What are the differences between Continuous Veno-Venous Hemofiltration (CVVH), Continuous Veno-Venous Hemodialysis (CVVHD), Continuous Veno-Venous Hemodiafiltration (CVVHDF), and Continuous Veno-Venous Hemodialysis with Hemofiltration (CVVHDH)?

Continuous Renal Replacement Therapy Modalities: Key Differences

Primary Mechanism of Solute Removal

CVVHD (Continuous Veno-Venous Hemodialysis) uses diffusion as its primary mechanism, where dialysate flows countercurrent to blood across a semipermeable membrane, while CVVH (Continuous Veno-Venous Hemofiltration) relies on convection with ultrafiltrate production and replacement. 1, 2

CVVHD (Continuous Veno-Venous Hemodialysis)

• Operates through diffusion where dialysate solution flows countercurrent to blood flow at rates typically 1-2 L/hour 1, 3
• Fluid replacement is not routinely administered 1
• Superior for small molecular weight solute removal (urea, creatinine, uric acid, phosphate) compared to CVVH 4
• Provides higher ammonia clearance rates than CVVH, making it preferable for hyperammonemia management 5

CVVH (Continuous Veno-Venous Hemofiltration)

• Operates through convection where ultrafiltrate is produced and replaced with replacement solution 1, 2
• Solute removal occurs through convective transport as plasma water is filtered across the membrane 6
• More efficient at removing middle and high molecular weight solutes (such as beta-2 microglobulin) compared to CVVHD 1, 7
• Ultrafiltration in excess of replacement results in patient volume loss 1

CVVHDF (Continuous Veno-Venous Hemodiafiltration)

• Combines both diffusive and convective solute removal mechanisms 2, 3
• Uses both dialysate flow (for diffusion) and ultrafiltration with replacement (for convection) 3
• Generally provides the highest overall solute clearance across all molecular weight ranges 6, 4
• Demonstrates interaction between convection and diffusion that enhances overall efficiency 4

CVVHDH (Continuous Veno-Venous Hemodialysis with Hemofiltration)

• This is essentially another term for CVVHDF - there is no distinct modality called "CVVHDH" in standard nomenclature 2
• The terminology may vary by institution, but the mechanism remains the same: combined diffusion and convection 2

Comparative Efficiency

The expected efficiency hierarchy for drug and solute removal is CVVHDF > CVVH > CVVHD for most compounds, though this varies based on molecular weight. 6

Small Molecular Weight Solutes (Urea, Creatinine)

• CVVHD demonstrates greater clearance than CVVH for small solutes 7, 4
• Clearances during CVVH and CVVHD at 35 ml/kg/h are comparable, with no significant difference in urea (31.6 vs 35.7 ml/min) or creatinine (38.1 vs 35.6 ml/min) 7
• CVVHDF provides the highest overall small solute clearance due to combined mechanisms 4

Middle to Large Molecular Weight Solutes

• CVVH shows higher beta-2 microglobulin clearance (16.3 ml/min) compared to CVVHD (6.27 ml/min), though this difference did not reach statistical significance 7
• Convection is more efficient than diffusion for large solute removal 4
• When middle to large molecular weight solute removal is prioritized (such as in inflammatory conditions with cytokine removal), CVVH may be preferred 1

Clinical Outcomes and Practical Considerations

CVVHDF may offer survival advantages in specific populations, particularly septic patients with oliguric/anuric acute kidney injury. 8

Mortality and Survival

• In septic patients with AKI without preserved renal function, CVVHDF demonstrated longer mean survival time compared to CVVH 8
• CVVH was associated with higher overall mortality in oliguric/anuric patients 8
• CVVHDF is the modality of choice for septic patients with AKI where renal function is no longer preserved 8

Hemodynamic Stability

• CVVHD is superior to conventional HD and PD in infants for maintaining hemodynamic stability by removing isotonic fluid 5
• All CRRT modalities result in fewer cardiovascular complications compared to intermittent HD 5
• Warmed dialysate in neonates provides added hemodynamic stability 5

Filter Lifespan

• CVVHD demonstrates significantly longer median filter lifespan (37 hours) compared to CVVH (19 hours) 7
• Longer filter lifespan translates to fewer circuit changes and potentially lower costs 7

Dosing and Prescription

Both modalities typically aim for an effluent volume of 20-25 mL/kg/h for adequate solute clearance in acute kidney injury. 1

Standard Dosing

• Typical dialysate flows in CVVHDF are 1-2 L/hour 3
• For hypercatabolic states or increased clearance needs, flows up to 20-25 mL/kg/hour can be considered 3
• High-volume hemofiltration employs ultrafiltration volumes greater than 35 mL/kg/h 1

Pharmacokinetic Implications

• Extracorporeal clearance during CVVHD (30.5 ml/min) significantly exceeds CVVH (17.5 ml/min) for drugs like fluconazole 9
• Hydrophilic antimicrobials (beta-lactams, aminoglycosides, glycopeptides) with dominant renal clearance require significant dosage increases during CRRT 6
• Lipophilic compounds (fluoroquinolones, oxazolidinones) that are nonrenally cleared require minimal dosage modification 6
• Therapeutic drug monitoring is essential for optimizing drug exposure during CRRT 6

Technical Setup Differences

CVVHD Configuration

• Dialysate solution delivered at 1-2 L/hour countercurrent to blood flow 1, 3
• No routine replacement fluid administration 1
• Clearance can be predicted by the formula: Kd = Q(D)/60 4

CVVH Configuration

• Ultrafiltrate production with replacement solution (pre-dilution or post-dilution) 3, 6
• Clearance formula: K(UF) = (Q(UF)/60) × Q(B)/(Q(B) + Q(UF)/60) 4
• Pre-dilution administration improves ultrafiltration rates and may reduce filter clotting 3

CVVHDF Configuration

• Combined dialysate flow plus ultrafiltration with replacement 3
• Requires both dialysate delivery system and replacement fluid system 3
• Demonstrates interaction between convective and diffusive mechanisms 4

Common Pitfalls and Caveats

• Avoid subclavian veins for vascular access due to increased risk of thrombosis and stenosis, regardless of modality 1, 3
• Regional citrate anticoagulation is first choice for patients without increased bleeding risk for all modalities 1
• Avoid supraphysiological glucose concentrations in dialysate/replacement fluids as they cause hyperglycemia 3
• Bicarbonate is preferable to lactate in patients with lactic acidosis and/or liver failure 3
• Hourly urine output is the strongest positive predictor of survival in septic AKI patients, suggesting CRRT should be initiated earlier while renal function is still partially preserved 8