Dialysis Flashcards
What are the three concepts that underlie dialysis?
Diffusion, convection and adsorption
What does dialysis allow?
Removal of toxins which build up with ESRD (e.g urea, sodium, potassium) and infusion of bicarbonate
What is haemodialysis?
Blood is removed from the body and filtered through a man-made membrane called a dialyser
Filtered blood is then returned to the body
What are the contents of the dialysate when it is removed and returned to the body?
Leaving = urea, creatine, Na+, K+, other toxins Entering = pure H2O, Na+, K+, HCO3-
How does haemodialysis get rid of water?
Ultrafiltration = negative pressure of 100-200 mmHg needed to cause convective solute drag
What is ultrafiltration?
Movement of water and the solutes dissolved in it across a semi-permeable membrane in response to a pressure gradient
What is principally affected by adsorption?
Plasma proteins and any solutes that might be bound to them
What happens to plasma proteins?
Stick to the membrane surface and are removed by membrane binding (especially those of low molecular weight)
Which membranes adsorb protein-bound solutes best?
High flux membranes are better at adsorbing than low flux membranes
What is the difference in mechanism between haemodialysis and haemodiafiltration?
Haemodialysis is primarily diffusive whilst haemodiafiltration is increasingly convective in nature
What effect does increasing the convective force in haemodiafiltration have?
The greater the convective force, the greater the generated volume of the pressure-driven ultrafiltrate
What effect does large volumes of ultrafiltrate have?
Adds enormously to solute drag, especially for the larger middle molecule solute class
How does diffusion occur in haemodiafiltration?
Down engineered concentration gradients
What are some factors that affect the efficiency of convective transport?
Water flux (rate and volume) Viscosity of fluid within membrane pores Membrane pore size = big/little holes, and their ratios Hydrostatic pressure difference across membrane Size, shape and electrical charge of molecules
What is the key difference between haemodialysis and haemodiafiltration?
Replacement of extra-convective ultrafiltrate, throughout the dialysis period (minus any intended ultrafiltration volume)
Why is the composition and purity of the replacement fluid given in haemodiafiltration important?
Re-infusate is given directly into the patient’s circulation
What is high volume haemodifiltration defined as?
Replacement volumes >20 litres
What are the benefits of haemodiafiltration?
Offers smoother, less symptomatic treatment than HD
Enhances recovery time and improves survival
Achieves results at a similar cost to conventional HD
What is the minimum dialysis prescription?
4hrs, 3 times a week
What effect does decreasing dialysis time have?
Increases risk of death = 1% for every 30mins
What are the restrictions put on dialysis patients?
Fluid = if anuric then 1L/day (including food based fluid) Salt = low salt diet to reduce thirst and help fluid balance Potassium = low potassium diet Phosphate = low phosphate diet, phosphate binders with meals (6-12 pills per day)
What are different ways of gaining vascular access for dialysis?
Scribner shunt, tunnelled venous catheter, fistula, arteriovenous graft, haemodialysis reliable outflow graft
What are some features of a Scribner shunt?
Original form of vascular access to the blood stream to allow maintenance haemodialysis, allows dialysis for both AKI and ESRD
How are tunnelled venous catheters inserted?
Catheter inserted into a large vein, usually the internal jugular vein
What are the pros and cons of tunnelled venous catheters?
Pros = easy to insert, can be used immediately Cons = high risk of infection, can become blocked, can stenose/thrombose making future line insertion difficult
What are some features of infections caused by tunnelled venous catheters?
Staph aureus = endocarditis, discitis, death
Do blood cultures, FBC, CRP and exit site swab
Treat with vancomycin +/- gentamicin
May need to remove or exchange line
What is the gold standard method of gaining vascular access for dialysis?
Fistula = artery and vein are surgically connected
What happens to the venous part of the fistula?
Develops to create an enlarged, thick walled vessel called an arteriovenous fistula (AVF)
Where are some common sites for fistulas to be formed?
In both upper limbs = radio-cephalic, brachio-cephalic, brachio-basilic transposition
What are the pros and cons of fistulas?
Pros = good blood flow, less likely to cause infection Cons = requires surgery, needs maturation of 6-12 weeks before use, thrombosis/stenosis
What is steal syndrome?
Limited blood flow to distal arm caused by a fistula
What are the risks of dialysis?
Hypotension and cardiac arrest
Haemorrhage = life threatening if ruptured AVF
Loss of vascular access
Arrhythmia = electrolyte imbalance, ischaemia
What causes intra-dialytic hypotension?
Myocardial stunning on dialysis = underfilling of intravascular space and low BP due to removal of large volume of H20 three times a week (rather than continuously)
How does peritoneal dialysis work?
Solutes removed by diffusion across peritoneal membrane, water removed by osmosis driven by high glucose concentration of dialysate fluid
What are the types of peritoneal dialysis?
Continuous ambulatory peritoneal dialysis and automated peritoneal dialysis
How is continuous ambulatory peritoneal dialysis carried out?
Four 2L bag exchanges per day, PD dialysate drained then fresh bag instilled, 20-30 mins per exchange
How is automated peritoneal dialysis carried out?
1 bag of fluid stays in all day, overnight machine controls fluid drainage in and out for 9-10hrs per night
What are the complications of peritoneal dialysis?
Peritonitis or exit site infection
Peritoneal membrane failure
Hernias = require repair and smaller fill volumes
What are some features of peritonitis and exit site infections caused by PD?
Contamination (staph, strep, diptheroids) or gut bacteria translocation (E.coli, klebsiella)
Must culture PD fluid and may need to remove line
Give intra-peritoneal antibiotics
What is peritoneal membrane failure?
Inability to remove enough water (causing fluid overload) and solutes (causing uraemia) = requires switch to haemodialysis
What blood test would be indicative of starting dialysis?
Resistant hyperkalaemia, eGFR <7 ml/min, urea >40 mmol/l, unresponsive metabolic acidosis
What symptoms may be indicative of starting dialysis?
Nausea, anorexia, vomiting, profound fatigue, itch, unresponsive fluid overload
How is haemodialysis started?
Gradual build up = first session of 90-120 mins then subsequent sessions building up to 4hrs
Why is it important to start haemodialysis slowly?
Too rapid a correction of uraemic toxin levels can lead to disequilibrium syndrome
What are the features of disequilibrium syndrome?
Cerebral oedema, possible confusion, seizures, death
How is peritoneal dialysis started?
Training = 3-6 weeks after catheter insertion
Start with smaller fill volumes = increase to 2-2.5L
Regular clinic and nurse follow up
What effect does age have on dialysis survival in patients with ESRD?
As age increases, survival with dialysis decreases
Does dialysis make a huge improvement for patients over 75 with many co-morbidities?
Not really = expected to have similar number of hospital free days whether starting HD or not
What are some reasons for withdrawing dialysis?
Haemodynamic instability, progressive dementia, CV event, terminal cancer, inability to remain on therapy for full duration due to agitation, increasing frailty and inability to cope at home
