Nephrology Question 40 Instructor

Describe the acid-base diagnosis in the following case (for example, metabolic acidosis with compensatory respiratory alkalosis):

A 41 year old male presents to the ER with the following lab results (hint: look for multiple simultaneous problems):

Na: 140 mmol/L
K: 4.0 mmol/L
Cl: 110 mmol/L
pH: 7.0
pCO₂: 35 mmHg
pO₂: 75 mmHg
HCO₃: 8 mmol/L

The primary disturbance in this patient is an acidosis, as indicated by the acidemic pH in the blood (pH 7.0). The normal bicarbonate concentration is 24 mmol/L but this patient’s bicarbonate concentration is 8 mmol/L. This indicates a fall in bicarbonate of 16 mmol/L.

Thus, there is a primary metabolic acidosis.

The anion gap is 22.
Recall, anion gap is calculated by the formula:

AG = Na – HCO₃ – Cl
AG = 140 – 8 – 110
AG = 22

The normal anion gap is about 12, so the anion gap is increased.

Thus, there is an anion gap metabolic acidosis.

The anion gap is increased by 10 but the bicarbonate has fallen by 16 mmol/L. Therefore, there is also a fall in bicarbonate that is not accounted for by the H+ ions that accompanied the unmeasured anions in this case – this means there is also a non-anion gap metabolic acidosis.

The bicarbonate has decreased by 16.

We would expect that in a metabolic acidosis, there would be a 1 mmHg fall in pCO₂ for every 1 mmol/L of bicarbonate.

Therefore, we would expect that the pCO₂ would be 24 mmHg. Since it is 35 mmHg, it is too high and this represents a respiratory acidosis.

Therefore, this is a case of a

1. Anion gap metabolic acidosis
2. Non-anion gap metabolic acidosis
3. Respiratory acidosis