Definition
P.E. is an occlusion of a pulmonary artery(ies) by a blood clot.

Pathogenesis
A fragment of a thrombus usually located in the deep leg veins breaks off and travels to the lung.

Pathology

• Large or small blood clots may occlude major or minor branches of the pulmonary arterial circulation.
• Large emboli may cause acute cor pulmonale. Saddle thrombus is a blood clot which is seen at the bifurcation of the pulmonary artery. This could result in sudden death with no pathologic change in the lungs
• Smaller emboli lodge distally depending on their size. There is hemorrhage into the adjacent lung parenchyma Ten percent of emboli cause wedge-shaped hemorrhagic infarctions which extend to the peripheral lung.
• Infarction occurs when there is underlying heart failure or other diseases causing inadequacy of bronchial arterial supply. Infarcts are typically pyramidal with base at the pleura. The lung parenchyma dies (there are no nuclei in the septa) and there is often hemorrhage into the infarcted lung tissue If the cardiovascular function is adequate, the bronchial circulation will maintain tissue viability resulting in hemorrhage but no infarct.
• Emboli may contain tumor, infection, fat or bone marrow
• Paradoxical embolus occurs when there is either an ASD or VSD through which the embolus goes to the left side of the heart and then into the systemic arteries

Pathophysiology

• Varies from no consequence to instantaneous death.
• Respiratory compromise: dyspnea, hemoptysis, chest pain.
• Hemodynamic compromise: acute cor pulmonale.