Describe and interpret this arterial blood gas:

History: 25-year-old man brought in by police in a highly agitated state.

pH 7.59 Na 148 mmol/L
pCO2 17 mmHg K 3.8 mmol/L
pO2 126 mmHg (FiO2 0.21) Cl 112 mmol/L
HCO3 17 mmol/L Glucose 5.5 mmol/L
Lactate 3.8 mmol/L

Interpretation

 

What is the pH?

7.59 = alkalaemia

What is the primary process?

pCO2 17 = primary respiratory alkalosis

Is there compensation?

Expected HCO3 = 24 – 2 x {(40 – Measured pCO2)/10}

= 24 – 2 x {(40 – 17)/10}

= 19.4

Actual bicarbonate is 17 suggesting a co-existing metabolic acidosis

Are there other clues to diagnosis?

Expected PAO2 = 150 – (pCO2 x 1.25)

= 150 – (17 x 1.25)

= 128.75

A-a gradient = PAO2 – PaO2

= 128.75 – 126

= 2.75

Therefore there is no significant A-a gradient.

Anion gap = Na – (Cl + HCO3)

= 148 – (112 + 17)

= 19

Therefore there is an elevated anion gap acidosis.

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

= (19 – 12) ÷ (24 – 17)

= 1

Consistent with a high anion gap acidosis.

Electrolyte clues:

Chloride, sodium and lactate are elevated. Potassium and glucose are within normal limits.


 

Formulation:

Description: This arterial blood gas shows an alkalaemia secondary to a primary respiratory alkalosis, with coexisting high anion gap metabolic acidosis. There is no significant A-a gradient and oxygenation is appropriate. Lactate is elevated, contributing to the anion gap. Chloride is elevated, which is normally associated with a non-anion gap metabolic acidosis. Sodium is also elevated.

Interpretation: This gas is most consistent with primary hyperventilation, causing a respiratory alkalosis. In view of the metabolic acidosis, underlying causes of agitation may include toxicological causes (salicylates, stimulants) or sepsis with delirium and end organ perfusion deficit causing the elevated lactate. Increased ICP with increased respiratory drive secondary to meningitis/encephalitis should also be considered. A respiratory source of sepsis is less likely given appropriate oxygenation on room air and the lack of an elevated A-a gradient.

Additional information: Strong ion gap Na – Cl = 36, revealing a non-anion gap component to acidosis which is not calculated elsewhere.

Diagnosis: Agitation and hyperventilation secondary to ingestion of an illicit stimulant substance.

Blood Gas #8
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