Haemodialysis

Basic principles:

• Blood is pumped from the patient through an array of semipermeable membranes, which bring the blood into close contact with diasylate, flowing counter-current with the blood
• The plasma biochemistry changes towards that of the diasylate owing to diffusion of molecules down their concentration gradients
• Blood flow during dialysis is usually 200-300ml per minute and the diasylate flow is usually 500ml per minute

Access for haemodialysis:

• Adequate dialysis requires a blood flow of at least 200ml per minute
• The most reliable long-term way of achieving this is surgical construction of an arteriovenous fistula using the radial or brachial artery and the cephalic vein
• This results in distension of the vein and thickening (‘arterialization’) of its wall, so that after 6-8 weeks, large-bore needles may be inserted to take blood to and from the dialysis machine
• If dialysis is needed immediately, a large-bore double-lumen cannula may be inserted into a central vein
• Stenosis of the subclavian vein is common and the jugular route is, in general, preferred

Complications:

• Hypotension during dialysis (major problem)
• Haemolytic reactions
• Air embolism