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
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.