Lung ventilation/perfusion scan, also called pulmonary ventilation/perfusion scan or VQ (ventilation quotient) scan, in medicine, a test that measures both air flow (ventilation) and blood flow (perfusion) in the lungs. Lung ventilation/perfusion scanning is used most often in the diagnosis of pulmonary embolism, the blockage of one of the pulmonary arteries or of a connecting vessel. Pulmonary embolism is caused by a clot or an air bubble that has become lodged within a vessel or by the accumulation of fat along the inner walls of the vessel, thereby narrowing the passageway and hindering the flow of blood. The procedure is also used to accurately identify damaged regions of lung tissue prior to surgery to remove the tissue. This approach may be taken for patients with advanced or rapidly spreading lung cancer.