Hypoxia is most important feature of acute pulmonary embolism.
It occurs due to variety of mechanisms
- Ventilation perfusion mismatch is the major mechanism ( Normal ventilation /Reduced perfusion)
- Atelectasis of lung ( Left to right shunt)
- Loss of lung volume due to pulmonary infarct contribute later
- Low mixed venous Oxygen saturation (Tissue hypoxia -more extraction )
- One more important cause is right to left shunting across PFO due to sudden elevation of right atrial mean pressure reflected from RVEDP .
Can acute pulmonary embolism be diagnosed with out Hypoxia ?
Surprisingly many standard text books mention hypoxia is a soft sign . In fact , Braunwald’s text book of cardiology do mention about it .
Significant acute pulmonary embolism can not occur without affecting o2 saturation .
However , it is possible sub acute pulmonary embolism could occur with normal oxygen saturation.
Hypoxia is indeed a hard sign for most events of major pulmonary embolism . It can even be termed as an essential criteria .A hypoxic , tachypenic patient in sinus tachycardia with echo evidence of new onset RA or RV dilatation is almost 100 % specific for acute pulmonary embolism . ( This becomes 200 % if he or she has DVT as well !)