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What are the mechanisms of hypoxia in Acute pulmonary embolism ?

Posted May 31 2012 12:11pm

Hypoxia is most important feature of acute pulmonary embolism.

It occurs due to variety of mechanisms

  1. Ventilation perfusion mismatch is the major  mechanism  ( Normal ventilation /Reduced perfusion)
  2. Atelectasis of lung  ( Left to right shunt)
  3. Loss of lung volume due to pulmonary infarct  contribute later
  4. Low mixed venous Oxygen saturation  (Tissue hypoxia -more extraction )
  5. 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.

Final message

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 !)


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