Mechanisms of dialysis

Basics of dialysis

• Diffusion: the process by which particles, atoms, or molecules move from an area of high concentration to an area of low concentration across a semi-permeable membrane
• Factors affecting diffusion: concentration gradient, molecular weight of solute
• Diffusion via dialysis allows the removal of toxins which build up with ESKD - urea and creatinine, potassium, sodium
• Also allows the infusion of bicarbonate

Establishing venous access

• Tunnelled central venous catheter - catheter inserted into a large vein, typically the internal jugular vein
  • Easy to insert and can be used immediately
  • High risk of infection, can become blocked, can cause damage to central veins making future line insertion difficult
• Ateriovenous fistula - 'gold standard' for dialysis vascular access
  • Artery and vein in the upper limb surgically connected, venous part develops to create an enlarged, thick-walled vessel
  • Good blood flow, less likely to cause infection
  • Requires surgery, usually requires maturation of 6-12 weeks before can be used, can limit blood flow to distal arm ('steal syndrome'), can thrombose or stenose
• Arteriovenous graft - next best option to AVF, graft inserted to connect the artery and vein and it is the graft that can be needled for HD/HDF
  • Being replaced by HeRO graft
• Temporary venous catheter

Haemodialysis

• Mainly removes solutes via diffusion
  • Diffusion → convection → adsorption
• Adsorption affects plasma proteins - stick to the membrane surface and are removed by membrane binding
  • High flux membranes adsorb protein-bound solutes better than low flux membranes

Haemofiltration

• Mainly removes solutes via convection
  • Convection → diffusion → adsorption
• Factors affecting convective transport:
  • Water flux
  • Membrane pore size
  • Pressure difference (hydrostatic pressure) applied to and across the membrane
  • Viscosity of the fluid within the membrane pores
  • The size, shape, and electrical charge of each molecule
• There is replacement of the ultra-convective ultrafiltrate throughout the dialysis period - key difference between HD and HDF

Peritoneal dialysis