Acute Kidney Injury

Acute kidney injury is where there is an abrupt loss of kidney function, in a person that previously had functioning kidneys. A loss of kidney function is detected by a rise in creatinine and urea levels in the blood.

Causes
Causes of acute kidney injury can be classified into three groups – prerenal, intrinsic renal and postrenal.

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Prerenal – this is due to a disorder that occurs before the kidney, causing inadequate blood supply to the kidney. Without adequate blood supply to the kidney, the kidney is not able to function correctly. Some examples of prerenal causes are:

  • Low blood volume (hypovolemia), for example severe dehydration or hemorrhage
  • Diminished effective circulating blood volume, for example severe heart failure or septic shock 

Intrinsic renal – this is a disorder within the kidney itself, causing it to not function correctly. There is adequate blood supply to the kidney; however, the kidney itself has a disorder. Some examples of intrinsic renal causes are:

  • Toxins that damage the kidney (called nephrotoxins)
  • Medications that damage the kidney (called nephrotoxic drugs)
  • Prolonged inadequate blood supply to the kidney, causing the nephrons to become ischemic

Postrenal – this is due to a disruption to the flow of urine after it has left the kidney. There is adequate blood supply to the kidney, and the kidney is functioning correctly, however the urine is not able to pass through the urinary tract as it should. Some examples of post renal causes are:

  • Obstruction to the urinary tract, such as kidney stone, tumor, or an enlarged prostate
  • Bladder dysfunction due to damage to the nervous system, or due to certain medications (such as anticholinergic medications)

Signs and Symptoms
There may be a variety of different clinical presentations, depending on the underlying cause. Symptoms can range, from very mild to severe. Severe symptoms may include nausea, vomiting, anorexia, fatigue and confusion. Other patients may have no symptoms at all.

Investigations
A thorough physical examination and history taking is required to determine the underlying cause of acute kidney injury.

Once the underlying cause is suspected, specific investigations may then be required, for example, analysis of the urine, an ultrasound of the urinary tract or an abdominal x-ray. The investigations ordered will depend on the suspected underlying cause.

Blood tests for kidney function (creatinine and urea) should be performed, and regularly monitored. Blood tests should also include electrolyte levels, especially potassium levels.

An electrocardiogram (ECG) can be performed to assess for conduction abnormalities associated with high potassium levels (hyperkalemia).

Treatment
The treatment required depends on the underlying cause. Prerenal disease requires the restoration of blood flow to the kidney. Intrinsic renal disease requires specific treatment depending on the specific disease. Postrenal disease requires removal of the obstruction and/or the restoration of urine flow.

General considerations, in all types of acute kidney injury, should be:

Electrolyte abnormalities may require treatment, especially high potassium levels (hyperkalemia). Treatment of hyperkalemia may include intravenous glucose and insulin, salbutamol nebulizers and intravenous sodium bicarbonate. Intravenous calcium (such as calcium gluconate) should also be administered for hyperkalemia, as it offers some protection to the heart from arrhythmias.

Fluid balance abnormalities may require treatment (according to whether the patient is volume overloaded or volume depleted). Volume overload requires fluid restriction, and volume depletion requires administration of fluids. In either situation, the fluid input and output should be monitored. Urine output can be monitored by requesting that the patient passes urine into a measuring container, and then the amount recorded each time on a chart. If urine output is very low, then inserting a urinary catheter is beneficial, as it allows regular and accurate monitoring of the urine output.

If metabolic acidosis is present, then this may require treatment. Sodium bicarbonate may be required.

Any medications that are damaging to the kidneys (nephrotoxic drugs) should be ceased, such as non-steroidal anti-inflammatory drugs (NSAIDs) and certain antibiotics. 

If the complications of kidney disease are severe and not responding to treatment, then dialysis may be required. Indications for dialysis include any one of the following: dangerously high potassium levels (hyperkalemia), severe pulmonary edema, severe metabolic acidosis, very high levels of urea. Correct management of acute kidney injury usually enables the kidney function to return and long term dialysis is usually not required.