A matched perfusion defect in ventilation perfusion scans occurs where there is both altered ventilation and perfusion over the same area of lung. In this situation the diagnosis should be apparent from the chest radiograph; there may a region of water density - which includes the possible diagnosis of an established infarction - or there may be overinflation.
Note that a process which supposedly causes a pure ventilation defect will invariably result in some alteration in perfusion since the hypoxia results in altered blood flow.
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