Describe and interpret this venous blood gas:

History: 20-year-old women presents to the emergency department 6 hours after taking an overdose of 100 tablets.

pH 7.38 Na 145 mmol/L
pCO2 25 mmHg K 3.9 mmol/L
HCO3 15 mmol/L Cl 116 mmol/L
Lactate 1.2 mmol/L Glucose 5.8 mmol/L



What is the pH?

7.38 = within normal range 

What is the primary process?

HCO3 15 = metabolic acidosis

pCO2 25 = respiratory alkalosis

Is there compensation?

Expected pCO2 = 1.5 x HCO3 + 8

= 1.5 x 15 + 8

= 30.5

Actual pCO2 is lower, confirming primary respiratory alkalosis.

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

= 24 – 2 x {(40 – 25)/10}

= 21

Actual HCO3 is lower, confirming primary metabolic acidosis.

Are there other clues to diagnosis?

Anion gap = Na – (HCO3 + Cl)

= 145 – (15 + 116)

= 14

There is a normal anion gap metabolic acidosis.

Electrolyte clues:

Sodium, potassium, lactate and glucose are within normal limits. Chloride is elevated.



Description: This VBG shows coexisting primary non-anion gap metabolic acidosis and primary respiratory alkalosis with overall normal pH. Chloride is elevated but other electrolytes and lactate are within the normal range.

Interpretation: In this clinical scenario this VBG is most consistent with a large, late presentation salicylate overdose with mixed primary respiratory alkalosis and metabolic acidosis. The normal pH is of concern, and indicates requirement for urgent urinary alkalisation (+/- dialysis) to prevent progression to cerebral oedema and death. While salicylate overdose is normally listed as a cause for high anion gap acidosis, an artefactual, normal anion gap acidosis with elevated chloride is frequently seen due to cross-reactivity of the chloride assay with salicylate.

Additional information: 28.8g aspirin overdose (480 mg/kg). Good response to urinary alkalinisation with resolution of salicylism.

Blood Gas #13
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