Acid-Base Challenge

Tackle the acid-base problems below, using the pLACO method!

  1. A 24-year old comatose woman.

    Blood gas: arterial blood pH: 7.39 | CO2: 27 mm Hg
    Serum HCO3: 16 meq/L | Cl: 100 meq/L | Na: 137 meq/L| Albumin: 4.5 g/L

p(H): acidemic or alkalemic?
A pH of < 7.4 = acidemia
L: Based on the labs, what’s the primary acid-base disturbance?
The bicarbonate is LOW and the carbon dioxide is LOW. Our primary process is a metabolic acidosis. If both values were HIGH, we would have a primary respiratory acidosis with an acidemic pH.
A: What’s the anion gap?
¬†First, let’s calculate this patient’s normal anion gap: albumin x 2.5 = 4 x 2.5 = 10.
Now, the patient’s anion gap at this time is: Na – Cl – HCO3 = 137 – 100 – 16 = 21. The anion gap is elevated and the increase in the anion gap from baseline is 21 – 10 = 11.
C: Is there appropriate compensation?
We have a metabolic acidosis, so we need to use Winter’s formula to look for compensation. The expected pCO2 in this case is 1.5 (16) + 8 = 32, give or take 2. The patient’s pCO2 is 27 – lower than the expected. Thus, we can diagnosis a respiratory alkalosis.
O: Is there anOther acid-base disturbance?
Since we now have an anion gap metabolic acidosis, we need to look for a second metabolic process that may be present. Does the increase in the anion gap equally account for the drop in bicarbonate? If it doesn’t, then we have a second metabolic process. Here, the bicarbonate has decreased by 9 (24 – 16) and the anion gap has increased by 11. 9 is not far from 11, so we can call this a pure anion gap metabolic acidosis with a respiratory alkalosis.

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