A PE usually occurs when a thrombus in the deep veins of the lower extremities loosens or dislodges and begins to move in the bloodstream. The thrombus (now an embolus because it is

790 Pulmonary Embolism moving in the bloodstream) floats to the heart, moves through the right side of the heart, and enters the pulmonary circulation through the pulmonary artery. Major risk factors for the development of PE include any condition that produces venous stasis, increased blood coagulability, or venous endothelial (vessel wall) changes. Situations resulting in these pathological changes include immobility, dehydration, injury, or decreased venous return. Conditions associated with these risk factors include varicosities, pregnancy, obesity, tumors, thrombocytopenia, atrial fibrillation, multiple trauma, presence of artificial heart valves or vessels, sepsis, and congestive heart failure.

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