Principles of renal replacement therapy in brief summary

This chapter struggles to comprehend the aims of Section H2(i) from the 2023 CICM Primary Syllabus, which expects the exam candidate to "describe the principles of dialysis and filtration". Only two questions have ever approached this area.  One was Question 4 from the first paper of 2024, in which 73% of the candidates were successful; and the other was Question 24 from the first paper of 2011,  which was failed dismally by the vast majority of the candidates. Both questions requested nothing less than "the PHYSICAL PRINCIPLES that are involved in the flow of blood through a dialysis circuit, and, in the movement of solutes across a dialysis membrane". To clarify their expectations, examiners listed the following essential elements in the model answer:

• Factors contributing to the flow of a fluid through a hollow tube
• Relationship between pressure, fluid viscosity and tube resistance
• Factors that determine blood viscosity
• Factors that determine circuit pressures
• Physical chemistry involved in diffusion across a semipermeable membrane
• Electrochemical forces affecting solute and water movement across a membrane
• Some practical examples; "the factors that influence the performance of dialytic therapies in practical application"

If the following summary accomplishes only one thing, let it be to impress upon the reader how unrealistic it is to expect somebody to explain all of this in writing over ten minutes. Where possible, links to the long-form discussions of the highlighted material are offered as bait to the poor souls who want to learn something instead of rote-cramming densely packed examinable material.

• Blood flow through the CRRT circuit (Q) is described by the equation Q = (P_a- P_r) / R,
  where
  P_a = access pressure
  P_r = return pressure
  R = circuit resistance
  • Access pressure and return pressure are determined by:
    • Set blood pump speed
    • Vascular access device properties
    • Pressure at the access points (eg. central venous pressure, arterial pressure for CAVHD, or ECMO circuit pressure for dialysis via the ECMO circuit)
  • Circuit resistance is described by the Poiseuille equation, 
    • R = (8 l η) / πr⁴, where 
    • r = radius of the circuit, which is affected by
      • Vascular access
      • Filter design
      • Circuit dimensions
      • presence of clots and fibrin (age of the circuit), and thus
      • anticoagulation strategy
    • η = viscosity of blood (3.5-5.5 cP, which depends on:
      • Haematocrit
      • Blood protein and lipid content
      • Temperature of the blood
      • Pre- or post-filter replacement fluid
      • The Fahraeus–Lindquist effect, where blood becomes less viscous in smaller vessels (less than 0.5mm diameter) because of the deformability of red cells
    • l = length of the dialysis circuit (~ 3.5m)
• Dialysis is a diffusional clearance strategy, and depends on:
  • Counter-current mechanism (an essential design feature)
  • Factors which influence the passive diffusion of molecules across membranes, which include:
    • Concentration gradient, which is influenced by
      • Blood concentration of the specific solute, and
      • The composition of the dialysate fluid
    • Filter membrane characteristics: 
      • Thickness
      • Porosity
      • Available surface area of the filter
    • Temperature of the solution
    • Diffusivity coefficient for the solute, which is turn influenced by:
      • the gas constant
      • the viscosity of the solvent
      • size of the solute particles
    • Solute properties, including:
      • Particle size (molecular weight as well as shape)
      • Particle charge
    • Electrochemical forces influencing the movement of solute and water across a membrane, which are described by the Gibbs-Donnan effect:
      • The product of diffusible ions on one side of the membrane will be equal to the product of diffusible ions on the other side of the membrane
      • The electrochemical gradients produced by unequal distribution of charged ions produces a transmembrane potential difference which can be calculated using the Nernst equation
      • The presence of impermeant ions on one side of the membrane creates an osmotic diffusion gradient attracting water into that compartment.
• Ultrafiltration is a conventional clearance strategy;
  • Clearance of water by ultrafiltration depends on:
    • Transmembrane Pressure (TMP), =  (Pf+Pr)/2 - Pe,  where
      • Pf = pre-filter pressure
      • Pr = return pressure
      • Pe = effluent pressure
    • Plasma oncotic pressure, which opposes TMP, and which can be altered by pre or post-replacement fluid
  • Clearance of solutes by ultrafiltration (convection), or the "convective flux" (Jc),  is described by the relationship Jc = Qf × Cb × S,  where
    • Qf = ultrafiltration rate,
    • Cb = solute concentration in plasma water, and
    • S = sieving coefficient: the ratio of a specific solute concentration in the ultrafiltrate (removed only by a convective mechanism), divided by the mean plasma concentration in the filter.