Analyzing ABG's: Identifying Isolated Or Mixed Metabolic Disorder

Analyzing ABG's: How To Identify An Isolated Or Mixed Acidosis Or Alkalosis.

When analyzing ABG's, arterial blood gasses, we do pretty good at determining if our gasses represent a metabolic or respiratory acidosis or alkalosis.  We look at the pH, then the PaCO2, then the Bicarb, and the we make our "diagnosis."  But if that's all we do, we're missing a HUGE part of the equation.

The Scenario:

Say you're patient has a pH of 7.1, a PaCO2 of 32 , and a BiCarb of 10.  What does this gas tell you?

Well, we know the patient is in a metabolic acidosis because the pH and Bicarb are low.  We also know it's partially compensated because the PaCO2 is low. But is this the whole story?

We know the respiratory system is the 2nd fastest mechanism for compensating to acid-base disorders, but how do we know if the respiratory system is keeping up with the demand to buffer the low bicarb?  How do we know there isn't another problem contributing to the patients metabolic acidosis?

The answer can be found using the "Winters" formula.  (Don't confuse this with De Winters T-wave).

Winters formula is used for identifying mixed acid-base disorders of the Metabolic system...

  • Metabolic Acidosis
  • Metabolic Alkalosis

There's a slight variation in the formula for each, so pay attention, it's quite easy.

Metabolic Acidosis

The formula for calculating EXPECTED PaCO2 for MetabolicAcidosis looks like this...

PACO2=1.5(BICARB)+8, PLUS OR MINUS 2

Using the ABG's from the previous example, we would get an expected PaCO2 of N=1.5(16)+8 where n=expected PaCO2.

In this case our expected PaCO2 is 23 (plus or minus 2)

However, our measured PaCO2 is 32.  Why?

Well, the answer is you're dealing with a MIXED ACIDOSIS.  Something else is driving up the CO2 and contributing to your acidosis.  The most likely culprit is the the patients hypoventilating.

Metabolic Alkalosis

The formula for calculating EXPECTED PaO2 for MetabolicAlkalosis is just a little different and looks like this...

PAO2=0.9(BICARB)+16, PLUS OR MINUS 2

The application of the formula is the same.  Do your calculation to determine what you'd expect your PaCO2 to be, then compare it to you measured PaCO2.  If their the same, or close, you're probably dealing with an isolated acid-base disorder.

If the different then you're dealing with a mixed disorder, and you need to do some more investigation as to why. NOTE: The greater the difference between your measured PaCO2 and your expected PaCO2 the more likely it is you're dealing with a mixed disturbance.

Check here to read an article published in the Journal of Critical Care Nurses for additional information about Winders Formula and how to identified mixed respiratory acid-base disorders.

For additional information about how the acid-base compensatory system works, click here to check out a related post from FlightCrit.

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Fly Safe, and Live Well!

Sean

Sean Eaton

ProMedic Project LLC, 608 Hoyt Lane, Lafayette, CO, 80026