Dialysis

Question 1

describe the general concept of diffusion in dialysis

Answer 1

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

Question 2

dialysis allows the movement of what substances in end stage renal disease?

Answer 2

removal of toxins from the blood
- urea
- potassium
- sodium
infusion of bicarbonate into the blood

Question 3

what is required for dialysis?

Answer 3

vascular access
flow rate of 300-350 mls/min (far higher than a normal vein)
artificial kidney-like function

Question 4

how does dialysis machine achieve kidney like function?

Answer 4

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

Question 5

how are the toxins etc removed from the blood and transferred into the dialysate?

Answer 5

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

Question 6

absorption vs adsorption?

Answer 6

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)

Question 7

how does adsorption occur in dialysis?

Answer 7

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

Question 8

haemodialysis vs haemodiafiltration (HDF)?

Answer 8

haemodialysis = primarily works by diffusion
HDF = primarily works by convection

Question 9

how does HDF work?

Answer 9

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

Question 10

what 5 things can affect the efficacy of HDF?

Answer 10

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

Question 11

high volume HDF is defined by replacement volume of what?

Answer 11

> 20 litres

Question 12

dialysis is not very efficient, why is this?

Answer 12

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

Question 13

what dietary restrictions must patients on dialysis abide by?

Answer 13

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)

Question 14

most common method of vascular access used in dialysis?

Answer 14

tunnelled venous catheter
- catheter inserted into large vein
- usually internal jugular, rarely the femorals are used
easy insertion and can be used immediately

Question 15

risks of tunnelled venous catheter?

Answer 15

infection
can become blocked
can damage central veins making future line insertion difficult

Question 16

most common infection with tunnelled venous catheterisation?

Answer 16

staph aureus
can cause
- endocarditis
- discitis
- death

Question 17

how is staph aureus infection from tunnelled venous catheter investigated and managed?

Answer 17

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

Question 18

gold standard for dialysis vascular access?

Answer 18

arteriovenous fistula (surgical connection between artery and vein)

• radio-cephalic
• brachio-cephalic
• brachio-basilic transposition

Question 19

how does arteriovenous fistula work?

Answer 19

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

Question 20

risks of arteriovenous fistula?

Answer 20

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

Question 21

what is used for venous access if fistula fails or patient doesn’t have a good enough vein for fistula for dialysis?

Answer 21

arteriovenous graft (AVG)
artificial graft used to connect artery and vein

Question 22

what is used if the patient runs out of native options for venous access for dialysis?

Answer 22

HeRO graft (haemodialysis reliable outflow graft)
graft between artery which goes straight into the atrium
like a hybrid between a line and a graft

Question 23

5 complications of dialysis?

Answer 23

hypotension
haemorrhage (ruptured fistula)
loss of vascular access (thrombosis/stenosis/infection)
arrhythmia (electrolyte imbalance)
cardiac arrest

Question 24

what is myocardial stunning?

Answer 24

hypotension during dialysis

removal of too much fluid faster than it can be refilled from the ICF > ECF > intravascular

Question 25

what is peritoneal dialysis?

Answer 25

method of at home dialysis which is performed daily by the patient themselves
utilises peritoneal cavity by inserting/removing dialysate into peritoneal cavity across peritoneum which acts as a semi-permeable membrane
water inserted into peritoneal cavity and solutes/toxins dissolve in it
high glucose conc in dialysate creates a concentration gradient causing water from blood on other side of peritoneum to move out of the peritoneal cavity with dissolved toxins via osmosis

Question 26

what are the 2 types of peritoneal dialysis?

Answer 26

continuous ambulatory peritoneal dialysis (CAPD)
- 4 2L fluid exchanges per day
- 20-30 mins per exchange
automated peritoneal dialysis
- 1 bag of fluid stays in during day
- ADP machine controls fluid drainage in/out overnight for 9-10 hours

Question 27

common complication of peritoneal dialysis? how is this managed?

Answer 27

peritonitis or exit site infection
- contamination with skin commensals
- translocation of gut bacteria (E.coli, klebsiella)
management
- culture peritoneal dialysis fluid
- intraperitoneal antibiotics (vancomycin and gentamicin)
- may need catheter removal

Question 28

2 other complications of peritoneal dialysis?

Answer 28

peritoneal membrane failure (cant remove water or solutes)
- requires switch to haemodialysis
hernia (due to increased intra-abdominal pressure from fluid)
- requires hernia repair and smaller fill volumes

Question 29

when should dialysis be started (based on blood test results)?

Answer 29

resistant hyperkalaemia
eGFR <7ml/min
urea >40
unresponsive metabolic acidosis

Question 30

how is haemodialysis started?

Answer 30

gradual build-up

• 1st session = 90-120 mins
• subsequent sessions build up to 4 hours

Question 31

too rapid a correction of uraemic toxin levels can lead to what?

Answer 31

disequilibrium syndrome

- cerebral oedema and possible confusion, seizures and occasionally death

Question 32

how is peritoneal dialysis started?

Answer 32

3-6 weeks training
starts with smaller fill volumes
fill volume then increases to 2-2.5L
regular clinic and nurse follow up