CRRT

CRRT offers theoretical advantages in the critical care setting such as better hemodynamic tolerance, enhanced fluid balance, and enhanced uremic solute homeostasis; however, the superiority over IRRT has not been definitively proven.

1. Ultrafiltration - the removal of fluid over time, generally targeting a rate of 50-400 mL/hr which is less than the plasma-refill rate.
2. Hemofiltration is plasma water exchange, or fluid replacement either prefilter (predilutional HF), or after the filter (postdilutional HF).
3. In CRRT, the dialysis flow rate is generally 15-30 mL/min and is the limiting factor for exchange, as opposed to the bloodflow rate in IRRT.
4. Clearance of LMW solutes is flow-dependent, higher LMW particles are membrane-dependent.

Modalities of CRRT

1. CVVH - hemofiltration, woks by convection
2. CVVHD - hemodialysis, mainly works by diffusion
3. CVVHDF - Hemofiltration + hemodialysis, works by convective and diffusion

Timing of CRRT

See Timing of RRT in AKI. Discontinuation is a more subjective and evidence-free decision such as is the discontinuation of IRRT.

Prescription Variables of CRRT

1. Dose
   1. Neither the ATN nor the RENAL study showed a survival or renal recovery benefit from more intensive RRT in critically ill patients.
   2. There is no evidence that high-volume hemofiltration (>35 mL/kg per hour) compared with conventional CRRT dose is beneficial in mpatients with septic or post-cardiac surgery vasoplegic shock.
   3. Accordingly, the current recommendation for a minimum dose of RRT supports the delivery of at least 20 to 25 mL/kg per hour of CVVH, CVVHD, or CVVHDF.
2. Volume - determined by the clinician. Takes into account infusions, GI/GU input and output, insensible fluid losses, current volume status, etc.

Technical Considerations

1. Access - >12Fr double lumen catheter is used. Avoid use of existing AVF/AVGs due to possibility of injury with continuous needling. Both IJV and femoral are acceptable but the right IJV is preferred for clear reasons. Subclavian access discouraged due to risk of stenosis.
2. Solutions - either dialysate or replacement fluid (in hemofiltration).
   1. Monitor Ca/Mg/PO4 closely and replace early
   2. Typically buffered with lactate or bicarbonate. Avoid acetate buffered solutions.
3. Anticoagulation
   1. Circuit clotting is the most frequent cause of therapy interruption
   2. Anticoagulation is necessary and ideally limited only to the circuit.
   3. Most commonly this is UFH infused at the arterial side of the circuit; alternatively, regional citrate anticoagulation with systemic calcium infusion

References

1. Parrillo Critical Care Medicine (2019) - Chapter 19