Describe and interpret this arterial blood gas:

History: 53-year-old woman found collapsed at home.

pH 7.58 Na 138 mmol/L
pCO2 12.6 mmHg K 3.7 mmol/L
pO2 160 mmHg (FiO2 0.25) Cl 108 mmol/L
HCO3 12.1 mmol/L Glucose  6.3 mmol/L
Lactate 3.5 mmol/L

Interpretation

 

What is the pH?

7.58 = alkalaemia

What is the primary process?

pCO2 12.6 = respiratory alkalosis

Is there compensation?

Expected HCO3 = 24 – 2 x {(40 – Measured pCO2)/10}

= 24 – 2 x {(40 – 12.6)/10}

= 18.5

Actual bicarb is lower, indicating a coexisting metabolic acidosis.

Are there other clues to diagnosis?

Anion gap = Na – (Cl + HCO3)

= 138 – (108 + 12.1)

= 17.9

Therefore the anion gap is high.

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

= (17.9 – 12) ÷ (24 – 12.1)

= 0.49

Suggests coexisting anion gap and non-anion gap acidoses.

Electrolyte clues:

Lactate is elevated, other electrolytes are within the normal range.

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

= (713 x 0.25) – (12.6 x 1.25)

= 162.5 mmHg

Measured pO2 is 160, therefore there is a normal A-a gradient.


 

Formulation:

Description:  This ABG shows an alkalaemia with respiratory alkalosis, combined with anion gap and non-anion gap metabolic acidoses. Lactate is elevated and there is no A-a gradient.  Other electrolytes are within normal limits.

Interpretation:  In this clinical context, this ABG is highly suggestive of a salicylate overdose.  Elevated lactate will also contribute to the anion gap.  Hyperventilation due to pain, anxiety or another central cause should be considered.  Salicylate levels should be requested and enhanced elimination commenced where appropriate.

Additional information: The patient had taken unknown amounts of aspirin, lithium and olanzapine. Salicylism is normally associated with a high anion gap acidosis, however many gas machines read salicylate as chloride, giving a falsely elevated chloride and a non-anion gap acidosis.

Blood Gas #2
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