This is a cause of obstructive shock, which presents as sustained low blood pressure, slowed heart rate, or pulselessness. Risk factors
About 90% of emboli are from proximal leg deep vein thrombosis (DVTs) or pelvic vein thromboses. The rare venous thoracic outlet syndrome can also be a cause of DVTs, especially in young men without significant risk factors. DVTs are at risk for dislodging and migrating to the lung circulation. The conditions are generally regarded as a continuum termed venous thromboembolism (VTE).VTE is much more common in immunocompromised individuals as well as individuals with comorbidities including:

Those that undergo orthopedic surgery at or below the hip without prophylaxis.This is due to immobility during or after the surgery, as well as venous damage during the surgery. Pancreatic and colon cancer patients (other forms of cancer also can be factors, but these are the most common)This is due to the release of procoagulants.Risk of VTE is at its greatest during diagnosis and treatment, but lowers in remission. Patients with high-grade tumors
Pregnant individualsAs the body puts itself into what is known as a "hypercoagulable state" the risk of a hemorrhage during childbirth is decreased and is regulated by increased expression of factors VII, VIII, X, Von Willebrand, and fibrinogen. Those on estrogen medicationThe development of thrombosis is classically due to a group of causes named Virchows triad (alterations in blood flow, factors in the vessel wall, and factors affecting the properties of the blood). Often, more than one risk factor is present.
CTPA is non-inferior to VQ scanning, and identifies more emboli (without necessarily improving the outcome) compared to VQ scanning. Ventilation/perfusion scan
A ventilation/perfusion scan (or V/Q scan or lung scintigraphy) shows that some areas of the lung are being ventilated but not perfused with blood (due to obstruction by a clot). This type of examination is as accurate as multislice CT, but is less used, due to the greater availability of CT technology. It is particularly useful in people who have an allergy to iodinated contrast, impaired kidney function, or are pregnant (due to its lower radiation exposure as compared to CT). The test can be performed with planar two-dimensional imaging, or single photon emission computed tomography (SPECT) which enables three-dimensional imaging. Hybrid devices combining SPECT and CT (SPECT/CT) further enable anatomic characterization of any abnormality. Low probability diagnostic tests/non-diagnostic tests
Tests that are frequently done that are not sensitive for PE, but can be diagnostic. Chest X-rays are often done on people with shortness of breath to help rule-out other causes, such as congestive heart failure and rib fracture. Chest X-rays in PE are rarely normal, but usually lack signs that suggest the diagnosis of PE (for example, Westermark sign, Hamptons hump). Ultrasonography of the legs, also known as leg doppler, in search of deep venous thrombosis (DVT). The presence of DVT, as shown on ultrasonography of the legs, is in itself enough to warrant anticoagulation, without requiring the V/Q or spiral CT scans (because of the strong association between DVT and PE).