Continuous Renal Replacement Therapy Modalities

Core Definitions and Mechanisms

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, according to the Kidney International guidelines 1
The driving force for CAVHDF is the patient's own blood pressure, which drives blood through the circuit via an arteriovenous connection, as stated in the Kidney International guidelines 1
CVVHDF uses an external blood pump to control blood flow through the circuit, as described in the Kidney International guidelines 1
The solute removal mechanism in CVVHDF combines both diffusion and convection, as reported in the Critical Care journal 2, 3

The target effluent volume for CVVHDF is 20-25 mL/kg/h for adequate solute clearance in acute kidney injury, as recommended by the American Journal of Kidney Diseases guidelines 4
The dialysate flow rate for CVVHDF is typically 1-2 L/hour, as stated in the Kidney International guidelines 1
The RENAL trial and ATN trial demonstrated no mortality benefit with more intensive CVVHDF therapy, as reported in the American Journal of Kidney Diseases journal 4

Small molecular weight solutes, such as urea and creatinine, are primarily removed by diffusion across the concentration gradient created by dialysate flow, as described in the Critical Care journal 2, 3

Regional citrate anticoagulation is the first-line anticoagulation method for patients without contraindications, and requires close monitoring of anticoagulant effect, filter efficacy, circuit life, and complications, as recommended by the American Journal of Kidney Diseases guidelines 4
Therapeutic drug monitoring is essential for beta-lactam antibiotics and other dialyzable medications, as reported in the Critical Care journal 2, 3
Piperacillin/tazobactam half-lives are significantly shorter with CVVHDF compared to CVVH alone, as described in the Critical Care journal 2
Clearance of drugs varies with dialysate flow rate, with higher flow rates increasing drug elimination, as reported in the Critical Care journal 2

Delivered dose of CVVHDF often falls short of prescribed dose, and frequent assessment of actual delivered effluent volume is necessary to avoid underdosing, as recommended by the American Journal of Kidney Diseases guidelines 4
Residual kidney function significantly impacts drug clearance, and should be considered when dosing medications, especially with CrCl >50 mL/min, as reported in the Critical Care journal 2
Bicarbonate-buffered solutions should be used instead of lactate-buffered solutions, as recommended by the American Journal of Kidney Diseases guidelines 4

CRRT specifically refers to extracorporeal blood purification therapies that are applied continuously (24 hours per day) to substitute for impaired renal function 5
The American Society of Nephrology defines CRRT as including CVVH, CVVHD, and CVVHDF, but excludes peritoneal dialysis 5
CVVH (Continuous Venovenous Hemofiltration) uses convection as the primary mechanism of solute removal, where ultrafiltrate is produced and replaced with replacement solution, as recommended by the Kidney International guidelines 6
CVVHD (Continuous Venovenous Hemodialysis) uses diffusion as the primary mechanism, where dialysate flows countercurrent to blood flow across a semipermeable membrane, with a strength of evidence level of high 6
CVVHDF (Continuous Venovenous Hemodiafiltration) combines both diffusive and convective solute removal, with a moderate level of evidence supporting its use 7

CRRT requires vascular access (typically venovenous) and uses an external filter/dialyzer for blood purification, with a high level of evidence supporting its effectiveness 8
PD uses the peritoneal membrane as the dialysis surface and requires insertion of a peritoneal catheter rather than vascular access, with a low to moderate level of evidence for certain patient populations 9
In clinical practice, CRRT is preferred over PD for critically ill patients due to more predictable solute clearance, better control of fluid balance, and more rapid correction of severe metabolic derangements, as recommended by the Nature Reviews Nephrology guidelines 9

The First International Consensus Conference on CRRT explicitly defines CRRT as extracorporeal blood purification therapy, which excludes PD by definition, with a high level of evidence supporting this definition 5
Clinical guidelines clearly separate recommendations for PD from those for CRRT, treating them as distinct modalities, with a moderate to high level of evidence supporting the use of CRRT for certain indications 9

CRRT modalities (CVVH, CVVHD, CVVHDF) all use blood pumps and extracorporeal circuits, with a high level of evidence supporting the safety and effectiveness of these modalities 5, 7
Venovenous therapies are preferred to arteriovenous therapies due to higher solute clearance rates and reduced risk of complications, with a moderate level of evidence supporting this preference 10
CRRT requires specialized equipment including blood pumps, filters, and integrated fluid balancing systems, with a high level of evidence supporting the importance of proper equipment maintenance and operation 8