Describe and interpret this arterial blood gas:

History: A 75-year-old man with end-stage renal failure on home dialysis and a history of a right-sided total hip replacement has a mechanical fall. He sustains a periprosthetic fracture of his chronically osteomyelitic right femur. He is hypotensive and requiring noradrenaline to maintain a mean arterial pressure ≥ 65 mmHg.

pH 7.271 Na 139 mmol/L
pCO2 28.5 mmHg K 4.9 mmol/L
pO2 112 mmHg (FiO2 0.25) Ca 1.18 mmol/L
HCO3 12.9 mmol/L Cl 111 mmol/L
 Albumin 26 g/L Glucose 13.2 mmol/L
Hb 105 g/L Lactate 2.5 mmol/L


What is the pH?

7.271 = acidaemia

What is the primary process?

HCO3 12.9 = primary metabolic acidosis

Is there compensation?

Expected pCO2 = 1.5 x HCO3 + 8 (± 2)

= 1.5 x 12.9 + 8

= 27.4 (25.4 – 29.4)

Therefore there is maximal respiratory compensation.

Are there other clues to diagnosis?

Anion gap = Na – (HCO3 + Cl)

= 139 – (12.9 + 111)

= 15.1

Correct anion gap = calculated anion gap + (normal albumin – measured albumin)/4

= 15.1 + (40 – 26)/4

= 18.6

Therefore there is an elevated anion gap and thus a high anion gap metabolic acidosis.

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

= (18.6 – 12) ÷ (24 – 12.9)

= 0.59

This suggests a coexisting non-anion gap metabolic acidosis.

(= 0.38 if calculated using uncorrected anion gap, which is consistent with a pure NAGMA)

Corrected Na = Na + (glucose – 5)/3

= 139 + (13.2 – 5)/3

= 142

= normal range

Electrolyte clues:

Lactate is slightly elevated. Glucose is slightly elevated. Sodium, potassium, and calcium are within the normal range. Chloride is slightly elevated. Mild anaemia. Hypoalbuminaemia.


Description: This arterial blood gas shows a metabolic acidosis which, when the anion gap is corrected for hypoalbuminaemia, is likely a combination of a non-anion gap metabolic acidosis and a high anion gap metabolic acidosis. There is mild anaemia, hyperchloraemia, mild hyperglycaemia, and an elevated lactate.

Interpretation: In the clinical context, this arterial blood gas is consistent with septic shock (“a subset of sepsis in which particularly profound circulatory, cellular, and metabolic abnormalities are associated with a greater risk of mortality than with sepsis alone”) resulting in a high anion gap metabolic acidosis and confirmed clinically by the requirement of a noradrenaline infusion to maintain a mean arterial pressure of ≥ 65 mm Hg and a serum lactate > 2 mmol/L. The coexisting non-anion gap metabolic acidosis is most likely the result of volume resuscitation with a 0.9% sodium chloride solution resulting in hyperchloraemia.

Albumin is the major unmeasured anion and contributes almost the whole of the value of the anion gap. A normally high anion gap metabolic acidosis in a patient with hypoalbuminaemia may appear as a non-anion gap metabolic acidosis; a lactic acidosis in a hypoalbuminaemic patient will often appear as a non-anion gap metabolic acidosis. 

Blood Gas #14
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