Overview of Renal Replacement

Renal replacement is the use of extracorporeal blood purification to control water and solute load when the kidneys are unable to do so. Methods include:

• Intermittent Haemodialysis (IHD)
  Rapid removal of volume and toxins.
  • Most commonly used by ESRD patients in the community
• Continuous Renal Replacement Therapy (CRRT)
  Slower removal, preferred in critically ill due to haemodynamic stability.
  • Typical effluent rate of 25-30mL/kg/hr
• Sustained Low-Efficiency Dialysis (SLED)
  Similar toxin control to CRRT, with potentially better acid-base control and HDx stability. Evidence is lacking to recommend SLED over CRRT.
• Peritoneal Dialysis (PD)

SLED is often referred to as SLEDD, for Sustained Low-Efficiency Daily Dialysis.

I have elected to use SLED because during renal recovery second-daily (or less) use is common, and so the “daily” is a misnomer.

Principles

Physical

Principles:

• Ultrafiltration
  Generation of a transmembrane pressure that exceeds the oncotic pressure, resulting in net loss of water across the dialysis membrane. This is achieved by:
  • Relative pressurisation of the blood relative to the dialysate, ↑ hydrostatic pressure
    Volume cleared is proportional to the transmembrane pressure.
  • ↑ Osmolality of the dialysate, drawing water across the membrane

\(TMP = {P_{Filter} + P_{Return} \over 2} - {P_{Effluent}}\)

Where:

• \(TMP\) = Transmembrane Pressure
• \(P\) = Pressure

• Convection (Solvent drag)
  Passive transport of solute particles across a semipermeable membrane along with the movement of solvent.
  • Better clearance of mid-sized molecules
• Diffusion
  Generation of an electro-chemical gradient across the membrane.
  • Rate of diffusion is affected by:
    • Molecular weight
    • Membrane porosity
    • Blood flow rate
  • Better clearance of small molecules

Values:

• Sieving Coefficient
  Describes how effectively a given solute is removed.
  • A high value indicates effective clearance

\(SC = {[UF] \over [Blood]}\)

Where:

• \(SC\) = Sieving coefficient
• \(UF\) = Ultrafiltrate

• Filtration Fraction
  Fraction of plasma removed from blood during filtration.

\(FF = {Rate_{UF} \over Rate_{Blood}} = {Rate_{UF} \over Rate_{Blood \ Pump} \times (1 - Hct)}\)

Where:

• \(FF\) = Filtration Fraction
• \(UF\) = Ultrafiltrate
• \(Hct\) = Haematocrit

Practical

• Dose
  Effluent production in mL/kg/hr.
  • CRRT dose should generally be 20-25mL/hr
    
    • This typically requires a prescription of 25-30mL/hr due to interruptions
    • Higher doses are not associated with improved outcome, but do ↑ the incidence of other electrolyte abnormalities
      e.g. ↓ PO₄.
    • Higher doses may be used (with little evidence) in profound metabolic derangements refractory to standard RRT
      • Rhabdomyolysis with intractable hyperkalaemia
      • Liver failure to ↑ ammonia clearance
  • Hypotension and electrolyte disturbances are more common at higher intensities (dosing) and are not associated with improved outcome
  • Composition of effluent (and therefore the nature of “dose”) varies depending on the:
    • Use of pre-filter dilution
    • Mechanism of effluent production
      • Ultrafiltration
        i.e. TMP driven.
      • Diffusion
        i.e. Dialysis flow rate driven.
  • There is no good evidence suggesting which measure of weight should be used

Note this definition of dose is peculiar to CRRT; dosing intermittent dialysis is different due to the alterations in kinetics and fluid shifts that occur.

• Fluid removal
  Difference between:
  • Effluent production
  • Pre-filter diluent
  • Post-filter replacement

Comparison of Techniques

Methods of Renal Replacement

Technique	Clinical Setting	Advantages	Disadvantages
IHD	HDx stability Planned for continued dialysis in ward environment	Rapid Cheaper	Hypotension due to rapid volume shifts May delay renal recovery in the ICU patient. Cerebral oedema due to rapid fluid shifts (dialysis disequilibrium syndrome)
CRRT	HDx instability ↑ ICP	Technically simple(r) HDx stability Continuous removal of toxins	Slower clearance Prolonged anticoagulation/immobilisation Expensive
SLED	Encourage activity	HDx stability Reduced anticoagulation requirement Facilitates mobilisation with overnight dialysis	Slow clearance of toxins ↑ Technical complexity
PD	HDx instability Coagulopathic Difficult venous access High ICP Austere setting	Simple Cheap No anticoagulation requirement No vascular access requirement	Reduced clearance in catabolic patients Protein loss No control over rate of fluid removal Risk of peritonitis Hyperglycaemia Respiratory impairment
SCUF	Removal of fluid 100-500mL/hr of UF.	Haemodynamic stability

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References

1. Kidney Disease: Improving Global Outcomes (KDIGO) Acute Kidney Injury Work Group. KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney inter., Suppl. 2012; 2: 1–138.
2. Bersten, A. D., & Handy, J. M. (2018). Oh’s Intensive Care Manual. Elsevier Gezondheidszorg.