Dialysis Flashcards
What are the three concepts of dialysis?
Diffusion.
Convection.
Adsorption.
What happens during the diffusion aspect of dialysis?
Movement of solutes from high to low concentration until the concentration is equalised.
What is dialysis?
Dialysis allows the removal of toxins which build up with end-stage renal disease (ESRD) e.g. urea, potassium and sodium.
It also allows the infusion of bicarbonate.
How does haemodialysis work?
People have a blood flow rate of ~300-350mls/min.
Vascular access is achieved to connect an artificial kidney containing lots of very small filaments (~1 cell thick).
Dialysate (dialysis fluid) flows in the opposite direction to blood flow through this artificial kidney to allow diffusion of ions across the filaments.
- Dialysate in: pure water, Na, HCO3 , K, glucose.*
- Dialysate out: urea, creatinine, sodium, potassium, other ‘toxins’.*
What is a reverse osmosis machine?
Creates the dialysate (dialysis fluid).
How does haemodialysis remove water?
Through convection.
What happens during the convection aspect of dialysis?
The movement of water (and all solutes dissolved in it -> convective solute drag) across a semi-permeable membrane in response to a pressure gradient.
Water moves from high to low hydrostatic pressure.
Known as ultrafiltration in dialysis.
What happens during the adsorption aspect of dialysis?
Principally affects plasma proteins and any solutes that might be bound to them.
Plasma proteins (especially those of low molecular weight) stick to the membrane surface and are removed by membrane binding.
High flux membranes adsorb protein-bound solutes better than low flux membranes - low flux membranes no longer used.
What is haemodiafiltration?
More convective than haemodialysis.
The greater the convective force, the greater will be the generated volume of the pressure-driven ‘ultrafiltrate’.
Large volumes of ultrafiltrate add enormously to solute drag - especially for the larger “middle molecule” solute classes.
There is still diffusion down engineered concentration gradients.
What main process drives haemodialysis?
Diffusion.
What main process drives haemodiafiltration?
Convection.
What are the most important things that affect the efficiency of the convective ‘transport’ of a molecule across a membrane in haemodiafiltration?
Water flux (rate and volume).
Membrane pore size (big or little holes and their respective ratios).
The (hydrostatic) pressure difference applied to and across the membrane.
Viscosity of the fluid within the membrane pores.
The size, shape and electrical charge of each molecule.
What is the basis of haemodiafiltration?
Replacement of extra-convective ultrafiltrate, throughout the dialysis period (minus any intended ultrafiltration volume) is a key difference between HD and HDF.
As this replacement fluid must be given back directly to the patients’ circulation, the composition and purity of this replacement fluid (re-infusate) are pivotal.
High volume HDF is defined by replacement volumes of >20L.
Is dialysis efficient?
Not really; a minimum of 4 hours 3 times per week is required.
What restrictions are given to people on dialysis?
Fluid restriction to (if anuric [<100ml urine produced] ~1L per day including food-based fluid).
Low salt to reduce thirst and help fluid balance.
Low potassium diet.
Low phosphate diet at CKD stage 3b and phosphate binders with meals.
What is an arteriovenous fistula for venous access?
Artery and vein are surgically connected.
Essentially a left to right shunt.
The venous part will develop to create an enlarged, thick-walled vessel called an arteriovenous fistula (AVF).
What is a HeRO graft?
Haemodialysis reliable outflow graft (HeRO).
Used in patients running out of native options
A line is connected from the brachial artery directly into the right atrium.
