Dialysis Flashcards
describe the general concept of diffusion in dialysis
diffusion = movement of solutes from high to low concentration down a concentration gradient in order to equalise concentrations across a semi-permeable membrane
blood = high concentration of urea, solutes, toxins etc
dialysate (dialysis fluid) has low concentration
= a concentration gradient is formed
dialysis allows the movement of what substances in end stage renal disease?
removal of toxins from the blood - urea - potassium - sodium infusion of bicarbonate into the blood
what is required for dialysis?
vascular access
flow rate of 300-350 mls/min (far higher than a normal vein)
artificial kidney-like function
how does dialysis machine achieve kidney like function?
contains many filaments which are 1 cell thick and contain a semi-permeable membrane
dialysate pumped in and diffusion occurs across this semi-permeable membrane and then dialysate containing all the solutes, toxins, urea etc comes back out
how are the toxins etc removed from the blood and transferred into the dialysate?
convective solute drag
- pressure gradient between blood (high pressure) and dialysate (Low pressure) causes water to be dragged out of the blood and into the dialysate across the semi-permeable membrane
therefore anything which is dissolved in the water of the blood is also removed and transferred into the dialysate = convective solute drag
process is known as ultrafiltration in dialysis
absorption vs adsorption?
absorption = where molecules of one substance pass into or through the bulk of another medium adsorption = where molecules of one substance stick to the surface of another medium (reversible)
how does adsorption occur in dialysis?
plasma proteins (and any solutes stuck to them) stick to the membrane surface and are removed by membrane binding high flux membranes adsorb better than low flux therefore the plasma proteins don't pass into dialysate but aren't stuck permanently to the membrane either
haemodialysis vs haemodiafiltration (HDF)?
haemodialysis = primarily works by diffusion HDF = primarily works by convection
how does HDF work?
basically same as haemodialysis
large volumes of ultra-pure water (ultrafiltrate) are added to the body to increase the pressure gradient and therefore increase the convective solute drag
- removing water/salt/toxin mixture from plasma and replacing with toxin free ultra-pure water so gradient and convective solute drag is preserved but toxins still decreasing
what 5 things can affect the efficacy of HDF?
water flux (rate and volume)
membrane pore size
pressure difference (hydrostatic pressure) applied to and across the membrane
viscosity of the fluid
size, shape and electrical charge of each molecule
high volume HDF is defined by replacement volume of what?
> 20 litres
dialysis is not very efficient, why is this?
requires long treatment times for optimum efficacy
minimum = 4 hrs 3 time per week
whole circulation is removed 12-15 times per session and doesn’t even remove everything
what dietary restrictions must patients on dialysis abide by?
1L fluid per day (if anuric) (including fluid from food)
low salt
low potassium
low phosphate
- take phosphate binders with meals (6-12 pills per day)
most common method of vascular access used in dialysis?
tunnelled venous catheter
- catheter inserted into large vein
- usually internal jugular, rarely the femorals are used
easy insertion and can be used immediately
risks of tunnelled venous catheter?
infection
can become blocked
can damage central veins making future line insertion difficult
most common infection with tunnelled venous catheterisation?
staph aureus can cause - endocarditis - discitis - death
how is staph aureus infection from tunnelled venous catheter investigated and managed?
investigation
- blood cultures
- FBC, CRP
- exit site swab
management
- vancomycin + gentamicin = empirical treatment as covers gram +ve and -ve
- specific treatment once cultures are back
- line removal or exchange once infection is gone
gold standard for dialysis vascular access?
arteriovenous fistula (surgical connection between artery and vein)
- radio-cephalic
- brachio-cephalic
- brachio-basilic transposition
how does arteriovenous fistula work?
surgically created
over 6-12 weeks the vein will develop/hypertrophy to create an enlarged, thick walled vessel resulting in good blood flow for dialysis to occur
risks of arteriovenous fistula?
requires surgery
requires 6-12 weeks of maturation before it can be used
can limit blood flow to distal arm causing “steal syndrome”
can thrombose or stenose
what is used for venous access if fistula fails or patient doesn’t have a good enough vein for fistula for dialysis?
arteriovenous graft (AVG) artificial graft used to connect artery and vein
what is used if the patient runs out of native options for venous access for dialysis?
HeRO graft (haemodialysis reliable outflow graft) graft between artery which goes straight into the atrium like a hybrid between a line and a graft
5 complications of dialysis?
hypotension haemorrhage (ruptured fistula) loss of vascular access (thrombosis/stenosis/infection) arrhythmia (electrolyte imbalance) cardiac arrest
what is myocardial stunning?
hypotension during dialysis
removal of too much fluid faster than it can be refilled from the ICF > ECF > intravascular
