Principles of Hemodialysis


Outline: Introduction, Definition, Physical principles, Hemodialysis procedure, Indications and Contraindications, Complications of hemodialysis

Definition of Hemodialysis

  • The dialysis process is the removal of solutes from the blood, through a semi-permeable membrane – eliminating them from the body
  • An artificial replacement for lost kidney function in people with renal failure
The goals include
  • Fluid balance & regulation 

  • Electrolyte balance and restoration 

  • Removal of toxins & metabolic waste

  • Regulation of pH

Physical Principles of Hemodialysis

  1. Semipermeable membrane
  2. Diffusion
  3. Ultrafiltration
  4. Convention
Semipermeable Membrane
  • A semipermeable membrane is a thin layer of material that contains holes of various sizes, or pores. 
  • Smaller solutes and fluid pass through the membrane, but the membrane blocks the passage of larger substances (for example, red blood cells, large proteins).
  • This replicates the filtering process that takes place in the kidneys, when the blood enters the kidneys and the larger substances are separated from the smaller ones in the glomerulus.

Diffusion in hemodialysis

ultrafiltration in hemodialysis

  • Virtually all inter-dialytic fluid retained by the patient must be removed during the next dialysis session.
  • Fluid removal during hemodialysis is by ultrafiltration. The amount of fluid removed is the ultrafiltrate (QF)
  • This is effected by technically creating a hydrostatic pressure difference across the dialyzer membrane -Transmembrane pressure (TMP) 
  • TMP is determined by average pressure in blood compartment minus the average pressure in the dialysate compartment (TMP = ave.PB – ave.PD)
convention in hemodialysis

Hemodialysis Procedure

  • The dialysis procedure involves circulating patient’s blood through an extracorporeal circuit, during which small molecular weight waste product solutes are removed from the circulation.

Prerequisites for hemodialysis
  1. Vascular access
  2. Dialysis machine
  3. Dialyzer
  4. Dialysate
  5. Water treatment unit
  6. Anticoagulation

Vascular Access

  • To maintain the blood flow required for dialysis, a vascular access for haemodialysis is necessary.
  • Generally, vascular access types are classified as either temporary or permanent, depending on the expected duration of it’s usefulness.
  • Temporary
  1. Most catheters (Femoral, Internal Jugular, Subclavian)
  2. A-V Shunt (Schribner, Thomas, Allen-Brown, and Busselmeier shunts)
  • Tunnelled cuffed venous catheters
  • Autogenous subcutaneous Arterio-venous fistula
  • Prosthetic graft.

  • For urgent dialysis, a temporary (and untunnelled) large-bore, double-lumen dialysis catheter may be inserted into a central vein – usually the subclavian, jugular or femoral vein.

  • Effective dialysis needs blood flows of between 250 and 450 mL/min.

  • In order to achieve this, a surgically fashioned arteriovenous fistula  is formed, using the radiocephalic, brachiocephalic or transposed basilic vein(upper arm or forearm). 

  • Large-bore needles are inserted into the arterialized vein of the AVF to take blood to and from the dialysis machine.
  • In patients with poor-quality veins or arterial disease (e.g. diabetes mellitus), synthetic arteriovenous grafts offer an alternative.
  • For many, an AVF is not an immediate or appropriate solution, and a semipermanent dual-lumen venous catheter can be inserted under a skin tunnel into the jugular or femoral vein.
  • Although easy to place and offering immediate use, there is a significant risk of bloodstream infection (with a foreign body directly accessing the circulation), catheter malfunction(thrombosis), or venous stenosis or occlusion.

Mature A-V Fistula