A 56 year old female presents with 4 days of shortness of breath and chest pain. She is tachypnoeic and has a mild haemdynamic derangement.

Her blood gas shows:

Acute on chronic (4 days) compensated respiratory alkalosis

pH = 7.61, pCO2 = 17

expected HCO3 for acute (using 2/10 rule) = 20 and chronic (using 5/10 rule) = 14

HCO3 is 17 which is slightly too low for pure acute compensation, probable chronic component (4 day history of symptoms)

Normal electrolytes, lactate, glucose

A-a gradient significantly elevated:

A-a gradient : (760-47)FiO2 – (1.25XpCO2) – pO2 = 140

This implies VQ mismatch or shunt.

With the history given, a shunt caused by large Pulmonary embolus or significant embolic load must be at the top of the list of DDx. Other cause – Pneumonia