History: An 83-year-old woman presents with an altered conscious state and a fever. Below is her arterial blood gas on arrival:
|pH 7.24||Na 142 mmol/L|
|pCO2 22 mmHg||K 4.7 mmol/L|
|pO2 115 mmHg (FiO2 0.4)||Cl 114 mmol/L|
|HCO3 9 mmol/L||Glucose 13 mmol/L|
|Lactate 12 mmol/L|
What’s the pH?
7.24 = acidaemia
What’s the primary process?
HCO3 = 9
∴ this is a primary metabolic acidosis
Is there any compensation?
Expected pCO2 = 1.5 x HCO3 + 8
= 1.5 x 9 + 8
The measured pCO2 is 22, therefore there is maximal respiratory compensation.
Are there any other clues?
Anion gap = Na – (Cl + HCO3)
= 142 – (114 + 9)
Delta gap = (Anion gap – 12) ÷ (24 – HCO3)
= (19 – 12) ÷ (24 – 9)
This suggests a coexisting NAGMA.
Lactate is severely elevated. Chloride and glucose are slightly elevated. Sodium and potassium are within normal limits.
Expected PAO2 = (713 x FiO2) – (pCO2 x 1.25)
= (713 x 0.4) – (22 x 1.25)
A-a gradient = PAO2 – PaO2
= 257.7 – 115
= elevated A-a gradient
What’s the differential diagnosis?
Description: This arterial blood gas demonstrates a compensated metabolic acidosis with mixed HAGMA and NAGMA components. There is severe hyperlactataemia with elevated chloride and glucose. Sodium and potassium are within normal limits. There is a significantly elevated A-a gradient.
Interpretation: In this clinical context, the blood gas suggests severe sepsis / septic shock from a respiratory source, with impairment of oxygenation and severe lactic acidosis contributing to the anion gap. Non-anion gap acidosis (elevated chloride) is likely associated with saline hydration or adrenal insufficiency. Sources of sepsis should be sought and treated and the patient resuscitated appropriately.
Additional information: Community-acquired pneumonia with sepsis.