Pulmonary embolism is a blockage in the pulmonary arteries usually caused by blood clots. It can be diagnosed based on risk factors, symptoms, and tests like a D-dimer blood test, CT scan, or lung scan. Treatment depends on severity but generally involves blood thinners like heparin or warfarin to prevent further clots. For severe cases, thrombolysis can dissolve clots while a vena cava filter may be placed for recurrent clots or if blood thinners are contraindicated.