Describe and interpret this arterial blood gas:

History: A 62-year-old man presenting with chest pain and dyspnoea. He has a cardiac history and is peripherally oedematous.

pH 7.50 Na 122 mmol/L
pCO2 13 mmHg K 4.8 mmol/L
pO2 68 mmHg (FiO2 0.21) Cl 101 mmol/L
HCO3 10 mmol/L Glucose 20 mmol/L
Albumin 27 g/L Lactate 6.1 mmol/L
Urinary Na <10 mmol/L Urea 5.0 mmol/L
Interpretation

 

What is the pH?

7.50 = alkalaemia

What is the primary process?

pCO2 = 13 mmHg ∴ this is a primary respiratory alkalosis

Is there compensation?

Expected HCO3 = 24 – 2((40 – pCO2)/10)

= 24 – 5.4

= 18.6

Measured HCO3 = 10 mmol/L ∴ there is a coexisting primary metabolic acidosis.

Are there other clues to diagnosis?

Anion gap = Na – (HCO3 + Cl)

= 122 – (10 + 101)

= 11

AG corrected for hypoalbuminaemia = AG + (40 – albumin)/4

= 11 + (40 – 27)/4

= 14.25

∴ this is a non-anion gap metabolic acidosis

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

= (14.25 – 12) ÷ (24 – 10)

= 0.16

This is consistent with a NAGMA.

Strong Ion Difference (SID) = Na – Cl

= 122 – 101

= 21

This is consistent with a NAGMA.

Corrected Na = Na + (glucose – 5)/3

= 122 + (20 – 5)/3

= 127

Calculated osmolality = (2 x Na) + urea + glucose

= (2 x 122) + 5 + 10

= 259 mmol/kg (Normal 275-295 mmol/kg)

Expected PAO2 = (713 x FiO2) – (pCO2 x 1.25)

= 150 – (13 x 1.25)

= 133.75

A-a gradient = PAO2 – PaO2

= 133.75 – 68

= 65.75 mmHg (normal = Age/4 + 4 = 19.5)

∴ there is an elevated A-a gradient.

Electrolyte clues:

Hyponatraemia, hyperglycaemia, hyperlactataemia, hypoalbuminaemia, normokalaemia, normochloraemia, normal urea, and a low urinary sodium.


Formulation:

Description: This arterial blood gas shows an alkalaemia due to a primary respiratory alkalosis. There is a coexisting non-anion gap metabolic acidosis. There is marked hyponatraemia, despite correction for hyperglycaemia. There is hyperlactataemia, hypoalbuminaemia, normal urea, and a low urinary sodium. There is normokalaemia, and normochloraemia.

Interpretation:

Hypervolaemic hyponatraemia with low urinary sodium is consistent with the stated history of cardiac failure. The differential diagnosis includes cirrhosis, nephrotic syndrome, and secondary hyperaldosteronism. Rapid correction of sodium should be avoided (≤8 mmol/day is safe) in order to avoid osmotic demyelination syndrome (also known as central pontine myelinolysis); this will require fluid and salt restriction and the judicious use of diuretics. There is significant hypoxia with an elevated A-a gradient associated with chest pain. The potential causes include pneumonia ± sepsis, cardiogenic pulmonary oedema secondary to myocardial ischaemia ± infarction, or pulmonary embolism with hyperventilation due to pain, hypoxia, and V/Q mismatch.

Blood Gas #20
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