Describe the principle and types of dialysis. Give its indications.

Definition

Principles Involved

• Diffusion — solutes move from blood to dialysate down their concentration gradient. The chief mechanism
• Ultrafiltration — water is removed by a hydrostatic pressure gradient
• Osmosis — water removal in peritoneal dialysis, using a hypertonic glucose dialysate
• Counter-current flow of blood and dialysate maintains the gradient along the whole membrane

Composition of Dialysate

• Contains Na+, K+, Ca2+, Mg2+, Cl−, bicarbonate and glucose at near-physiological concentrations
• Contains NO urea, creatinine, phosphate or uric acid — so these diffuse out of the blood down a maximal gradient
• K+ is kept low (0–2 mEq/L) to remove the excess of renal failure
• No protein and no cells — the membrane pores retain them

Types

Indications

• Remembered as AEIOU:

– A — refractory Acidosis

– E — Electrolyte disturbance, especially hyperkalaemia

– I — Intoxication (methanol, lithium, salicylate, ethylene glycol)

– O — fluid Overload refractory to diuretics

– U — Uraemia with pericarditis or encephalopathy

Applied Aspects

• Dialysis cannot replace the endocrine functions of the kidney — erythropoietin and vitamin D activation must be supplied separately
• Disequilibrium syndrome — too rapid urea removal → osmotic shift of water into the brain → cerebral oedema, headache, fits
• Renal transplantation remains the definitive treatment; dialysis is a bridge to it