Pulmonary thromboembolism (PE) is a blood clot (thrombus) in the lung. It is not a disease in and of itself, but rather an often fatal complication of underlying venous thrombosis - a condition of abnormal blood clotting where clots form in the veins, usually veins in the legs. PE occurs when these propagating clots break loose and embolize to block pulmonary blood vessels. Studies suggest that nearly every patient with thrombus in the upper leg or thigh will have a PE if a sensitive enough test is done to look for it, even though most have no clinical symptoms.
Death from massive PE is one of the most common causes of unexpected death, being second only to coronary artery disease as a cause of sudden unexpected natural death at any age. Most clinicians do not appreciate the extent of the problem, because the diagnosis is unsuspected until autopsy in approximately 80% of cases. Although PE often is fatal, prompt diagnosis and treatment can reduce the mortality dramatically.
It is estimated that approximately 10% of patients in whom acute PE is diagnosed die within the first 60 minutes. Of the remainder, one-third will eventually be diagnosed and treated and two-thirds will remain undiagnosed. Among the group who are correctly diagnosed and treated, only about one-twelfth will die from massive PE or its complications. Among the group who are undiagnosed and therefore untreated, roughly one-third will die of PE.
Whilst PE can arise from deep vein thrombosis anywhere in the body, thrombus in the popliteal segment of the femoral vein (the segment behind the knee) causes PE in more than 60% of cases. One important autopsy study showed that more than 35% of patients who died from PE had isolated calf vein thrombosis. Fatal PE more often results from thrombus that originates in the axillary or subclavian veins (deep veins of the arm or shoulder) or in veins of the pelvis. Thrombus that forms around indwelling central venous catheters is a common cause of fatal PE.
Pulmonary embolism is the third most common cause of death in the United States, with at least 650,000 cases occurring annually. It is the first or second most common cause of unexpected death in most age groups. The highest incidence of recognized PE occurs in hospitalized patients. Autopsy results show that up to 60% of patients dying in the hospital have had a PE, but the diagnosis has been missed in about 70% of the cases.
Patients who survive an acute PE are at high risk for recurrent PE and for the development of pulmonary hypertension and chronic pulmonary heart disease, attendant mortality and morbidity.
Although the frequency of PE increases with age, age is not an independent risk factor. Rather, it is the accumulation of other risk factors, such as underlying illness and decreased mobility, that causes the appearance of PE with increased frequency in older patients.Several papers suggest that the incidence of PE may differ substantially from country to country, but there are no prospective controlled studies to lend support to this notion. It is possible that the observed variance is due to differences in the rate of diagnosis more than differences in the rate of the disease. If there is a real difference, it is not known whether the difference is due to genetic variation, or whether it can be ascribed to population differences in diet and activity. There may be subtle population differences in the incidence of DVT and PE, but the incidence is high in all racial groups.
In the UK, an average of one passenger per month dies of a pulmonary embolism caused by deep vein thrombosis soon after landing at London's Heathrow airport.