What causes pulmonary embolism?

Small clot fragments: Common cause is separation of small clots in the venous system (such as the leg, etc) which return to heart and lodge in the lungs. Trauma, some infections, varicose veins with poor draining, phlebitis are recognized as common causes of the clotting.

Phlebitis: Pulmonary embolism is the complication of inflammation and blood clots forming in veins (phlebitis) and then traveling in the venous system to the heart. The right side of the heart pumps these clots into the pulmonary arteries causing the syndrome of pulmonary embolism. Usually the clots form in the legs or pelvic veins, but can form in the arms as well.

Blood clot: A pulmonary embolism is the result of a blood clot travelling to your lungs. The blood clot (referred to usually as a DVT) is usually found in the veins in one or both of your legs. This breaks loose and travels up through the IVC to the heart and then to your lungs.