DIAGNOSIS OF PULMONARY EMBOLISM: A CONTINUING DILEMMA. ELGAZZAR, A.H. MD, FCAP and AL ENIZI, E. MD, KBNM Department of Nuclear Medicine, Kuwait University.

Basic considerations: 1 The vast majority of pulmonary emboli are thromboemboli originating from deep veins. Fat, air, or tumor emboli are rare [1]. Fat emboli are reported with long bone fractures and liposuction while air emboli occur with cardiac and neurosurgeries. Renal cell carcinoma with invasion reaching inferior vena cava is a clinical setting that may lead to tumor emboli. Data indicate that 90% of pulmonary thromboemboli originate from the lower extremities and pelvis. The remainder come from thrombi that occur in the right side of the heart or in bronchial or cervical veins. Embolization and symptomatology are proportional to how proximal is the vein that contains the thrombus. The vast majority of pulmonary thromboemboli originating from thrombi of the lower extremities come more frequently from the thigh and pelvis (75%) than from smaller veins of the calf and feet [2,3]. The risk of pulmonary embolus is also directly related to the presence of a residual clot at the site of a venous thrombus [4].
One of three events can happen during the natural history of venous thrombi. First, the red thrombus grows explosively and obstructs the vein completely. This can happen even within a few minutes. Second, partial venous obstruction may occur. Blood flow therefore continues over the thrombus surface. Under this circumstance, thrombus growth tends to occur by the progressive layering of platelets and fibrin on the clot surface, pathologically seen as the lines of Zahn. Third, probably the most common scenario, a small thrombus is swept away before it reaches an appreciable size. It lodges in the pulmonary vasculature without symptoms.
Unless fibrinolytic resolution is prompt, organization of the thrombus begins within hours of formation. What was a thrombus is slowly replaced by granulation tissue. This process anchors the thrombus to the venous wall. The dynamic battle between fibrinolysis and thrombus formation is fought out over a period of 7–10 days, at the end of which time either complete resolution has occurred or an endothelialized residual is present. At any time during this period, a portion or all of the thrombus can detach as an embolus. This risk is highest early, before significant dissolution or organization occurs [3].