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I think you might be misreading some of the language in the question. I'm sure it was trying to say there was NOT a ventilation defect. That's the diagnostic criteria for PE on a VQ scan. I think how they meant it was as a perfusion defect that has not been matched.

This gets at a key piece of lung physiology: the lungs are the only organ that will vasoCONSTRICT in response to hypoxia (aka ventilation defect). So lets imagine we had a foreign body somewhere in the bronchial tree; the VQ scan in that case would show a ventilation defect distal to the obstruction AND a perfusion defect in that same area.
Then why does supplemental o2 not work in low v/q if blood is moved from poorly ventilated area to well ventilated area?
 
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