent hyperglycemia 5
• Standard electrolyte composition should match physiologic needs 5

5. Net Ultrafiltration Rate

Calculate net fluid removal based on patient's volume status and hemodynamic tolerance. 5

Calculation:

• Net UF rate = Desired hourly fluid removal (mL/hour)
• Example: If patient needs 2 L removed over 24 hours = 83 mL/hour net UF
• Avoid aggressive fluid removal in hemodynamically unstable patients 5
• Monitor closely for hypotension and adjust accordingly 8

6. Filter Selection

Use a dialyzer with biocompatible membrane material. 5

• Standard surface area: 0.9-1.5 m² for most adults 4
• High-flux membranes are preferred for CVVHD 5

7. Sample CVVHD Prescription (70 kg patient)

Written Order Format:

• Mode: CVVHD
• Access: Right internal jugular dialysis catheter
• Blood flow rate: 180 mL/min
• Dialysate flow rate: 2,000 mL/hour (33 mL/min)
• Net ultrafiltration: 100 mL/hour (adjust based on volume status)
• Total effluent dose: 2,100 mL/hour = 30 mL/kg/h
• Dialysate composition: Bicarbonate-buffered, standard electrolytes
• Anticoagulation: Regional citrate anticoagulation (specify institutional protocol)
• Filter: Biocompatible high-flux membrane, 1.4 m² surface area

8. Monitoring and Adjustments

Frequently assess actual delivered dose and adjust prescription to achieve target effluent volume, as circuit downtime reduces effective therapy. 5

Key Monitoring Parameters:

• Filter performance: Watch for rising transmembrane pressures indicating clotting 5, 8
• Electrolyte balance: Check sodium, potassium, calcium, magnesium every 6-12 hours 8
• Acid-base status: Monitor pH and bicarbonate every 6-12 hours 5
• Fluid balance: Track cumulative fluid balance hourly 5
• Anticoagulation efficacy: Monitor circuit pressures and filter life 7, 8

When to Adjust:

• If filter clotting occurs frequently (<24 hours filter life): Increase anticoagulation or consider pre-dilution 5
• If delivered dose <80% of prescribed: Increase dialysate flow rate to compensate for downtime 5
• If electrolyte abnormalities persist: Adjust dialysate composition 8

9. Common Pitfalls to Avoid

• Never rely on single BUN/creatinine thresholds alone for initiating CRRT; consider the broader clinical context including volume overload, severe acidosis, hyperkalemia, and uremic complications 5
• Do not use topical antibiotics over catheter insertion sites or antibiotic locks for infection prevention 5
• Do not use diuretics to enhance kidney recovery or reduce CRRT duration 5
• Avoid lactate-buffered solutions in patients with liver failure or lactic acidosis 5
• Do not underdose: Frequently verify actual delivered dose matches prescription, as circuit downtime significantly reduces effective therapy 5, 8

10. CVVHD vs CVVH: When to Choose CVVHD

CVVHD is superior for hyperammonemia management due to higher ammonia clearance rates compared to CVVH. 6

• CVVH removes middle and large molecular weight solutes more efficiently, though this has not proven to improve clinical outcomes 6
• CVVHD provides more predictable small solute clearance (urea, creatinine, potassium) through diffusion 6, 3
• Both modalities provide similar hemodynamic stability and are superior to intermittent hemodialysis in critically ill patients 6