Specific Acid Base Confusion - Please Help! (UWorld vs. DIT)

[caprisun]

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A UWorld question gave me a patient with:

pH 7.23
PaO2 88
PaCO2 40
HCO3- 16

and said this was a mixed acid-base disorder because according to the Winter's Formula (PaCO2 = 1.5(HCO3-) +8), the PaCO2 is "inappropriately normal and should instead be 32. I can understand this.

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I received a question from a different source that gave me:

pH 7.50
PaCO2 42
HCO3- 35

and called this simply metabolic alkalosis. Using Winter's Formula, the PaCO2 should be 60 but is instead normal.

MY QUESTION: How can you tell if an "inappropriately NORMAL" value is due to a:
1) mixed acid-base disorder, or
2) single acid-base disturbance that has yet to be compensated?

 

[Ty Webb]

Your question is pretty broad. In general, there are two approaches to determining the nature of acid/base disturbances: you can use an algorithmic approach such as the one in FA (renal chapter), or you can try to view the blood gases holistically. I'd stick with the algorithm, it's more systematic and less error-prone.

For your first example, it would go like this:

1. Look at pH -> it's decreased -> acidosis
2. Look at CO2 -> it's normal/increased -> respiratory acidosis
3. Look at HCO3- -> it's decreased -> must also be metabolic acidosis present (or alternatively, use Winter's Formula here, and keep in mind that Winter's Formula only applies when trying to determine respiratory compensation in the presence of metabolic acidosis)

For your second example:

1. Look at pH -> it's increased -> alkalosis
2. Look at CO2 -> it's increased/normal -> metabolic alkalosis
3. Look at HCO3- -> it's increased -> further evidence of metabolic alkalosis

The pH and CO2 changes should be intuitive, and even if they're not, you can't really fail if you follow an algorithmic approach. The HCO3- should be intuitive as well, but maybe if you write down what disturbances you would expect for each scenario and review it until it makes sense, it will be easier to apply during an exam.

 

[FutureInternist]

A UWorld question gave me a patient with:

pH 7.23
PaO2 88
PaCO2 40
HCO3- 16

and said this was a mixed acid-base disorder because according to the Winter's Formula (PaCO2 = 1.5(HCO3-) +8), the PaCO2 is "inappropriately normal and should instead be 32. I can understand this.

---
I received a question from a different source that gave me:

pH 7.50
PaCO2 42
HCO3- 35

and called this simply metabolic alkalosis. Using Winter's Formula, the PaCO2 should be 60 but is instead normal.

MY QUESTION: How can you tell if an "inappropriately NORMAL" value is due to a:
1) mixed acid-base disorder, or
2) single acid-base disturbance that has yet to be compensated?

Winter's formula is used in metabolic acidosis only.
In the 2nd question, you obviously have an alkalosis and with a HCO3 of 35 it has to be metabolic. There are other formulae for expected values of pCO2 or HCO3 when the other one is known, depending on whether it is acute or chronic etc.
For the Step 1, just knowing Winter's should be enough

Also, regarding compensation, respiratory compensation is almost instantaneous while metabolic compensation takes 24-48 hours.

 

[caprisun]

Example #1:
pH 7.42
HCO3- 32
pCO2 64

Example #2:
pH 7.46
HCO3- 35
pCO2 53

Why is the first one considered a mixed metabolic alkalosis + respiratory acidosis while the second is considered metabolic alkalosis + respiratory compensation? I remember hearing something about "a compensation will never bring it back to normal pH." Is this true?


Also:
In Step Up 2 Step 2, there is a table for Compensation Formulas in Acid-Base Disturbances (p.99). Under the section for Metabolic Alkalosis that states
-If PCO2 >50, then additional respiratory acidosis
-if PCO2 <50, then additional respiratory alkalosis

My question is: when it says "additional," is that referring to a compensation or an additional mixed disorder? If this is referring to an additional mixed disorder, this does not coincide with example #2 because this would classify it as a mixed disorder and not just a regular compensation (because pCO2 >50).

 

[Phlame217]

Example #1:
pH 7.42
HCO3- 32
pCO2 64

Example #2:
pH 7.46
HCO3- 35
pCO2 53

Why is the first one considered a mixed metabolic alkalosis + respiratory acidosis while the second is considered metabolic alkalosis + respiratory compensation? I remember hearing something about "a compensation will never bring it back to normal pH." Is this true?

I have Dr. Goljan for my path teacher. One thing he has hammered into us is there is never full compensation. If you have a pH in normal range, then it has to be a mixed disorder.

Therefore:
pH 7.42 [range 7.35-7.45] this is normal = mixed
HCO3- 32 [range 22-28] this is alkalotic (metabolic)
pCO2 64 [range 35-45] this is acidosis (respiratory)

pH 7.46 [range 7.35-7.45] slighlt alkalotic = alkalosis
HCO3- 35 [range 22-28] heavily shifted to alkalosis (matches pH) = metabolic alkalosis
pCO2 53 [range 35-45] resp acidosis compensation

Also:
In Step Up 2 Step 2, there is a table for Compensation Formulas in Acid-Base Disturbances (p.99). Under the section for Metabolic Alkalosis that states
-If PCO2 >50, then additional respiratory acidosis
-if PCO2 <50, then additional respiratory alkalosis

My question is: when it says "additional," is that referring to a compensation or an additional mixed disorder? If this is referring to an additional mixed disorder, this does not coincide with example #2 because this would classify it as a mixed disorder and not just a regular compensation (because pCO2 >50).

No idea on this, Dr. Goljan didn't refer to or mention this.

 

[Ty Webb]

Example #1:
pH 7.42
HCO3- 32
pCO2 64

Example #2:
pH 7.46
HCO3- 35
pCO2 53

Why is the first one considered a mixed metabolic alkalosis + respiratory acidosis while the second is considered metabolic alkalosis + respiratory compensation? I remember hearing something about "a compensation will never bring it back to normal pH." Is this true?


Also:
In Step Up 2 Step 2, there is a table for Compensation Formulas in Acid-Base Disturbances (p.99). Under the section for Metabolic Alkalosis that states
-If PCO2 >50, then additional respiratory acidosis
-if PCO2 <50, then additional respiratory alkalosis

My question is: when it says "additional," is that referring to a compensation or an additional mixed disorder? If this is referring to an additional mixed disorder, this does not coincide with example #2 because this would classify it as a mixed disorder and not just a regular compensation (because pCO2 >50).

Look at pH, then PCO2, then HCO3-. Per your examples:

Example 1: pH is normal -> no alkalosis/acidosis, or mixed disorder (as the other poster alluded to, compensation will NOT completely correct the pH, thus this is NOT compensation. I'm sure there are rare circumstances that serve as exceptions, but this is a general rule and good guideline to follow)
PCO2 -> increased -> must be retaining CO2 -> respiratory acidosis
HCO3- -> increased -> metabolic alkalosis

You just have to apply this algorithm and use some logic. Since the pH is normal, we know there is either a mixed acid-base disturbance or there is no disturbance (not compensation, as already has been explained). Since the CO2 and HCO3- are not normal, we can assume in this case its a mixed disorder, and that acid-base disturbances are present that are counteracting each other. Since CO2 is significantly increased, this must mean that the patient is retaining CO2, and is in respiratory acidosis. Additionally, since HCO3- is increased, and since the pH is normal, we know this response is due to metabolic alkalosis (rather than also being due to respiratory acidosis, since the pH in this case would be low).

Example 1: pH is slightly elevated -> alkalosis
CO2 is high -> higher than expected compared to bicarb -> respiratory compensation (hypoventilation)
HCO3- is high -> due to both metabolic alkalosis and respiratory compensation/acidosis

With regards to your question about SU2S2, I think it is probably referring to an additional disturbance, but also would be of some value when attempting to quantify compensation. You just have to reason it out, and remember that with compensation there is never full correction of the pH disturbance.