Pulmonary thromboembolic disease refers to the condition in which blood clot(s) (thrombus or multiple thrombi) migrate from the systemic circulation to the pulmonary vasculature. Most of these thrombi arise from the deep veins of the lower and upper extremities (deep venous thrombosis [DVT]). From the clinical standpoint, DVT and pulmonary embolism (PE) can be considered a continuum of the same disease, and the two terms are often collectively referred to as venous thromboembolism.
The annual incidence of PE in the United States remains uncertain. In a retrospective analysis of data involving 2218 Olmsted County residents over a 10-year period, community residents who were not hospitalized within a 90 days period had an incidence of PE of 3.6 per 10,000 person-years.1 A slightly lower incidence of 2.3 per 10,000 person-years was reported in an earlier study in Massachusetts.2 This translates to an annual incidence of approximately 100,000 cases in the United States. However, the true incidence of PE is likely to be much higher since many cases remain undiagnosed. A recent systematic review revealed that silent PE was present in 32% of patients with DVT.3 An earlier report estimated that as many as 630,000 patients develop PE every year in the United States with 200,000 related deaths, the majority in patients in whom the diagnosis was never made (Fig. 73-1).4 Although considerable effort is directed toward the development of new diagnostic techniques and therapeutic agents, a considerable impact on mortality related to the disease would arise from the routine use of prophylactic strategies, an understanding of the often subtle clinical presentation of the disease, and the appropriate application of existing diagnostic techniques.
Estimated incidence and survival statistics for pulmonary embolism in the United States. (Reproduced with permission from Dalen JE, Alpert JS. Natural history of pulmonary embolism. Prog Cardiovasc Dis. 1975;17(4):259–270.)
Most cases (80%–95%) of PE occur as a result of thrombus originating in the lower extremity. Thrombus often begins at a site where blood flow is turbulent, such as at a venous bifurcation, or behind a venous valve (Fig. 73-2). When thrombus propagation exceeds the rate of thrombus organization and adherence to the endothelium, part or all of thrombus may break away and migrate via the venous system to the lungs. Most thrombi originate in the deep veins of the calf and propagate proximally to the popliteal and femoral veins. Calf-limited thrombi pose a minimal embolic risk while those that extend into and above the popliteal vein represent the most common source of acute symptomatic PE. This is not meant to imply that calf-limited thrombosis represents a benign condition. Proximal propagation may occur in as many as 15% of untreated patients along with a higher risk of thrombotic recurrence and postphlebitic syndrome.5