The set up
|Picture by Nicolas Holzheu|
Coal miner presents to the ED with fever and vomiting
Step one: determine the primary disorder
the pH is down, the HCO3 and CO2 are up so this is a respiratory acidosis
Step two: check to see if the compensation is appropriate
The CO2 is 67, since his chief complaint is not respiratory and he is a coal miner we will assume black lung and chronic COPD. So we will use the estimate for chronic respiratory acidosis
67 is almost 30 above normal pCO2, and for every 10 the CO2 rises the HCO3 should go up 3 (1 is this was acute). So a pCO2 of 67 should have a HCO3 of 2.7 x 3 = 8.1 above a normal bicarb of 24 = 32.1.
His actual bicarb is 27 so he has an additional metabolic acidosis (bicarb lower than predicted means metabolic acidosis).
Step three: if there is a metabolic acidosis what is the anion gap
The patient has a metabolic acidosis, so the anion gap is relevant. We calculate it and it is 21.
Step four: if there is an anion gap, calculate the bicarbonate before
The patient has an anion gap so to calculate the bicarbonate before the anion gap we subtract 12 from the calculated anion gap and add the difference to the current bicarbonate:
21-12 = 9 add that to the bicarbonate of 27 to get a bicarbonate of 36. This is higher than the predicted compensated bicarbonate from step two (32.1) so the patient has an additional metabolic alkalosis.
Final step: put it all together
The patient has black lung and COPD. His largest acid-base disorder is chronic respiratory acidosis. He does have an acute illness. This illness is causing an anion gap metabolic acidosis. Sepsis and multi organ failure does this. Prior to developing the anion gap the vomiting caused a metabolic alkalosis.
Its a triple disorder!