OUWB M2 questions and Answers

The Prince William Question

Lets do this number by numbers. This is an algorithm that will allow you to map out any acid-base question.

1. Primary disorder: pH is up (<7.4), pCO12 is down (< 40) and HCO3 is up (>24)The H-H variables are moving in discordant direction so this is an respiratory disorder, the pH is elevated so this is a respiratory alkalosis.

If you are really on the ball you will note that this breaks one of the fundamental guidelines of acid-base in that compensation is not in the same direction as the primary disorder (pCO2 and HCO3 almost always move in the same direction). This only happens when there are two primary disorders.

2. Is there a second primary acid-base disorder affecting compensation?Yes. In Respiratory alkalosis, for every 10 the pCO2 falls the bicarb falls 2 acutely and 4 chronically, so the target HCO3 is 22 for acute respiratory alkalosis and 20 for chronic respiratory alkalosis, well the bicarb did not drop at all, in fact it went up, so there is an additional primary metabolic alkalosis.

3. What is the anion Gap?148-(98+28)=22.So we did not talk about this, but the presence of an anion gap means there is an metabolic acidosis buried deep in the ABG.

4. Calculate the bicarb beforeBicarb before = HCO3 + (Anion gap -12)Bicarb before = 28 (22-12) = 38So without the anion gap the bicarb would be 38, revealing a pretty severe metabolic alkalosis, that is mostly hidden or covered up by the anion gap metabolic acidosis. The severity of the metabolic alkalosis by looking at the electrolytes without the anion gap.

Put it all together and you have: A respiratory alkalosis, likely from the respiratory stimulant effect of feverA metabolic alkalosis from vomiting and/or some antacids he may have taken to soothe his stomach. An anion gap metabolic acidosis from the sepsis.

September 4, 2007

Coal Miner

The set up

Picture by Nicolas Holzheu

Coal miner presents to the ED with fever and vomiting

pH 7.23
pCO2 67
pO2 88

Na 144
Cl 96
BUN 8

K 3.2
Bicarb 27
Creatinine 0.6

glucose 128

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!