Solution for Blood Gas #4

History: An 83-year-old woman presents with an altered conscious state and a fever. Below is her arterial blood gas on arrival:

pH 7.24 Na 142 mmol/L
pCO2 22 mmHg K 4.7 mmol/L
pO2 115 mmHg (FiO2 0.4) Cl 114 mmol/L
HCO3 9 mmol/L Glucose  13 mmol/L
Lactate 12 mmol/L  

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

What’s the pH?

7.24 = acidaemia

What’s the primary process?

HCO3 = 9

∴ this is a primary metabolic acidosis

Is there any compensation?

Expected pCO2 = 1.5 x HCO3 + 8

= 1.5 x 9 + 8

= 21.5

The measured pCO2 is 22, therefore there is maximal respiratory compensation.

Are there any other clues?

Anion gap = Na – (Cl + HCO3)

= 142 – (114 + 9)

= 19

= HAGMA

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

= (19 – 12) ÷ (24 – 9)

= 0.47

This suggests a coexisting NAGMA.

Electrolyte clues:

Lactate is severely elevated. Chloride and glucose are slightly elevated. Sodium and potassium are within normal limits.

Expected PAO2 = (713 x FiO2) – (pCO2 x 1.25)

= (713 x 0.4) – (22 x 1.25)

= 257.7

A-a gradient = PAO2 – PaO2

= 257.7 – 115

= 142.7

= elevated A-a gradient

What’s the differential diagnosis?

Description: This arterial blood gas demonstrates a compensated metabolic acidosis with mixed HAGMA and NAGMA components. There is severe hyperlactataemia with elevated chloride and glucose. Sodium and potassium are within normal limits. There is a significantly elevated A-a gradient.

Interpretation: In this clinical context, the blood gas suggests severe sepsis / septic shock from a respiratory source, with impairment of oxygenation and severe lactic acidosis contributing to the anion gap. Non-anion gap acidosis (elevated chloride) is likely associated with saline hydration or adrenal insufficiency. Sources of sepsis should be sought and treated and the patient resuscitated appropriately.

Additional information: Community-acquired pneumonia with sepsis.

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