Solution to Blood Gas #6

History: A 33-year-old indigenous man presents with general malaise. Below is his ABG shortly after arrival:

 pH 6.95  Na 128 mmol/L
 pCO2 21 mmHg  K 4.7 mmol/L
HCO3 4 mmol/L Cl 97 mmol/L
Lactate 0.9 mmol/L Glucose 5.9 mmol/L

5-step-approach-to-blood-gas-analysis

What’s the pH?

6.95 = severe acidaemia 

What’s the primary process?

HCO3 4 = primary metabolic acidosis

Is there any compensation?

Expected pCO2 = 1.5 x HCO3 + 8

= 1.5 x 4 + 8

= 14 mmHg

Actual pCO2 is higher, suggesting incomplete compensation or co-existing respiratory acidosis.

Are there any other clues?

Anion gap = Na – (Cl + HCO3)

= 128 – (97 + 4)

= 27

Therefore the anion gap is markedly elevate.

Delta Gap = (Anion gap – 12) ÷ (24 – HCO3)

= (27 – 12) ÷ (24 – 4)

= 0.7

Consistent with a co-existing anion and non-anion gap acidoses.

Electrolyte clues:

Sodium is low, chloride is borderline low. Potassium, glucose and lactate are all within normal limits.

What’s the differential diagnosis?

Description: This arterial blood gas shows a severe acidaemia due to primary mixed high anion gap and non-anion gap acidoses. There is either incomplete respiratory compensation, or a coexisting primary respiratory acidosis. The sodium is low. The chloride is borderline low, which is normally associated with a metabolic alkalosis. The potassium, glucose and lactate are all within normal limits.

Interpretation: This is a potentially life threatening acidaemia. The markedly high anion gap limits the diagnostic possibilities in this clinical context to diabetic ketoacidosis, however the patient has a normal serum blood glucose which essentially excludes this possibility, alcoholic ketoacidosis, toxic alcohol ingestion and possibly uraemia. The severity of the acidosis suggests a sub-acute development consistent with this differential diagnosis. Renal tubular acidosis or Addison’s disease in the context of sepsis (although the lactate is low) should also be considered.

Additional information: Acute on chronic renal failure secondary to sepsis. Urea 63.1 mmol/L, creatinine 2880 μmol/L (previously 441).

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