Describe and interpret this venous blood gas:

History: A 40-year-old man with history of alcohol misuse, abdominal pain and vomiting. He has recently commenced metformin.

pH 7.51 Na 132 mmol/L
pCO2 18 mmHg K 4.2 mmol/L
HCO3 12 mmol/L Cl 88 mmol/L
Lactate 5 mmol/L Glucose 20 mmol/L
Interpretation

 

What is the pH?

7.51 = alkalaemia

What is the primary process?

pCO2 = 18 mmHg = primary respiratory alkalosis

Is there compensation?

Expected HCO3 = 24 – 2((40 – pCO2)/10)

= 24 – 4.4

= 19.6

Measured HCO3 = 12 mmol/L, therefore there is a coexisting metabolic acidosis.

Are there other clues to diagnosis?

Anion gap = Na – (HCO3 + Cl)

= 132 – (88 + 12)

= 32

Therefore this is an anion gap metabolic acidosis.

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

= (32 – 12) ÷ (24 – 12)

= 1.67

This is consistent with a pure HAGMA.

Strong Ion Difference (SID) = Na – Cl

= 132 – 88

= 44

This is consistent with a metabolic alkalosis.

Corrected Na = Na + (glucose – 5)/3

= 132 + (20 – 5)/3

= 137

Electrolyte clues:

There is an elevated lactate, marked hypochloraemia, and hyperglycaemia.


Formulation:

Description: This venous blood gas shows an alkalaemia due to a  primary respiratory alkalosis. There is a coexisting high-anion gap metabolic acidosis. There is marked hypochloraemia resulting in an elevated strong ion difference, resulting in a coexisting metabolic alkalosis. There is hyperglycaemia-induced hyponatraemia, which corrects to normal range, and hyperlactataemia.

Interpretation: This is a complex triple acid-base disorder. The primary respiratory alkalosis is consistent with hyperventilation secondary to pain or anxiety. The high anion gap acidosis is likely to be due to alcoholic or diabetic ketoacidosis secondary to pancreatic insufficiency secondary to chronic alcoholic pancreatitis, although alcohol/toxic alcohol ingestion should be considered in the differential diagnosis. Urinary and/or serum ketones should be measured. Intra-abdominal sepsis, metformin toxicity, and severe salicylate poisoning should also be considered in the differential diagnosis. The primary metabolic alkalosis is likely the result of gastrointestinal chloride depletion secondary to repeated vomiting with or without significant intravenous volume contraction and should resolve with appropriate intravenous fluid resuscitation. Metformin should be ceased.

Additional information: Lipase 3000 and serum ketones 5.2 (normal < 0.5). Abnormalities resolved with analgesia and intravenous fluid resuscitation.

Blood Gas #17
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