Lab Case 43 – Interpretation

Young female with SOB and SVT which has resolved after Valsalva. She is currently haemodynamically stable.

1. Describe the arterial blood gas results:

mild respiratory alkalosis – pH 7.46, pCO2 27

compensation if acute: expected HCO3 = 21, actual 19, therefore

additional normal anion gap metabolic acidosis  (anion gap = 9), hyperchloraemia

Severe hypoxia pO2 = 49, Sats of 84% on 15 litres

Very large A-a gradient = 560 -assume O2 of 90%

Very high Hb = 194

2. Interpretation:


Hyperchloraemic metabolic acidosis – normal saline administration

Severe hypoxia and large A-a gradient not responding to Oxygen with very high Hb (?chronic hypoxia) – likely to indicate large shunt, the commonest of which is a large ASD/PFO or VSD with a right to left shunt.

The large A-a gradient is unlikely to be due to VQ mismatch or diffusion defect in this patient however other differentials include PE (if present would be large to cause this picture and you would expect haemodynamic instability)

3. The most important investigation in this patient is a Cardiac Echo – which in this patient showed a large ASD/PFO. Her CTPA did not show a PE.



5 thoughts on “Lab Case 43 – Interpretation

  1. Hi. With a pco2 of 27, would the expected hco3 in acute respiratory alkalosis not be 22 and chronic 19 (which it is). 24 – 5 = 19.

    • Hi Simon,

      I missed your comment earlier.

      The pH here is slightly elevated at 7.46 (or upper end of normal anyway) so its unlikely that compensation was complete. For compensation the HCO3 should be lower for this CO2.

  2. In fact on TTE, no ASD was visible on the cardiologist report. It was only through a flow artifact on CTPA and cardiac MRI that the ASD with Eisenmenger’s was confirmed!

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