Describe and interpret this blood gas:

History: 33-year-old indigenous man presents with general malaise.

 pH 6.95  Na 128 mmol/L
 pCO2 21 mmHg  K 4.7 mmol/L
HCO3 4 mmol/L Cl 97 mmol/L
Lactate 0.9 mmol/L Glucose 5.9 mmol/L

Interpretation

 

What is the pH?

6.95 = severe acidaemia 

What is the primary process?

HCO3 4 = primary metabolic acidosis

Is there compensation?

Expected pCO2 = 1.5 x HCO3 + 8

= 1.5 x 4 + 8

= 14 mmHg

Actual pCO2 is higher, suggesting incomplete compensation or co-existing respiratory acidosis.

Are there other clues to diagnosis?

Anion gap = Na – (Cl + HCO3)

= 128 – (97 + 4)

= 27

Therefore the anion gap is markedly elevate.

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

= (27 – 12) ÷ (24 – 4)

= 0.7

Consistent with a co-existing anion and non-anion gap acidoses.

Electrolyte clues:

Sodium is low, chloride is borderline low. Potassium, glucose and lactate are all within normal limits.


 

Formulation:

Description: This ABG shows a severe acidaemia due to primary mixed high anion gap and non anion gap acidoses. There is either incomplete respiratory compensation, or a coexisting respiratory acidosis component. Sodium is low. Chloride is borderline low, which is normally associated with metabolic alkalosis. Potassium, glucose and lactate are all within normal limits.

Interpretation: This is a potentially life threatening acidaemia. The markedly high anion gap limits the diagnostic possibilities in this clinical context to DKA (but patient has normal blood sugar which would argue against this), AKA, toxic alcohol ingestion and possibly uraemia. The severity of the acidosis suggests subacute development consistent with this differential. Further investigation should include measurement of creatinine and urea, urinary sodium and chloride, with further assessment for toxic alcohol ingestion or AKA as indicated by history and other results. Renal tubular acidosis or Addison’s disease in the context of sepsis (although the lactate is low) should be considered in contribution to the non-anion gap component of the acidosis.

Additional information: Acute on chronic renal failure secondary to sepsis. Urea 63.1mmol/L, Creatinine 2880 umol/L (previously 441). Remains dialysis-dependent.

 

Blood Gas #6
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