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.