Acid Base Review, Part 2
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PART TWO OF THE ACID BASE GUIDE
FOR THE MENTALLY CHALLENGED

Okay, the next part you’ve all been waiting for is how to work out an acid base problem.

I WAS going to do a q&a kind of thing but I think I’ll just go through the process step by step of what the hell to do.

First:  Check out the blood pH. Remember normal is 7.4.  If the pH is <7.2 or >7.6 then we have to address the acid base disorder specifically.  If the pH is between 7.2 & 7.6 then handling the primary cause should be adequate.  To diagnose the primary cause, the following calculations may be very helpful so don’t think you don’t have to do them.

If the pH is <7.2 then you should use the formula to determine how many mEq of bicarb to add:  mEq to add = 0.3 x kg x base excess

If the pH > 7.6, then you ask Roger Gfeller what to do cuz I sure as hell don’t know.

Second:  Figure out if there is a primary acidosis or a primary alkalosis.  If pCO2 >46, you have resp. acidosis. If pCO2 < 36, you have resp alkalosis.  If base excess < -4 then you have metabolic acidosis.  If base excess > +4, you have metabolic alkalosis.  The blood pH should be on the side of the primary derangement.

Third:  Figure out if there is a mixed acid/base disorder.  What is a mixed acid/base disorder? Let me re-phrase. Figure out if there is an additional acid/base disorder in combination w/what you think is going on.

Why would there be a second (or third) problem? Murphy’s law, that’s why. Let me illustrate what I am talking about. Let’s say you have a patient with an obstructed pylorus.  He has a metabolic alkalosis from vomiting all his H+ away. He also has a metabolic acidosis from dehydration.  And he has a compensatory respiratory alkalosis.  Sounds like a nightmare of calculations, doesn’t it. (Vomiting patient, acidotic, dehydrated, resp. alkalosis.  You might be fooled into thinking you could just correct the dehydration & control the nausea. You might miss this very valuable tip off to a pyloric obstruction.  You might not push for that barium study or that exploratory surgery if you don’t know for a fact there’s more to the picture.)

You need to figure out what your expected compensatory mechanism should produce & if you didn’t get approx. what the equations below say you should have gotten, then you should look for another disease.

SAY YOU HAVE A PRIMARY METABOLIC ACIDOSIS:
PaCO2 (expected) = PaCO2(normal) - [ (normal bicarb - measured bicarb) x 0.8]

If PaC02 is greater than expected then you have a respiratory acidosis concurrently; if it is less than expected you have a respiratory alkalosis concurrently.

SAY YOU HAVE A PRIMARY METABOLIC ALKALOSIS:
PaC02(expected) = [0.7 x (measured bicarb -normal bicarb)] + PaC02(normal)

If PaC02 is greater than expected then you have a respiratory acidosis concurrently; if it is less than expected you have a respiratory alkalosis concurrently.

SAY YOU HAVE A PRIMARY RESPIRATORY ALKALOSIS:
Bicarb (expected)= normal bicarb - ([PaC02 normal - PaC02 measured] x 0.55)

If bicarb is greater than expected then you have a metabolic alkalosis concurrently; if it is less than expected you have a metabolic acidosis concurrently.

SAY YOU HAVE A PRIMARY RESPIRATORY ACIDOSIS:
Bicarb (expected) = [(PaC02 measured - PaC02 normal) x 0.37] + bicarb normal

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Causes of respiratory alkalosis you may have over-looked:  panting due to pain, hypoxemia, primary CNS lesion, sepsis/fever, drugs, excess mechanical ventilation, heat stroke.

Causes of respiratory acidosis you may have over-looked:  respiratory. depression due to anesthetics, a CNS lesion, impaired cardio-pulm function, laryngeal disease, impaired diaphragm movement.

 Causes of metabolic alkalosis you have have over-looked:  gastic origin vomiting, diuretics, bicarb treatment, excess antacids, massive blood transfusion

For causes of metabolic acidosis, figure out the anion gap  & then see step 4. 

How close do the measured values have to be relative to the expected ones? Within 2 mEq.

How do you use normal values in these equations when everyone knows that normal values are a range of values rather than one value you can plug into an equation?  You use the midpoint of the range.

ALSO DON’T FORGET THAT RESPIRATORY COMPENSATION TAKES ABOUT 12 HOURS & METABOLIC COMPENSATION TAKES 3-5 DAYS.  If your values aren’t what you expected, there may have been inadequate time for compensation.

Fourth: If you have a metabolic acidosis, calculate anion gap
(Na+  +  K+) - (Cl- + HC03-). Normal anion gap should be 10-12 mEq. Metabolic acidoses are classified by whether or not there is normal or abnormal anion gap.  If they have a normal anion gap they are called hyperchloremic metabolic acidoses.

Causes of hyperchloremic metabolic acidosis
diarrhea, carbonic anhydrase inhibitors, rapid IV hydration esp w/saline, acidifying agents, ketoacidosis (recovering), renal tubular acidosis, azotemia/early CRF, Addison’s

Causes of high anion gap metabolic acidosis
Hyperglycemia, ketoacidosis, azotemia, antifreeze, tissue hypoperfusion/lactic acidosis, aspirin toxicity, muscle trauma.