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Relationship Between Pco2 And Ph


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#1 Michael Berrier

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Posted 28 June 2010 - 07:01 PM

It is commonly taught that a change in pCO2 of 10 mmHg will cause a change in pH of 0.08 in the opposite direction. What I want to know is does this tasty nugget have a name (i.e. Bob's Law, Homer's Postulate, Newton's Apple, etc)? I suppose I'd ask the same of the relationship between pH and serum K.

Thanks,
mb
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#2 RT_TLP

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Posted 28 June 2010 - 08:12 PM

Haven't found a definitive name for the equation. However,


Acute Resp. Acidosis: Change in pH = 0.008 x (40 - PaCO2)
Chronic Resp. Acidosis: Change in pH = 0.003 x (40 - PaCO2)

Acute Resp. Alkalosis: Change in pH = 0.008 x (40 - PaCO2)
Chronic Resp. Alkalosis: Change in pH = 0.017 x (40 - PaCO2)

Reference: http://emedicine.med...301574-overview
http://emedicine.med...301680-overview
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#3 Gila

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Posted 28 June 2010 - 09:14 PM

I think you are talking about the Henderson Hasselbalch equation? How well do you understand the concept of an equilibrium constant and acid/base relationships along with the concept of a logarithm? If you're understanding is not great, the formula will be rather confusing. In addition, with a few basic assumptions, I can rearrange the formula for clinical applications. One such rearrangement allows us to solve for PH. Rather than attempting to explain, I simply wrote out the equation and calculated PH based on arbitrary bicarbonate and CO2 values, then increased the CO2 by 10 mmHg:

Posted Image
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#4 FloridaMedic

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Posted 28 June 2010 - 09:32 PM

Look at steps 2 and 3.

http://rtcorner.net/...es&directory=73

Good sites:

http://rtcorner.net/

http://www.acid-base.com/
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#5 Gila

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Posted 28 June 2010 - 09:33 PM

Just wanted to add, when doing these calculations we are looking at a few variables in a vacuum and using assumptions in the form of a constant to solve. Clearly, I would expect clinical variation.
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#6 RT_TLP

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Posted 28 June 2010 - 09:37 PM

Why was I thinking the Henderson-Hasselbalch equation was used to solve for Bicarb only? Posted Image
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#7 SerendepitySaki

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Posted 29 June 2010 - 03:57 AM

can solve for either- no biggee... = ionization



Why was I thinking the Henderson-Hasselbalch equation was used to solve for Bicarb only? Posted Image


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#8 Gila

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Posted 29 June 2010 - 04:25 AM

Provided you have two of the three variables, you can solve for either PH, HCO3-, or CO2. I assume bicarbonate is often discussed because most blood gas analysers will directly measure PH and PaCO2, but simply calculate the HCO3- based on the other two measurements.

I should also point out while this only measures the bicarbonate buffer system PH (we know many other buffers such as protein and phosphates exist), the PH of the bicarbonate buffer system pretty much translates to the PH of all the other systems and the overall PH of the blood. This is because all the various buffer systems are in equilibrium and the PH applies across the board in a concept known as the isohydric principle.
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#9 MSDeltaFlt

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Posted 29 June 2010 - 07:13 PM

Provided you have two of the three variables, you can solve for either PH, HCO3-, or CO2. I assume bicarbonate is often discussed because most blood gas analysers will directly measure PH and PaCO2, but simply calculate the HCO3- based on the other two measurements.

I should also point out while this only measures the bicarbonate buffer system PH (we know many other buffers such as protein and phosphates exist), the PH of the bicarbonate buffer system pretty much translates to the PH of all the other systems and the overall PH of the blood. This is because all the various buffer systems are in equilibrium and the PH applies across the board in a concept known as the isohydric principle.


One thing you can do is to compare HCO3 with BMP CO2 levels. If they're similar, then the Henderson-Hasselbach equation is accurate, especially if the ABG's are corrected to pt temp (Charles' Law works even in the blood). You can also have an Anion Gap acidosis going on: pretty good PaCO2 and HCO3 and pH is still low, however your BE is -9 or greater.

Don't want to hijack a good thread, but you get my point.
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#10 Gila

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Posted 29 June 2010 - 08:33 PM

One thing you can do is to compare HCO3 with BMP CO2 levels. If they're similar, then the Henderson-Hasselbach equation is accurate, especially if the ABG's are corrected to pt temp (Charles' Law works even in the blood). You can also have an Anion Gap acidosis going on: pretty good PaCO2 and HCO3 and pH is still low, however your BE is -9 or greater.

Don't want to hijack a good thread, but you get my point.


Also, H-H is not a complete way to look at acid/base balance. There essentially exist three methods of appreciating this concept per se.

*Henderson-Hasselbach is perhaps an incomplete and more qualitative method of looking at this subject.

*The base deficit theory that you discussed allows us to appreciate a bit more of the picture and place more of a quantitative value on the base excess versus deficit in an acidosis.

However, both of the above theories assume that cations and anions in the body remain unchanged. However, this is in fact false as we all know. Hence, a newer approach is known as the *strong ion approach. It essentially takes three core concepts into consideration:

1) CO2

2) Strong ion difference: Basically, the (+) ions less the (-) ions. Much like what you do when you calculate your anion gap.

3) Weak acid concentration

It is actually possible to reformat the classical H-H formula into a complicated monster that is a much better generalization, but I think using the H-H allows us to appreciate where these generalisations about CO2 and PH changes come from, while hopefully appreciating the reliability of such generalisations.
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#11 MSDeltaFlt

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Posted 29 June 2010 - 09:05 PM

Also, H-H is not a complete way to look at acid/base balance. There essentially exist three methods of appreciating this concept per se.

*Henderson-Hasselbach is perhaps an incomplete and more qualitative method of looking at this subject.

*The base deficit theory that you discussed allows us to appreciate a bit more of the picture and place more of a quantitative value on the base excess versus deficit in an acidosis.

However, both of the above theories assume that cations and anions in the body remain unchanged. However, this is in fact false as we all know. Hence, a newer approach is known as the *strong ion approach. It essentially takes three core concepts into consideration:

1) CO2

2) Strong ion difference: Basically, the (+) ions less the (-) ions. Much like what you do when you calculate your anion gap.

3) Weak acid concentration

It is actually possible to reformat the classical H-H formula into a complicated monster that is a much better generalization, but I think using the H-H allows us to appreciate where these generalisations about CO2 and PH changes come from, while hopefully appreciating the reliability of such generalisations.


Do you have a lay-out of that formula?
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#12 Gila

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Posted 29 June 2010 - 09:45 PM

The following explains the strong ion concept:

http://www.acid-base.com/strongion.php

I believe the re-arrangement will look like this:

Posted Image

I am pretty sure it is also explained fairly well on emedicine. In fact, I believe it was a source I used on a chemistry project last semester. I will need to pull the papers, but a google search may be faster for you.

Also, the strong ion concept is rather controversial, but it does take into account additional changes.
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Christopher Bare
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#13 MSDeltaFlt

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Posted 29 June 2010 - 09:56 PM

The following explains the strong ion concept:

http://www.acid-base.com/strongion.php

I believe the re-arrangement will look like this:

Posted Image

I am pretty sure it is also explained fairly well on emedicine. In fact, I believe it was a source I used on a chemistry project last semester. I will need to pull the papers, but a google search may be faster for you.

Also, the strong ion concept is rather controversial, but it does take into account additional changes.


Thanks, Chris.
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#14 Gila

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Posted 29 June 2010 - 10:14 PM

You are welcome. My apologies if this has gone too far off topic. This is something I find quite interesting and spent the better part of a semester doing an acid/base chemistry project last year.
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Christopher Bare
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