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Hey guys. What does this mean: VQ scan shows a large perfusion defect that is not does not match a ventilation defect in the lungs. This is from uworld, q. id 476. Like I understand the concept of VQ mismatch, but apparently this person has a pulmonary embolism, which means the perfusion is defective. Why would there be a ventilation defect that has "not been matched"?
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.
 
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