86. Renal replacement therapies

Definition

Renal replacement therapy (RRT) is a term for those modalities which can “replace” or aid the kidney in its functions. They’re generally indicated when the kidney transiently or persistently loses its function to remove toxins, metabolites, and water from the body. It is also used if the kidney function is normal but they kidneys cannot clear a certain toxin fast enough, like in poisoning and overdose.

There are four main modalities:

  • Haemodialysis
  • Peritoneal dialysis
  • Haemofiltration
  • Kidney transplant

Each modality has its own advantages and disadvantages. We won’t discuss haemofiltration because it wasn’t discussed in the lecture.

Indications

RRT can be used in both acute kidney injury and chronic kidney disease, if any indications are present. It can also be used in overdose/intoxication of methanol, ethylene glycol, isopropanol, lithium, paracetamol, etc. Most dialysis patients are end-stage kidney disease patients.

RRT can also remove excess fluid from the patient, which is often necessary in kidney failures.

The indications of renal replacement therapy are as follows:

  • Anuria
  • Therapy-resistant hypervolaemia (pulmonary oedema, etc.)
  • Therapy-resistant hyperkalaemia (usually w/ ECG changes)
  • Other severe electrolyte changes
  • Severe metabolic acidosis
  • Uraemic symptoms (encephalopathy, vomiting, pericarditis, etc.)
  • Severe anaemia

Haemodialysis

Roughly 90% of dialysis patients are treated with haemodialysis. Standard therapy for CKD is usually 3 sessions a week for 4 – 5 hours each. These sessions must be performed in a dialysis centre.

For haemodialysis, large amounts of blood must be removed and supplied to the body, so large-bore vascular access is required. To achieve this, a type of arteriovenous fistula called a Cimino fistula is formed by surgery, usually between the radial artery and the cephalic vein in the forearm of the non-dominant arm. High-pressure blood will flow from the artery to the vein, which dilates the vein. After around 4-6 weeks the vein is dilated and accustomed to the higher pressure and higher flow and can be punctured to be used for haemodialysis.

During a session of haemodialysis, both the radial artery and the (now high-flow) cephalic vein are punctured. Blood is led from the artery into the dialysis machine, where magic happens, before the cleaned blood is returned to the vein.

As part of the magic in the machine, blood extracted from the artery is added anticoagulants (LMWH) before it passes through the dialyser. The dialyser is a tube with many small capillaries with very high surface area, through which the dialysate fluid runs in the direction opposite to the blood. A semipermeable membrane separates the blood and the dialysate, causing waste molecules to travel down the concentration gradient from the blood to the dialysate. Blood is then returned to the patient.

If haemodialysis is needed urgently, two catheters can be placed in the internal jugular vein or another central vein instead.

Peritoneal dialysis

Roughly 10% of dialysis patients are treated with peritoneal dialysis. In this technique, the peritoneal membrane is used as the semipermeable membrane instead of an artificial one. Unlike haemodialysis, peritoneal dialysis can be performed at home. It has slightly lower efficacy than haemodialysis.

A so-called Tenckhoff catheter is surgically placed in the patient’s peritoneal cavity. During dialysis the patient connects the dialysate fluid to the catheter, which flows into the peritoneal cavity. Waste products diffuse from the blood in the peritoneal vessels through the peritoneal membrane and into the dialysate fluid. Later, the dialysate fluid is drained to a drainage bag.

Peritoneal dialysis can be performed continuously (continuous ambulatory peritoneal dialysis, CAPD), or intermittently during the night, by attaching the catheter to a special machine.

Because it requires compliance and motivation on the patient’s end for maintenance, it’s not a good choice for everyone.

Kidney transplant

Kidney transplant is the renal replacement therapy with the best outcome, but unfortunately the supply of donor limits its use (sign up to be an organ donor today!). Due to the chronic immunosuppressive therapy necessary with it, infections and tumours are more frequent.

It is indicated for end-stage renal disease patients who have no contraindications. Some contraindications include:

  • Recent malignancy
  • Active alcohol use
  • Active substance use (except tobacco)
  • Active infection
  • BMI > 50
  • Old age
  • General poor prognosis

The left kidney is preferred.

Department videos

The 2nd department of Internal Medicine have made short educational videos of surprisingly good quality which explain the most important aspects of RRT:


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