Why is high BUN bad?

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

DrMasochist

Full Member
10+ Year Member
Joined
Nov 7, 2010
Messages
51
Reaction score
0
So why exactly is a high BUN bad? I understand it indicates poor or failing renal function, but does the decrease secretion of UREA in the urine cause problems throughout the body? Hyperammonemia can cause CNS problems which eventually leads to death, but high levels of UREA floating around (a nontoxic nitrogen carrier) doesnt seem to be a big deal to me.
Now I did find something in the Lipincott Biochem which said renal failure will shunt UREA to be disposed in the feces, and subsequently allow the gut bacteria to break down some of it (via ureases) to ammonia which is then put back into the blood (which I guess may lead to hyperammonemia). But does UREA have some kind of direct osmotic effect or does it disrupt cell functioning/enzymatic reactions, or is this bacterial finding the only major concern?

Thanks

Members don't see this ad.
 
Members don't see this ad :)
Elevated BUN may precede uremia, but I don't think they are synonymous (sort of like hypoxemia precedes cyanosis but hypoxemia doesn't necessarily mean cyanosis).

The wiki article said the findings may or may not be associated with high urea... weird.

I also want to know the "why" not just a list of the signs and symptoms. But if it is still unknown, I guess I'll have to accept it.
 
hipoxemia means cyanosis when you reach a certain value of deoxygenated Hb ensue, I think
 
An elevated BUN is a marker of bad "uremic toxins" that are actually responsible for the symptoms of uremia. The actual uremic toxins are unknown, I believe.

This is true. They've actually taken urea and injected it into all sorts of animal models to cause supraphysiologic azotemia without seeing the effects of uremia.

The hypothesis is indeed that there are other "toxins" (I hate using the word, since sham alternative medicine programs beat it to death) which precipitate uremia.

Azotemia = high blood levels of nitrogen containing compounds
Uremia = symptoms

For example, when you have a patient with a GI bleed developing uremic symptoms, even though their BUN is through the roof? (Actually if anyone has, please fill me in). Not to be confused with GI bleeding secondary to uremia, because uremia prevents platelet clot formation.

Edit: Sorry! Thought this was the step 2 area for a second. Realized you haven't seen patients yet and my example is moot. Regardless, if someone (OveractiveBrain?) could comment on my example that would be great to help clarify this. I want to make sure I'm not spitting out falsehoods based off my own experience.
 
So why exactly is a high BUN bad? I understand it indicates poor or failing renal function, but does the decrease secretion of UREA in the urine cause problems throughout the body? Hyperammonemia can cause CNS problems which eventually leads to death, but high levels of UREA floating around (a nontoxic nitrogen carrier) doesnt seem to be a big deal to me.
Now I did find something in the Lipincott Biochem which said renal failure will shunt UREA to be disposed in the feces, and subsequently allow the gut bacteria to break down some of it (via ureases) to ammonia which is then put back into the blood (which I guess may lead to hyperammonemia). But does UREA have some kind of direct osmotic effect or does it disrupt cell functioning/enzymatic reactions, or is this bacterial finding the only major concern?

Thanks

Critical care fellow here. We definitely see BUNs over 100 in the ICU. The record high I've personally see is 236.

As mentioned above, there is an encephalopathy that results from azotemia. You will also see delirium in patients immediately after dialysis (if BUN drops too much too soon) because of the osmotic effect = temporary cerebral edema.

There are also negative effects on platelet aggregation. I've had to give DDAVP to uremic pts in the OR before who had normal coags and patelet counts but had a bleeding diathesis which was presumably related to their azotemia.
 
Critical care fellow here. We definitely see BUNs over 100 in the ICU. The record high I've personally see is 236.

As mentioned above, there is an encephalopathy that results from azotemia. You will also see delirium in patients immediately after dialysis (if BUN drops too much too soon) because of the osmotic effect = temporary cerebral edema.

There are also negative effects on platelet aggregation. I've had to give DDAVP to uremic pts in the OR before who had normal coags and patelet counts but had a bleeding diathesis which was presumably related to their azotemia.

Im going to second this, then take in the direction of Step 1.

BUN has osmotic effect. So does Na. So does glucose. In fact, you can estimate the serum osmoles with the equation

2*Na + Glc/18 + Bun/2.8

So at BUNs of 10 (normal) the osmotic effect is (divided by 2.8) meh, negligible. But at levels in the triple digits (triple digits are required to call it "uremic encephalopathy") they start to have effect. Do the math if you don't believe me:

BUN 10 = Osmolals of 10/2.8 = 3
BUN 150 = Osmolals of 150/2.8 = 50

Your serum osmoles are about 280 normally, so I hope you can feel that "3" is not the big of a deal while "50" is a substantial portion of your serum osmoles. If you haven't kept up to this point, you probably won't be able to continue...

You've got alot of BUN around. It has osmotic effect. It gets into the brain. Lots of osmoles in the blood, same amount as always in the brain. What does water do? It travels to the area of highest concentration to low concentration (flows from more water, low concentration, to less water, high concentration). The brain, seeing a high BUN in the blood, shifts water from the cells to the blood. Plump happy cells get dehydrated, going to sad crumpled floppy cells. Sad cells 🙁

Now lets take PMPMD's dialysis example. Body has a lot of BUN, sad cells have already occurred. But they aren't dead. So, the body has adjusted. Now, the happy renal fellow (in his first year) sees a BUN of 236 (osmoles of what? 236/2.8 = 80ish) and says "ILL SAVE THE DAY!" and takes the BUN from 236 to 10. You just lost 80 osmoles from the blood. Now in the blood is "less stuff" and in the cells is relatively "more stuff." What does water do? It travels from the blood and into the brain. At first you might think "rehydrating the cells, thats good." But oh, as with everything in medicine, it is about the rate. You take crumpled sad cells (who, in their constant depression have accepted their fate, and like a dog in an electric cage, have adjusted), and FLOOD them with water. Much like a brittle water balloon they swell. Brain swelling? Bad. WORSENING the encephalopathy by causing a rebound edema, or, worse, popping the cells altogether.

In short: BUN causes problems because of water shifts in the brain. Because every BUN is divided by 3 (while Na is multiplied by 2) it takes a lot of BUN to make an osmolar impact. It all boils down to general chemistry, water flowing from an area of more water to an area of less water.
 
Im going to second this, then take in the direction of Step 1.

BUN has osmotic effect. So does Na. So does glucose. In fact, you can estimate the serum osmoles with the equation

2*Na + Glc/18 + Bun/2.8

So at BUNs of 10 (normal) the osmotic effect is (divided by 2.8) meh, negligible. But at levels in the triple digits (triple digits are required to call it "uremic encephalopathy") they start to have effect. Do the math if you don't believe me:

BUN 10 = Osmolals of 10/2.8 = 3
BUN 150 = Osmolals of 150/2.8 = 50

Your serum osmoles are about 280 normally, so I hope you can feel that "3" is not the big of a deal while "50" is a substantial portion of your serum osmoles. If you haven't kept up to this point, you probably won't be able to continue...

You've got alot of BUN around. It has osmotic effect. It gets into the brain. Lots of osmoles in the blood, same amount as always in the brain. What does water do? It travels to the area of highest concentration to low concentration (flows from more water, low concentration, to less water, high concentration). The brain, seeing a high BUN in the blood, shifts water from the cells to the blood. Plump happy cells get dehydrated, going to sad crumpled floppy cells. Sad cells 🙁

Now lets take PMPMD's dialysis example. Body has a lot of BUN, sad cells have already occurred. But they aren't dead. So, the body has adjusted. Now, the happy renal fellow (in his first year) sees a BUN of 236 (osmoles of what? 236/2.8 = 80ish) and says "ILL SAVE THE DAY!" and takes the BUN from 236 to 10. You just lost 80 osmoles from the blood. Now in the blood is "less stuff" and in the cells is relatively "more stuff." What does water do? It travels from the blood and into the brain. At first you might think "rehydrating the cells, thats good." But oh, as with everything in medicine, it is about the rate. You take crumpled sad cells (who, in their constant depression have accepted their fate, and like a dog in an electric cage, have adjusted), and FLOOD them with water. Much like a brittle water balloon they swell. Brain swelling? Bad. WORSENING the encephalopathy by causing a rebound edema, or, worse, popping the cells altogether.

In short: BUN causes problems because of water shifts in the brain. Because every BUN is divided by 3 (while Na is multiplied by 2) it takes a lot of BUN to make an osmolar impact. It all boils down to general chemistry, water flowing from an area of more water to an area of less water.

Nice!
 
Im going to second this, then take in the direction of Step 1.

BUN has osmotic effect. So does Na. So does glucose. In fact, you can estimate the serum osmoles with the equation

2*Na + Glc/18 + Bun/2.8

So at BUNs of 10 (normal) the osmotic effect is (divided by 2.8) meh, negligible. But at levels in the triple digits (triple digits are required to call it "uremic encephalopathy") they start to have effect. Do the math if you don't believe me:

BUN 10 = Osmolals of 10/2.8 = 3
BUN 150 = Osmolals of 150/2.8 = 50

Your serum osmoles are about 280 normally, so I hope you can feel that "3" is not the big of a deal while "50" is a substantial portion of your serum osmoles. If you haven't kept up to this point, you probably won't be able to continue...

You've got alot of BUN around. It has osmotic effect. It gets into the brain. Lots of osmoles in the blood, same amount as always in the brain. What does water do? It travels to the area of highest concentration to low concentration (flows from more water, low concentration, to less water, high concentration). The brain, seeing a high BUN in the blood, shifts water from the cells to the blood. Plump happy cells get dehydrated, going to sad crumpled floppy cells. Sad cells 🙁

Now lets take PMPMD's dialysis example. Body has a lot of BUN, sad cells have already occurred. But they aren't dead. So, the body has adjusted. Now, the happy renal fellow (in his first year) sees a BUN of 236 (osmoles of what? 236/2.8 = 80ish) and says "ILL SAVE THE DAY!" and takes the BUN from 236 to 10. You just lost 80 osmoles from the blood. Now in the blood is "less stuff" and in the cells is relatively "more stuff." What does water do? It travels from the blood and into the brain. At first you might think "rehydrating the cells, thats good." But oh, as with everything in medicine, it is about the rate. You take crumpled sad cells (who, in their constant depression have accepted their fate, and like a dog in an electric cage, have adjusted), and FLOOD them with water. Much like a brittle water balloon they swell. Brain swelling? Bad. WORSENING the encephalopathy by causing a rebound edema, or, worse, popping the cells altogether.

In short: BUN causes problems because of water shifts in the brain. Because every BUN is divided by 3 (while Na is multiplied by 2) it takes a lot of BUN to make an osmolar impact. It all boils down to general chemistry, water flowing from an area of more water to an area of less water.

This is an EXCELLENT description of osmotic gradients and flow and should be learned by all medical students and residents.
 
Im going to second this, then take in the direction of Step 1.

BUN has osmotic effect. So does Na. So does glucose. In fact, you can estimate the serum osmoles with the equation

2*Na + Glc/18 + Bun/2.8

So at BUNs of 10 (normal) the osmotic effect is (divided by 2.8) meh, negligible. But at levels in the triple digits (triple digits are required to call it "uremic encephalopathy") they start to have effect. Do the math if you don't believe me:

BUN 10 = Osmolals of 10/2.8 = 3
BUN 150 = Osmolals of 150/2.8 = 50

Your serum osmoles are about 280 normally, so I hope you can feel that "3" is not the big of a deal while "50" is a substantial portion of your serum osmoles. If you haven't kept up to this point, you probably won't be able to continue...

You've got alot of BUN around. It has osmotic effect. It gets into the brain. Lots of osmoles in the blood, same amount as always in the brain. What does water do? It travels to the area of highest concentration to low concentration (flows from more water, low concentration, to less water, high concentration). The brain, seeing a high BUN in the blood, shifts water from the cells to the blood. Plump happy cells get dehydrated, going to sad crumpled floppy cells. Sad cells 🙁

Now lets take PMPMD's dialysis example. Body has a lot of BUN, sad cells have already occurred. But they aren't dead. So, the body has adjusted. Now, the happy renal fellow (in his first year) sees a BUN of 236 (osmoles of what? 236/2.8 = 80ish) and says "ILL SAVE THE DAY!" and takes the BUN from 236 to 10. You just lost 80 osmoles from the blood. Now in the blood is "less stuff" and in the cells is relatively "more stuff." What does water do? It travels from the blood and into the brain. At first you might think "rehydrating the cells, thats good." But oh, as with everything in medicine, it is about the rate. You take crumpled sad cells (who, in their constant depression have accepted their fate, and like a dog in an electric cage, have adjusted), and FLOOD them with water. Much like a brittle water balloon they swell. Brain swelling? Bad. WORSENING the encephalopathy by causing a rebound edema, or, worse, popping the cells altogether.

In short: BUN causes problems because of water shifts in the brain. Because every BUN is divided by 3 (while Na is multiplied by 2) it takes a lot of BUN to make an osmolar impact. It all boils down to general chemistry, water flowing from an area of more water to an area of less water.

BUN (Urea), to my understanding, is an INEFFECTIVE OSMOL - doesn't effect the shifting of electrolytes, fluids as it freely crosses blood-brain barrier and cell membranes. However, OveractiveBrain is correct in saying that if you remove BUN that high in his example (200+) too quickly as by dialysis, then it takes body time to equilibrate that reduced osmol 2/2 dec BUN load - it is a phenomenon called Dialysis Dysequilibrium Syndrome (mostly happens with new dialysis pt particularly with high BUN). But in general you won't see many excited Renal fellows when they see BUN ~ 100 as it is ineffective osmol and will not shift fluid status one way or the other.

Just my 2 cents.
 
Im going to second this, then take in the direction of Step 1.

BUN has osmotic effect. So does Na. So does glucose. In fact, you can estimate the serum osmoles with the equation

2*Na + Glc/18 + Bun/2.8

So at BUNs of 10 (normal) the osmotic effect is (divided by 2.8) meh, negligible. But at levels in the triple digits (triple digits are required to call it "uremic encephalopathy") they start to have effect. Do the math if you don't believe me:

BUN 10 = Osmolals of 10/2.8 = 3
BUN 150 = Osmolals of 150/2.8 = 50

Your serum osmoles are about 280 normally, so I hope you can feel that "3" is not the big of a deal while "50" is a substantial portion of your serum osmoles. If you haven't kept up to this point, you probably won't be able to continue...

You've got alot of BUN around. It has osmotic effect. It gets into the brain. Lots of osmoles in the blood, same amount as always in the brain. What does water do? It travels to the area of highest concentration to low concentration (flows from more water, low concentration, to less water, high concentration). The brain, seeing a high BUN in the blood, shifts water from the cells to the blood. Plump happy cells get dehydrated, going to sad crumpled floppy cells. Sad cells 🙁

Now lets take PMPMD's dialysis example. Body has a lot of BUN, sad cells have already occurred. But they aren't dead. So, the body has adjusted. Now, the happy renal fellow (in his first year) sees a BUN of 236 (osmoles of what? 236/2.8 = 80ish) and says "ILL SAVE THE DAY!" and takes the BUN from 236 to 10. You just lost 80 osmoles from the blood. Now in the blood is "less stuff" and in the cells is relatively "more stuff." What does water do? It travels from the blood and into the brain. At first you might think "rehydrating the cells, thats good." But oh, as with everything in medicine, it is about the rate. You take crumpled sad cells (who, in their constant depression have accepted their fate, and like a dog in an electric cage, have adjusted), and FLOOD them with water. Much like a brittle water balloon they swell. Brain swelling? Bad. WORSENING the encephalopathy by causing a rebound edema, or, worse, popping the cells altogether.

In short: BUN causes problems because of water shifts in the brain. Because every BUN is divided by 3 (while Na is multiplied by 2) it takes a lot of BUN to make an osmolar impact. It all boils down to general chemistry, water flowing from an area of more water to an area of less water.

I <3 you.
 
Top