Solution to Blood Gas #11

History: A 35-year-old woman presents with acute dyspnoea. She has been taking an over-the-counter analgesic for a sore throat. Below is her venous blood gas on arrival:

pH 7.17 Na 136 mmol/L
pCO2 20 mmHg K 1.7 mmol/L
HCO3 7 mmol/L Cl 114 mmol/L
Lactate 1.1 mmol/L Glucose 6.2 mmol/L

5-step-approach-to-blood-gas-analysis

What’s the pH?

7.17 = severe acidaemia

What’s the primary process?  

HCO3 7 = primary metabolic acidosis

Is there any compensation?

Expected pCO2 = 1.5 x HCO3 + 8 ± 2

= 1.5 x 7 + 8 ± 2

= 16.5 – 20.5

Measured pCO2 is 20, therefore there is maximal respiratory compensation.

Are there any other clues?

Anion gap = Na – (Cl + HCO3)

= 136 – (114 + 7)

= 15

Therefore there is a normal anion gap.

Electrolyte clues:

Potassium is severely low. Chloride is elevated. Sodium, lactate and glucose are within the normal range.

What’s the differential diagnosis?

Description: This venous blood gas reveals a severe, non-anion gap metabolic acidosis with maximal respiratory compensation. There is profound hypokalaemia and moderate hyperchloraemia. Sodium, lactate and glucose are all within normal limits.

Interpretation: In this clinical setting, this venous blood gas would be consistent with a renal tubular acidosis with hypokalaemia and hyperchloraemia. Such a picture could be seen with misuse of ibuprofen, or solvent abuse (toluene). Autoimmune conditions causing renal tubular acidosis such as Sjögren’s syndrome, renal abnormalities and hypercalciuric conditions should also be considered. Diarrhoea and adrenal insufficiency are possible differentials. Management is with potassium replacement +/- bicarbonate replacement and removal or control of the underlying cause.

Additional information: History of polysubstance misuse and chronic diarrhoea. The  NAGMA resolved over 24 hours with potassium and bicarbonate replacement.

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