Q: When I enter normal values (pH=7.4, HCO3=24, Na=140, PCO2=40, Cl=100) then it says Primary Disorder: metabolic alkalosis, Expected pCO2=30.6, so Concomitant respiratory acidosis. What's going on?

A: Actually the 'normal' values I use as well are ph 7.4, pco2 40, and hco3 24. But, when designing the program I designed it that > or = to 7.4 is always an alkalosis, while < 7.4 is an acidosis. The reason for this is that sometimes you can have a 'normal' pH and still have an acid base disorder.

Here is an example problem (I know, it's long)

Question: After dining in the hospital cafeteria, an intern develops 24 hours of severe vomiting and diarrhea. Pulse is tachycardic, and both the blood pressure and urine output are low. Based on the following data, is there an acid-base disorder? ABG: ph 7.39, PCO2 41, Na 135, Cl 85, HCO3 23, K 2.5.

Answer: There is an anion gap of 27, so there is a metabolic acidosis probably due to lactate (poor perfusion & hemoconcentration with increased serum proteins). Since the pH is normal, there must be a compensitory disturbance.

The expected PCO2 is 1.5(23) + 8 = 43 indicating that there is adequate respiratory compensation.

Using the delta-delta, the pre-existing bicarbonate is 23 + 27 - 12 = 38 which is >> 24.

Therefore, there is a metabolic alkalosis which exactly counterbalances the metabolic acidosis.

This picture can be explained by acid loss from emesis and loss of base from diarrhea.

Anyways, so in answer to the original question, 7.4 is considered first an alkalosis (> or = to 7.4), then the program looks for why. HCO3 is > or = to 24, so it presumes it must be metabolic. For a metabolic alkalosis, the expected PCO2 is equal to 0.9*(HCO3) + 9, or 0.9*(24) + 9 = 30.6. Since 30.6 is MUCH less than 40, there must also be a concominant respiratory acidosis, which is what the program says. I didn't create the expected PCO2 formula, I just lifted it from a textbook, and that is the values it gives you. Besides, in real life it would be very unlikely to get exact values of 7.4, 40, and 24.