ABGs and Compensation

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Does anyone understand the ABG compensation thing? I understand the basic concept of metabolic acidosis and alkalosis and respiratory acidosis and alkalosis. But when there are 3 values given.... Ph, PCO2, and HCO3 and then you have to decide whethere its metabolic or respiratory then acidosis or alkalosis THEN if its compensation or not I get so confused. Can anyone help??

Normal Ph range is 7.35-7.45 with 7.4 being the absolute value

Normal PaCo2 is 35-45. Below 35 is alkaline ( base) and above 45 is acidic

Normal HCO3- is 22- 26. Below 22 is acidic and above 26 is alkaline ( base)

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^^ lol I got that part down... im just having trouble applying the knowledge to figure out if its compensated and what kind of compensation

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ph 7.50

PCO2 50

HCO3 42

The ph and HCO3 are both increased ... so that should indicated metabolic alkalosis but nooo... its metabolic alkalosis with comepensation. I dont understand where this compensation comes in!!

It is metabolic alkalosis w/ compensation. The key here is to look at the Ph. It's 7.5 right...that's abnormal- it's alkaline ( base). Look at the PaCo2...it's in the acidic range @ 50.

So the PaCo2 is acidic because it's compensating ....but it's only partially compensated because the Ph is still alkaline.

The PaCo2 is trying to compensate for the alkalinic state of the blood ( 7.5) and so it's increased. More C02= more acid...hence the acidic PaCo2 value.

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OK I see what you mean.

Here is a quick little tutorial I wrote for some of my classmates.

Step #1.

Write down the values

Step #2.

Interpret them as Acid or Base ( alkaline)

Step #3

Look at the PaCo2. Is it acidic or base?

Step #4.

Look at the Ph. Is it acidic or base.

If the PaCo2 values MATCH the Ph value( acidic or base)...you have a RESPIRATORY disorder. If not...see step 5

ie

Ph 7.14 ( acidic)

PaCo2 50 ( acidic)

HCO3- 24 ( normal)

This is respiratory acidosis

Step #5.

Look at the HCO3- does it's value ( acid or base) match the Ph? If so you have a METABOLIC DISORDER

ie

Ph 7.14 ( Acidic)

PaCo2 36 ( normal)

HCO3- 17 ( acidic)

Metabolic Acidosis

Step #6.

Interpret your results as either Respiratory Acidosis/Alkalosis or Metabolic Acidosis/ Alkalosis

Step #7.

You have now determined that the Ph waswas acidic or base ( alkaline). You have already determined if it was the PaCo2 or the HCO3- that resulted your state. NOW look at the OTHER value. If you had an acidic state caused by PaCo2...is the OTHER value alkaline ( the HCO3-)? If so...you have compensation. It's only a PARTIAL compensation since the Ph is still in the abnormal range. It would be a FULL compensation if the Ph was in the normal range.

Does this help at all?

Basically you have compensation occuring here

Ph 7.14 ( acidic)

PaCo2 20 ( base- alkaline)

HCO3- 18 ( acid)

Metabolic acidosis w/ partial compensation

BUT NOT HERE

Ph 7.14 (Acid)

PaCO2 35 ( normal)

HCO3- 18 ( Acid)

Metabolic Acidosis

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Specializes in ICUs, Tele, etc..

What if the Co2 is 45 and the HCO3 26...

How do you choose between respiratory and metabolic?

Okay if the co2 is less than 35 and the hco3 is greater than 26, your pH would be GREATER than 7.45, you will call this a MIXED alkalosis.

Same thing if the CO2 is greater than 45 and the HCO3 is less than 22, if they occur at the same time, your pH would be less than 7.35 and you will call this a MIXED acidosis.

In essence if both of them are abnormal, and both of them point towards the alkalosis part or the acidotic part, then it's called MIXED acidosis or MIXED alkalosis depending upon which is abnormal. There would be no compensation from either or because both would be exhibiting either alkalosis or acidosis.

For example...

pH 7.22 - acidotic ph

co2 50 - acidotic

hco3 18 - acidotic

hence this is a MIXED acidosis

again if it's

pH 7.50 - alkalotic pH

co2 30 - alkalotic

hco3 29 - alkalotic

then this would be MIXED alkalosis

NOTE THAT THERE ARE NO COMPENSATION FOR EITHER BECAUSE BOTH OF THEM ARE EITHER ACIDOTIC OR ALKALOTIC.

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Specializes in CVICU.

I hope this will help everyone. Our teacher taught us to draw arrows next to each value. If the PH was elevated(Alkaline) then the arrow yould point up. If the PCO2 was decreased then the arrow would point down.

You determine if it is respiratory by looking at the PH and PCO2. Is the arrows were both pointing in different directions (one up and one down) Then it is a respiratory problem.

To determine if it was a metabolic problem you have to look at PH and HCO3. Both arrows should point in the same direction ( Both up or both down).

If compensation was involved you determine it after you determine the problem. Say you had a problem where the PH and the HCO3 were both elevated. Based on the rules you know that it is Metabolic Alkalosis. But what if the PCO2 was also elevated instead of within normal range? It would be Metabolic Alkalosis with respiratory compensation. You know that it could not be a respiratory problem because both the PH and PCO2 arrows are pointing in the same direction(Up) instead of opposite directions so it is respiratory compensation. It the PCO2 was in normal range it would be no compensation involved.

Use the arrow mwthod and see if that helps you work out the problems.

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When you say respiratory compensation do you mean that if this was a question on a test....

ph 7.55

pco2 30

hco3 40

The only correct answer would be respiratory alkalosis with compensation?

The reason im asking is because our instructor gave us this problem and the answer was respiratory alkalosis with compensation

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Specializes in ICUs, Tele, etc..
When you say respiratory compensation do you mean that if this was a question on a test....

ph 7.55

pco2 30

hco3 40

The only correct answer would be respiratory alkalosis with compensation?

The reason im asking is because our instructor gave us this problem and the answer was respiratory alkalosis with compensation

Maybe you misinterpreted the question or the answer wrong because

pH 7.55 - Alkalosis

pc02 30 - Alkalosis

hco3 40 - ALKALOSIS

therefore, this is a MIXED ALKALOSIS, or you can say Mixed respiratory and metabolic alkalosis. there's no compensation whatosever.

If there was a hc03 compensation then your bicarb should be lower than normal

EDIT: Here I found you a case study of Mixed Alkalosis http://www.anaesthesiamcq.com/AcidBaseBook/AB9_6Case4.php

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There was another with ph of 7.50

hco2 50

hco3 42

and that was metabolic alkalosis and lungs are trying to compensate

is that right?

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There was another with ph of 7.50

hco2 50

hco3 42

and that was metabolic alkalosis and lungs are trying to compensate

is that right?

Yes this is Metabolic Alkalosis w/ compensation..if you meant PaCo2 was 50

ph of 7.50 ( alkalosis)

PaCo2 50( acidosis)

hco3 42 ( alkalosis)

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Specializes in OR.

A classmate of mine showed me his technique and it helped me get the whole compensation concept.

1. Give it a last name(ie. acidosis or alkalosis)

2. Look at ph and values and apply the R.O.M.E. principle(respiratory opposite, metabolic equal.)

3. Look at your values again, this time checking to see if one is abnormal or both. If both are out of whack, you know you have some compensation happening

4. Look at the PH again, if it is out of normal range you know you have partial compensation. If it's in normal range but both the HCO3 and CO2 are wacky, it's full compensation.

Hope this helps. It's not fancy but it works for me....

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I can figure out if its acidosis or alkalosis and if theres compensation going on but when you use this principle:

2. Look at ph and values and apply the R.O.M.E. principle(respiratory opposite, metabolic equal.)

What if the PCO2 is opposite the PH

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