# Blood Gas #4

Describe and interpret this arterial blood gas:

History: 83-year-old women with altered conscious state and a fever.

 pH 7.24 Na 142 mmol/L pCO2 22 mmHg K 4.5 mmol/L pO2 115 mmHg (FiO2 0.4) Cl 114 mmol/L HCO3 9 mmol/L Glucose 13 mmol/L Lactate 12 mmol/L

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What is the pH?

7.24 = acidaemia

What is the primary process?

HCO3 9 = primary metabolic acidosis

Is there compensation?

Expected pCO2 = 1.5 x HCO3 + 8

= 1.5 x 9 + 8

= 21.5

Actual pCO2 is 22, therefore there is complete respiratory compensation

Are there other clues to diagnosis?

Anion gap = Na – (Cl + HCO3)

= 142 – (114 + 9)

= 19

Therefore there is a high anion gap acidosis.

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

= (19 – 12) ÷ (24 – 9)

= 0.47

Suggests coexisting non-anion gap acidosis.

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

Therefore there is an elevated A-a gradient.

Formulation:

Description: This ABG shows a compensated metabolic acidosis with mixed anion-gap and non-anion gap 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, this ABG 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/resuscitated as appropriate.

Additional information: LRTI and severe sepsis. Discharged home well at 48 hours.

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