Continuous Renal Replacement Therapy

Reviewed and revised 30 September 2015

OVERVIEW

Continuous Renal Replacement Therapy (CRRT) Circuits

USE

• renal replacement therapy
• therapeutic plasma exchange
• haemoperfusion

DESCRIPTION

• intravenous access with large bore double lumen catheters (vascaths)
• blood enters an extracorporeal blood circuit
• blood is driven to a semi-permeable membrane by peristaltic pumps

Types of RRT defined by the following:

• Frequency of treatment:
  — Intermittent, continuous, slow continuous
• Vascular access mode:
  — Arterio-venous (AV), veno-venous (VV)
• Technique of blood purification:
  — ultrafiltration, haemofiltration, haemodialysis, haemodiafiltration

All circuits have these components:

• Extracorporeal circuit tubing
• Semipermeable membrane or filter
• Dialysis and/or replacement/substitution fluids
• Peristaltic pumps
• Effluent collection system
• Anticoagulation
  – portals to connect drug infusions
  — specific design features such as: circuit closed to air, smooth surfaces without stagnant areas of flow; circuit may be heparin bonded
• Safety features:
  — Air bubble trap and pressure alarms

METHOD OF INSERTION/ USE

Method of anticoagulation

• None if autoanticoagulated
• regional (only the circuit is anticoagulated; e.g. citrate, heparin with protamine)
• systemic delivery (the patient is also anticoagualted; e.g. unfractionated heparin, low molecular weight heparin, prostacyclin, hirudin derivatives)

Membrane/filter size and material

• semi-permeable, which allows separation of solutes between blood and dialysate
• Flux rating affects the ultrafiltration coefficient
  — higher flux allows greater fluid movement acros the membrane
• cellulose  membranes
  — low flux, porosity effectively has a MWT cut off < 5000D, less biocompatible with a greater inflammatory response
• synthetic membranes
  — low or high flux, can clear much larger molecules
• Commonly used filter in Australasia is the AN 69
  – a synthetic membrane (polyacrylonitrile and sodium mathallyl sulfonate copolymer)
  — combines biocompatibility with solute and fluid clearance.
• Membrane surface area varies
  —  for high volume haemofiltration it needs to be ~ 1.6–2 m2

Dialysis and replacement fluids containing a buffer (lactate or bicarbonate)

• pre-prepared 5L bags
• The same fluid is used for dialysis and replacement
• composition aids diffusion and prevents derangement of blood chemistry when used as replacement
• Lactate based solutions are cheaper and more stable in solution, but can accumulate in liver dysfunction or if coexistent lactic acidosis
• special fluids needed if citrate is used for regional anticoagulation

COMPLICATIONS

Line related complications

• see central venous line / vascath

Circuit related complications

• air embolism
• clotting of circuits
• bleeding from systemic anticoagulation
• DIC
• anaemia
• thrombocytopenia
• hypothermia
• hypotension
  — due intravascular hypovolaemia or SIRS response
  — sometimes decreased vasopressor requirements may occur , presumably because cytokines are removed in patients with SIRS
• electrolyte derangements
  — hypokalaemia, hypomagnesaemia, hypophosphataemia
• dialysis disequilibrium syndrome (DDS)
  — much less common with CRRT than intermittent haemodialysis
  — if solute clearance from the blood compartment is too rapid to allow equilibration with other compartments (e.g. brain), fluid moves out of the blood compartment resulting in adverse fluid shifts (e.g. cerebral edema)

Filter specific complications

• charcoal haemoperfusion
  — hypocalcaemia, hypoglycaemia and thrombocytopaenia
• plasma exchange
  —  coagulopathy (if large exchanges without FFP), immune suppression, exposure to replacement fluid (e.g. anaphylaxis, pyrogenic reactions to FFP)

Anticoagulation complications

• heparin
  — haemorrhage, HIT
• citrate
  — metabolic alkalosis and hypocalcaemia

CIRCUIT DIAGRAMS

Great circuit diagrams from DerangedPhysiology.com:

• CVVH: Continous Veno-Venous Hemofiltration circuit diagram
• CVVHD: Continuous Veno-Venous Hemodialysis circuit diagram
• CVVHDF: Continuous Veno-Venous Hemodiafiltration circuit diagram

References and Links

LITFL

• RRT terminology and nomenclature
• RRT principles in fluid management
• Hemoperfusion