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There seems to be an association between the use of "normal" saline and acute kidney injury (when compared with other IV fluids).

Why would that be?

#medthread #medtwitter #tweetorial

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We need to first understand 2 things:

What is the difference between saline and other "balanced" crystalloid IV fluids?
What is the evidence that saline is associated with kidney injury?

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How is saline different from other "balanced" crystalloid IV fluids?

Saline haslevels of Na/Cl
Bc of this difference in composition, when used in large volumes salineserum Cl levels and causes a hyperchloremic metabolic acidosis

https://bit.ly/358PJah 

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What is the evidence that saline is associated with renal injury?

There are several studies suggesting this association. Here's one

https://bit.ly/2ZG8wbZ 

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Because there is significantly more chloride in saline than other IV fluids, let's zoom in and see how that might affect the kidney

Infusing high chloride-containing fluids into dogs decreased renal perfusion and glomerular filtration rate (GFR).

https://bit.ly/36ei7cy 

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Why would high serum chloride levels decrease renal blood flow and GFR?

The answer involves the macula densa.

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To review:

Macula densa cells sense Na/Cl levels in the distal convoluted tubule (DCT)
In response to increased Na/Cl levels in the DCT the macula densa decreases renal blood flow and GFR by inducing renal afferent arteriole vasoconstriction

https://bit.ly/2Qdwzf7 

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Look how close the macula densa and renal afferent arteriole are to each other

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2776136/

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When exposed to high levels of Cl (and Na) the macula densa cells literally swell.

https://www.ncbi.nlm.nih.gov/pubmed/12060602 

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Along with swelling the macula densa cells release ATP, signalling nearby afferent arterioles to constrict,renal blood flow and GFR.

This process is called tubuloglomerular feedback, allowing the kidney to regulate its own perfusion

https://bit.ly/2ZGbzRs 

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ATP release from the macula densa cells is actually stimulated directly by Cl

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2776136/

So,Cl sensed by the macula dense leads toATP release, which signals afferent renal arterioles to constrict, whichGFR

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Let's summarize:

"Normal" saline haslevels of Cl compared w/ other IV crystalloid fluids
Increased serum Cl levelsrenal blood flow and GFR
This occurs because of ATP release from macula densa cells, which causes renal afferent arteriolar vasoconstriction