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 |
[expand title=”Interpretation”]
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.
Formulation:
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.
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