Difference Between CVVHD and CVVH
The main difference between CVVHD (Continuous Veno-Venous Hemodialysis) and CVVH (Continuous Veno-Venous Hemofiltration) is their solute removal mechanism: CVVHD uses diffusion while CVVH uses convection, which affects their efficiency in removing different molecular weight solutes and their filter lifespan. 1
Key Differences
Mechanism of Solute Removal
- CVVHD uses diffusion as the primary mechanism of solute removal, where dialysate solution flows countercurrent to blood flow across a semipermeable membrane 1
- CVVH uses convection, where ultrafiltrate is produced and replaced with replacement solution, with solute removal occurring through convective transport 1
Fluid Management
- 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 CVVH, ultrafiltrate produced is replaced with replacement solution, and ultrafiltration in excess of replacement results in patient volume loss 1
Solute Clearance Efficiency
- CVVHD is more efficient at removing small molecular weight solutes (like urea and creatinine) compared to CVVH 2, 3
- CVVH is generally more efficient at removing middle and high molecular weight solutes compared to CVVHD, though this enhanced solute removal has not been proven to influence clinical outcomes 1, 3
Filter Lifespan
- CVVHD demonstrates significantly longer filter lifespan compared to CVVH (median 37 hours vs 19 hours in one study) 3
- A recent quasi-randomized trial found that filter life in CVVH was 79% of that observed in CVVHD, with fewer filters in CVVH reaching 72 hours of operation 4
Clinical Implications
When to Choose CVVHD
- When longer filter life is desired to minimize circuit interruptions 4
- When small solute clearance is the primary goal 3
- For patients with increased risk of filter clotting (CVVHD had 17.5% filter loss due to clotting vs 26.7% in CVVH) 4
When to Choose CVVH
- When middle to large molecular weight solute removal is prioritized, such as in certain inflammatory conditions 1, 3
- When there may be benefits from enhanced cytokine removal, though clinical outcome benefits remain unproven 1
Practical Considerations
Prescription Parameters
- Both modalities typically aim for an effluent volume of 20-25 mL/kg/h for adequate solute clearance in acute kidney injury 5
- High-volume hemofiltration is a variant of CVVH that employs ultrafiltration volumes greater than 35 mL/kg/h 1
Anticoagulation Requirements
- Both modalities require anticoagulation strategies, with regional citrate anticoagulation being the first choice for patients without increased bleeding risk 5
- Filter clotting is more common in CVVH than CVVHD, which may influence anticoagulation requirements 4
Common Pitfalls
- Failing to account for the difference in filter lifespan between modalities when planning treatment duration 4
- Not considering the impact of modality choice on drug clearance, particularly for antimicrobial agents 6
- Overlooking the interaction between convection and diffusion when using continuous venovenous hemodiafiltration (CVVHDF), which combines both mechanisms 2
- Using subclavian veins for vascular access, which increases risk of thrombosis and stenosis, regardless of modality choice 5
Evidence Quality
The evidence comparing CVVHD and CVVH is limited, with no consensus on which modality is superior for all clinical scenarios 1. While some studies suggest differences in filter lifespan and solute clearance profiles, there is insufficient evidence that these differences translate to meaningful differences in patient outcomes such as mortality or renal recovery 7.