Expansion
Diffusion for small solutes, convection for larger ones, ultrafiltration for water
Expansion
- Diffusion (dialysis): solute moves down a concentration gradient across a semipermeable membrane into dialysate flowing countercurrent. Most efficient for small molecules such as urea, potassium and creatinine
- Convection (filtration): a pressure gradient drags solute along with water, so-called solvent drag. Better for middle-sized molecules
- Ultrafiltration: removal of water, driven by a transmembrane pressure gradient
- Adsorption: binding of solutes to the membrane
Haemodialysis relies mainly on diffusion; haemofiltration on convection; haemodiafiltration on both.
Peritoneal dialysis uses the peritoneum as the membrane, with glucose in the dialysate providing the osmotic gradient for water removal.
Continuous therapies in intensive care remove fluid more slowly and are better tolerated in haemodynamic instability.
A drug or toxin is dialysable if it has low molecular weight, low protein binding, a small volume of distribution and low endogenous clearance: lithium, methanol, ethylene glycol, salicylate and metformin qualify, whereas digoxin and tricyclics do not.