Ventilation Perfusion (V/Q) scans are used primarily to see if someone may have a blood clot in the lungs, otherwise known as a pulmonary embolus (PE). If a blood clot forms in the pulmonary arteries or its branches, air and oxygen can get to the lung (+ventilation), but there is decreased or no blood flow to the part of the lung that is affected by the blood clot (-perfusion). This results in ventilation/perfusion mismatch and is used to rate the probability that someone has a PE. The test is read as "high probability, intermediate probability or low probability" for the presence of a PE.

The V/Q scan can be abnormal due to lung diseases like COPD, pneumonia, pneumonitis (inflammation of the lungs) and a host of other conditions. It can also be abnormal if someone has lung cancer, but cannot differentiate lung cancer from other lung conditions. Other imaging tests including high resolution computed tomography (CT) of the chest can better visualize the lung anatomy and structures.

Here is a reference from WebMD about V/Q scans:

http://www.webmd.com/lung/lung-scan