The natural history of venous thromboembolism: impact on ventilation/perfusion scan reporting

Deep vein thrombosis (DVT) and pulmonary embolism (PE) are separate but related aspects of the same dynamic disease process known as venous thromboembolism (VTE). Recent community studies have shown that VTE is a major health issue for the developed world, with at least 201,000 new cases each year in the United States, comprising 107,000 with DVT and 94,000 with PE. A quarter of PE cases die within 7 days, some so rapidly that treatment or intervention is impossible. Despite the availability of heparin prophylaxis, the annual incidence of VTE has remained constant at 1 event per 1,000 person-years since 1979 but reaches 1 event per 100 person-years for the over-85-year-olds. The most important risk factors for VTE are hemostatic and environmental. The recent discoveries of factor V Leiden, prothrombin 20210A, and high concentrations of factor VIII have highlighted the increasing importance of a genetic predisposition to thrombophilia. Acquired hemostatic factors include pregnancy and the puerperium, oral contraception, hormone-replacement therapy, malignant tumors, and antiphospholipid syndromes. Important environmental risk factors include hospitalization with previous surgery or trauma, confinement in a care facility, neurologic disease or paraplegia after stroke, current or recent central venous catheter or transvenous pacemaker, and long airplane flights. Internists may be confused about the risk of PE after ventilation/perfusion (VQ) imaging. This may well arise from their use of the relative risk of PE after a low-probability category scan rather than the absolute risk obtained by incorporating the PE prevalence for their particular patient in the risk analysis. Ideally, personal communication with an experienced referring physician provides this clinical information for nuclear medicine. Diagnostic tools or checklists can be used as an alternative. A general knowledge of the natural history of VTE will encourage the nuclear medicine physician to provide an appropriate clinical signal to complement VQ categorical analysis. Combination of these 2 dynamic elements of the art and science of VQ scan reporting-the clinical pretest probability of PE and lung scan category-will permit an accurate prediction of the absolute risk of PE posttest.