15.6 RRT

Question 1

a) What are the indications for renal replacement therapy (RRT) in the intensive care setting? (40%)

Answer 1

> > Acute kidney injury (AKI) with:
• Refractory fluid overload.

• Hyperkalaemia (greater than 6.5 mmol/l, failing to respond to medical management or rapidly rising).
• Metabolic acidosis.

• Rapidly increasing urea or creatinine.

• Symptomatic uraemia:
encephalopathy, pericarditis, bleeding, nausea.
• Oliguria/anuria.

> > Overdose with a dialysable drug or toxin.

> > Severe sepsis.

> > Management of pre-existing chronic kidney disease.

Question 2

b) List the types of RRT available on intensive care. (30%)

Answer 2

> > Intermittent haemodialysis
(cheaper, efficient, but more rapid fluid
shifts may not be tolerated in
cardiovascularly unstable or head injured
patients. Not available in all ICUs).

> > Continuous renal replacement therapies (CRRTs):
• Continuous venovenous haemofiltration (CVVHF).

• Continuous venovenous haemodialysis (CVVHD).
• Continuous venovenous haemodiafiltration (CVVHDF).

> > Peritoneal dialysis of a patient
already using this form of RRT is feasible
in the ICU but does not have the efficiency of haemodialysis, may cause
problems with diaphragmatic
splinting in ventilated patients, and is not
appropriate in patients with intra-abdominal pathology.

Question 3

c) Outline the principle mechanisms of solute and water removal by RRT. (30%)

Answer 3

Haemodialysis for removal of solutes:

> > Blood pumped through an
extracorporeal circuit that incorporates
a dialyser
(semipermeable membrane separating blood from crystalloid solution, the dialysate).

> > Solutes move from high
concentration to low according
to Fick’s laws of diffusion.

> > To maintain concentration gradients,
the dialysate flows countercurrent to
the flow of the blood

Haemofiltration for removal of water:
» Blood pumped through an extracorporeal system that incorporates a semipermeable membrane.

> > Hydrostatic pressure drives
plasma water across the membrane –
this is ultrafiltration.

> > Small molecules (less than 50 kDa)
are dragged across the membrane
with the water by convection.

> > Ultrafiltrate is discarded and
replacement fluid added according to
desired fluid balance.