Describe and interpret this arterial blood gas:
History: 83-year-old women with altered conscious state and a fever.
| pH 7.24 | Na 142 mmol/L |
| pCO2 22 mmHg | K 4.5 mmol/L |
| pO2 115 mmHg (FiO2 0.4) | Cl 114 mmol/L |
| HCO3 9 mmol/L | Glucose 13 mmol/L |
| Lactate 12 mmol/L |
[expand title = “Interpretation”]
What is the pH?
7.24 = acidaemia
What is the primary process?
HCO3 9 = primary metabolic acidosis
Is there compensation?
Expected pCO2 = 1.5 x HCO3 + 8
= 1.5 x 9 + 8
= 21.5
Actual pCO2 is 22, therefore there is complete respiratory compensation
Are there other clues to diagnosis?
Anion gap = Na – (Cl + HCO3)
= 142 – (114 + 9)
= 19
Therefore there is a high anion gap acidosis.
Delta gap = (Anion gap – 12) ÷ (24 – HCO3)
= (19 – 12) ÷ (24 – 9)
= 0.47
Suggests coexisting non-anion gap acidosis.
Electrolyte clues:
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)
= 257.7
A-a gradient = PAO2 – PaO2
= 257.7 – 115
= 142.7
Therefore there is an elevated A-a gradient.
Formulation:
Description: This ABG shows a compensated metabolic acidosis with mixed anion-gap and non-anion gap 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, this ABG 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/resuscitated as appropriate.
Additional information: LRTI and severe sepsis. Discharged home well at 48 hours.
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