What is the difference between Continuous Veno-Venous Haemofiltration (CVVH), Continuous Veno-Venous Haemodialysis (CVVHD), and Continuous Veno-Venous Haemodiafiltration (CVVHDF)?

Differences Between CVVH, CVVHD, and CVVHDF

The primary difference between CVVH, CVVHD, and CVVHDF is their mechanism of solute removal: CVVH uses convection, CVVHD uses diffusion, and CVVHDF combines both mechanisms to optimize solute clearance. 1, 2

Core Mechanisms of Action

• CVVH (Continuous Veno-Venous Hemofiltration) uses convection as its primary mechanism, where ultrafiltrate is produced and replaced with replacement solution, with solute removal occurring through convective transport 1
• CVVHD (Continuous Veno-Venous Hemodialysis) relies on diffusion, where dialysate solution flows countercurrent to blood flow across a semipermeable membrane 1
• CVVHDF (Continuous Veno-Venous Hemodiafiltration) combines both diffusive and convective solute removal mechanisms to maximize clearance efficiency 2

Fluid Management Differences

• In CVVH, ultrafiltrate produced is replaced with replacement solution, and ultrafiltration in excess of replacement results in patient volume loss 1
• In CVVHD, dialysate solution is delivered across the membrane at rates typically 1-2 L/hour, and fluid replacement is not routinely administered 1
• In CVVHDF, both replacement fluid and dialysate are used, combining the fluid management approaches of both CVVH and CVVHD 2

Solute Clearance Efficiency

• CVVH is generally more efficient at removing middle and high molecular weight solutes compared to CVVHD 1, 3
• CVVHD demonstrates better clearance of small molecular weight solutes 4
• CVVHDF potentially offers the best of both worlds with enhanced clearance across the molecular weight spectrum, though there is interaction between convection and diffusion 4

Clinical Considerations for Modality Selection

• When middle to large molecular weight solute removal is prioritized (such as in certain inflammatory conditions), CVVH may be preferred 1
• For enhanced small solute clearance (urea, creatinine), CVVHD may be equally effective or superior 3, 4
• Filter lifespan tends to be significantly longer during CVVHD compared to CVVH (median 37 hours vs. 19 hours in one study), which may be clinically important for treatment continuity 3
• Acid-base balance may be better maintained with CVVH compared to extended daily dialytic techniques 5

Technical Aspects

• All three modalities typically aim for an effluent volume of 20-25 mL/kg/h for adequate solute clearance in acute kidney injury 1
• High-volume hemofiltration is a variant of CVVH that employs ultrafiltration volumes greater than 35 mL/kg/h 1
• All modalities require vascular access (typically venovenous) and use an external filter/dialyzer for blood purification 2
• Regional citrate anticoagulation is the first choice for patients without increased bleeding risk for all modalities 1

Pharmacokinetic Implications

• The efficiency of drug removal generally follows: CVVHDF > CVVH > intermittent hemodialysis 6
• Drug clearance during these therapies is particularly relevant for hydrophilic antimicrobial agents (e.g., beta-lactams, aminoglycosides, glycopeptides) 6
• Therapeutic drug monitoring should be considered during all CRRT modalities to optimize drug exposure 6

Common Pitfalls and Caveats

• Using subclavian veins for vascular access increases risk of thrombosis and stenosis, regardless of modality choice 1
• Assuming all medications are equally affected by different CRRT modalities can lead to dosing errors 6
• Neglecting the impact of residual renal function can lead to miscalculations of total clearance 6
• The enhanced middle molecule clearance of CVVH has not been conclusively proven to influence clinical outcomes despite theoretical benefits 1