Describe and interpret this venous blood gas:

History: A 37-year-old woman with a history of non-insulin dependent diabetes mellitus, alcohol dependence, and recurrent depressive episodes is found unconscious at home. On arrival to the emergency department she remains comatose (GCS 3), hypotensive (BP 90/60), and tachycardic (HR 106 beats/minute, sinus). Her initial VBG is as follows:

pH 6.72 Na 138 mmol/L
pCO2 48 mmHg K 4.9 mmol/L
pO2 78 mmHg Cl 104 mmol/L
HCO3 5 mmol/L Glucose 20.2 mmol/L
Lactate 9.2 mmol/L Albumin 40 g/L
Ethanol 0.08% Urea 6 mmol/L
Serum Osmolality 416 mOsmol/kg Creatinine 2800 μmol/L
Interpretation

 

What is the pH?

6.72 = severe acidaemia

What is the primary process?

HCO3 = 5 mmol/L∴ this is a primary metabolic acidosis

Is there compensation?

Expected pCO2 = 1.5 x HCO3 + 8 ± 2

= 13.5 – 17.5

Actual pCO2 = 48 ∴ there is a coexisting primary respiratory acidosis

Are there other clues to diagnosis?

Anion gap = Na – (HCO3 + Cl)

= 138 – (5 + 104)

= 29

∴ this is a HAGMA

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

= (29 – 12) ÷ (24 – 5)

= 0.89

∴ this is consistent with a pure HAGMA

Corrected Na = Na + (glucose – 5)/3

= 138 + (20.2 – 5)/3

= 143 mmol/L

∴ within normal range

Calculated osmolality = (2 x Na) + urea + glucose + ethanol

= (2 x 138) + 6 + 20.2 + (0.08 x 218)

= 319.64 mOsm/kg

Osmolar gap = Measured osmolality – Calculated osmolality

= 416 – 319.64

= 96.36

∴ there is a markedly elevated osmolar gap

Electrolyte clues:

Hyperglycaemia, hyperlactataemia, greatly elevated creatinine with a normal urea. Normal sodium, potassium, chloride, and albumin.


Formulation:

Description: This venous blood gas shows a severe acidaemia due a mixed acidosis. The metabolic component predominates and is due to a high anion-gap metabolic acidosis. There is a significantly elevated osmolar gap, hyperglycaemia, and marked hyperlactataemia.

Interpretation: Normal serum or urinary ketones would reliably exclude diabetic or alcoholic ketoacidosis. Lactic acidosis, from either sepsis, seizure, or metformin poisoning, could be contributing to the anion-gap metabolic acidosis. However, the elevated osmolar gap in this context suggests toxic alcohol poisoning (methanol, ethylene glycol, propylene glycol). The primary respiratory acidosis is mild and could be due to aspiration or CNS depression from other toxic co-ingestions (e.g. opioids or benzodiazepines).

Case Resolution: The patient was intubated and admitted to ICU. After several hours of dialysis she was extubated and admitted to attempting suicide by ingesting vodka and “Blue Thunder”, a fuel for radio-controlled racing cars containing methanol and nitromethane. Nitromethane is responsible for the spuriously elevated serum creatinine, as measured by the Jaffe method. This may have value as an early surrogate marker of methanol poisoning in those who ingest radio-controlled fuel.

Blood Gas #21
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