2025 – PAGE 365 – ACID-BASE DISORDERS

ABG RULES FOR A METABOLIC ACIDOSIS

NOTE: If you diagnose a metabolic acidosis as your OVERARCHING diagnosis, ALWAYS do this stuff!

• Calculate the anion gap. You should be doing this on ALL of your exam patients!
• For any overarching metabolic acidosis (gap or non-gap), you should also check for appropriate respiratory compensation by using Winter’s formula. Again, you do NOT do this unless the metabolic acidosis is your overarching disorder.
  • Expected value of pCO2 = (1.5 x Bicarb) + 8 + 2.
    
  • If the values match, respiratory compensation is adequate.
  • If the measured pCO2 is higher than the expected pCO2, there is also a concurrent respiratory acidosis since not enough pCO2 is being blown off.
  • If the measured pCO2 is lower than the expected pCO2, there is also a concurrent respiratory alkalosis since too much pCO2 is being blown off.
• For any patient with an OVERARCHING anion gap acidosis:
  • Calculate the delta gap (∆ GAP). Again, you ONLY do this when the overarching diagnosis is a GAP ACIDOSIS. The delta gap is the “delta,” or the difference, between the upper limit of a normal gap (usually 12), and the measured anion gap based on your patient’s chemistry. The ∆ GAP = Calculated Gap – 12. So if the anion gap is calculated at 16, the ∆ GAP is 16 – 12 = 4.
  • Now calculate the ∆ Bicarb. This is the difference between the upper limit of a normal bicarb level (24), and the bicarb level your patient has on the chemistry. The ∆ Bicarb = 24 – measured bicarb, so if the measured bicarb level is 20, the ∆ Bicarb is 24 – 20 = 4.Does the ∆ Bicarb = ∆ Gap? The drop in bicarb (∆ Bicarb) should be equal to the increase in the anion gap from 12 (the ∆ GAP) if there are no other metabolic disorders. So, if the ∆ Bicarb = ∆ Gap, there are no other metabolic disorders.Is the bicarb lower than expected? If so, then there’s less bicarb in the system. This represents an additional primary NON-GAP metabolic acidosis that has driven the bicarbonate level down.Is the bicarb higher than expected? If so, then there’s more bicarb (an alkaline substance) in the system. This represents an additional primary metabolic alkalosis.