What are Continuous Arteriovenous Hemodiafiltration (CAVHDF) and Continuous Veno-Venous Hemodiafiltration (CVVHDF)?

Continuous Arteriovenous Hemodiafiltration (CAVHDF) and Continuous Venovenous Hemodiafiltration (CVVHDF)

Core Definitions

CAVHDF and CVVHDF are continuous renal replacement therapy modalities that combine both diffusive and convective solute removal mechanisms, with the primary difference being the driving force for blood flow through the circuit. 1

CAVHDF (Continuous Arteriovenous Hemodiafiltration)

• Driving force: Patient's own blood pressure drives blood through the circuit via an arteriovenous connection 1
• Circuit configuration: Requires both arterial and venous access 1
• Dialysate delivery: Dialysate solution flows countercurrent to blood flow at rates substantially slower than blood flow (typically 1-2 L/hour) 1
• Solute removal: Combines both diffusion (from dialysate) and convection (from ultrafiltration) 1
• Fluid management: Ultrafiltration volumes are optimized to exceed desired weight loss and enhance solute clearance; fluid losses are replaced partially or completely with replacement solution 1

CVVHDF (Continuous Venovenous Hemodiafiltration)

• Driving force: External blood pump controls blood flow through the circuit 1
• Circuit configuration: Venovenous access only (typically via dual-lumen catheter) 1
• Dialysate delivery: Same as CAVHDF—dialysate flows countercurrent at 1-2 L/hour 1
• Solute removal: Identical dual mechanism combining diffusion and convection 1
• Fluid management: Same principles as CAVHDF with controlled ultrafiltration and replacement 1

Critical Operational Differences

CVVHDF has essentially replaced CAVHDF in modern clinical practice due to superior performance characteristics. 1

Why CVVHDF is Preferred:

• Higher and more predictable solute clearance rates because blood pump control allows precise flow regulation independent of patient hemodynamics 2
• Reduced vascular complications by avoiding arterial access 2
• Better hemodynamic stability as flow rates remain constant regardless of blood pressure fluctuations 1
• More reliable ultrafiltration control through pump-driven systems 1

Clinical Applications and Dosing

Standard Dosing Parameters:

• Target effluent volume: 20-25 mL/kg/h for adequate solute clearance in acute kidney injury 1, 3, 4
• Dialysate flow rate: 1-2 L/hour (typical range) 1, 4
• Replacement fluid: Administered to replace ultrafiltrate losses, with net ultrafiltration adjusted based on volume status 4

Evidence-Based Outcomes:

• No survival benefit from higher intensity dosing: The RENAL trial (CVVHDF at 40 vs 25 mL/kg/h) and ATN trial (CVVHDF at 35 vs 20 mL/kg/h) both demonstrated no mortality benefit with more intensive therapy 1
• Survival advantage over CVVH alone: Adding dialysate to hemofiltration (creating CVVHDF) improved 28-day survival (59% vs 39%, P=0.03) and 90-day survival (59% vs 34%, P=0.0005) compared to CVVH alone 5
• Comparable outcomes between CAVHDF and CVVHDF: In hepatic failure/hepatorenal syndrome patients, both modalities achieved similar biochemical improvements and 30% survival rates, though CVVHDF is operationally simpler 6

Mechanism of Solute Removal

Diffusive Component:

• Small molecular weight solutes (urea, creatinine, electrolytes) are primarily removed by diffusion across the concentration gradient created by dialysate flow 1, 3
• Enhanced ammonia clearance compared to pure convective therapies, making CVVHDF preferable for hyperammonemia 3, 4

Convective Component:

• Middle and large molecular weight solutes are removed through convective transport with ultrafiltrate 3
• Cytokine removal theoretically enhanced, though clinical outcome benefits remain unproven 3

Practical Implementation Considerations

Vascular Access:

• First choice: Right internal jugular vein 4
• Avoid subclavian veins due to increased thrombosis and stenosis risk 3, 4
• CAVHDF requires arterial access (typically femoral artery), which is a major disadvantage 1

Anticoagulation:

• Regional citrate anticoagulation is first-line for patients without contraindications 4
• Monitor closely: Anticoagulant effect, filter efficacy, circuit life, and complications 1

Drug Dosing Challenges:

• Therapeutic drug monitoring is essential for beta-lactam antibiotics and other dialyzable medications 1
• Piperacillin/tazobactam half-lives: Significantly shorter with CVVHDF (6.1±2h and 9.4±2.4h respectively) compared to CVVH alone 1
• Clearance varies with dialysate flow rate: Higher flow rates increase drug elimination 1

Common Pitfalls and How to Avoid Them

Underdosing:

• Delivered dose often falls short of prescribed dose 1
• Solution: Frequent assessment of actual delivered effluent volume, not just prescribed volume 1

Ignoring Residual Renal Function:

• Residual kidney function significantly impacts drug clearance but is rarely considered 1
• Solution: Account for residual creatinine clearance when dosing medications, especially with CrCl >50 mL/min where piperacillin clearance increases fivefold 1

Inadequate Monitoring:

• Filter performance degrades over time affecting solute clearance 4
• Solution: Monitor electrolytes, acid-base status, and fluid balance closely; adjust prescription as needed 4

Buffer Selection:

• Use bicarbonate-buffered solutions, not lactate-buffered 1, 4
• Critical in: Circulatory shock, liver failure, or lactic acidemia where lactate metabolism is impaired 4