V/Q Scan in COPD: Matched Defects

A. Low probability for PE — matched defects in COPD are characteristic of parenchymal disease, not vascular obstruction

In patients with COPD, areas of airway obstruction reduce both ventilation AND perfusion to the same lung regions (hypoxic vasoconstriction and/or loss of vascular bed), producing matched V/Q defects. By PIOPED II criteria, matched defects — particularly when they correspond to abnormalities on chest X-ray — represent LOW probability for PE. The V/Q scan is less specific in patients with COPD because the underlying lung disease creates matched defects that mimic the appearance of PE-related mismatches but without actual vascular occlusion. CT pulmonary angiography (CTPA) is often preferred in patients with severe COPD.