Friday, August 9, 2013

What is the difference between prerenal failure & acute tubular necrosis?

What is prerenal failure?
Prerenal acute renal failure (ARF) occurs when a sudden reduction in blood flow to the kidney (renal hypoperfusion) causes a loss of kidney function.
In prerenal acute renal failure, there is nothing wrong with the kidney itself.

What is acute tubular necrosis?
Acute tubular necrosis is a kidney disorder involving damage to the tubule cells of the kidneys, which can lead to acute kidney failure.

What is the difference between prerenal failure & acute tubular necrosis?

Difference between prerenal failure & acute tubular necrosis flashcard

The fractional excretion of sodium (FENa) is the percentage of the sodium filtered by the kidney which is excreted in the urine.

In prerenal failure, ADH is high which leads to water, urea and sodium reabsorption.
That is why, the fractional sodium excretion is less than 1%

In acute tubular necrosis, there is loss of epithelium of tubules that reabsorb sodium.
So that's why, the fractional sodium excretion is greater than 2%

This also explains why urinary sodium in prerenal failure is less than 20 while in acute tubular necrosis is greater than 40.

We know that ADH reabsorbs water and when it is high, it will make sure water is reabsorbed from urine.
This makes the urine highly concentrated - urine osmolality >500 in prerenal failure.

In acute tubular necrosis, the kidney has lost it's function completely - reabsorption of sodium and water is impaired.
That is why, urine osmolality is <350 in acute tubular necrosis.
(Water is being pulled into urine by solutes and can not be reabsorbed)

BUN is blood urea nitrogen (Nitrogen from amino acids is converted into urea in our body)
and creatinine is the end product of creatine metabolism.

In prerenal failure, you have less renal blood flow, you will filter less and GFR will decrease.
When GFR decreases, it gives the proximal tubule more time to reabsorb urea.
Thus, there is an increase in serum urea.

Creatinine is not reabsorbed, but you do get rid of it through the kidneys.
When GFR is decreased, there is a back up of creatinine and will not be able to clear it as fast.
Therefore, there will be an increase in serum creatinine.

Urea is increased more than creatinine (because urea is being reabsorbed)
There is a disproportionate increase of BUN and creatinine.
This leads to the high BUN Creatinine ratio.

If the patient truly has renal failure it will affect the BUN and Creatinine equally.
(Something is wrong with the kidney, therefore there is the same effect on the BUN and creatinine)

Kidney can not get rid of urea and can not get rid of creatinine.
They increase in proportion to each other
(because urea is not being reabsorbed anymore)
This leads to the low BUN Creatinine ratio

That's all!
Stay awesome <3



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