ACR Codes: 5.6
Pulmonary embolism (PE) can be associated with significant mortality if untreated. The clinical diagnosis of pulmonary embolism is unreliable. Symptoms of PE include tachypnea/dyspnea (most common), tachycardia, hypoxia, pleuritic chest pain, hemoptysis, syncope, and atrial fibrillation. Blood gas may be normal.
The D-dimer blood test is a screening tool for pulmonary embolism. A serum level less than 500 ng/L excludes pulmonary embolism with a 90% accuracy. A positive test is non-specific. Additionally, the test is unreliable in the presence of malignancy, sepsis, recent surgery, or trauma.
The source is most commonly from deep venous thrombosis in the lower extremity, however, about 10% arise from clot in the upper extremity. Risk factors are Immobilization, Recent Surgery, Underlying Malignancy, History of Deep Venous Thrombosis or Pulmonary Embolism, Estrogen use, or Pre-existing cardiac disease. A low percentage of cases of pulmonary embolism result in pulmonary infarction, due to the presence of the bronchial circulation.
Treatment for PE most commonly consists of anticoagulation with heparin or coumadin. Anticoagulation prevents clot propagation and allows endogenous fibrinolytic activity to dissolve existing thrombiAnticoagulation decreases mortality form 30-60% to <5%.
Thrombolytic agents are not routinely used for the treatment of acute PE. Thrombolytic treatment is generally reserved for patients with massive pulmonary embolism producing circulatory shock (hypotension).
For patients that cannot be anticoagulated, an inferior vena caval filter can be placed in order to prevent life-threatening PE. Major complications occur in about 1% of cases. Complications include central migration of the filter, filter fracture, inferior vena caval perforation, and vena caval thrombosis.
V/Q scanning has been the mainstay for screening symptomatic patients for the presence of pulmonary embolism. A negative V/Q scan essentially excludes PE, and a high probability study is associated with the presence of a PE in about 85% of cases at angiography. Confusion arises with low or intermediate probability examinations, and there is common disagreement among expert readers in the interpretation of scans in these categories. The problem with V/Q scanning is that it does not directly visualize thromboembolism, but rather its effects on perfusion and ventilation . This problem causes the need for probability criteria, which in turn causes confusing results. Nuclear medicine scanning for PE is probably most useful in previously healthy patients with a normal chest radiograph. As the complexity of the patients underlying cardiopulmonary disease increases, so does the likelihood that the scan will not be informative (intermediate probability). Using PIOPED criteria, intermediate probability V/Q scans occurred in 60% of patients with COPD, but in only 13% of patients with normal CXR's. However, a generalized abnormality on CXR such as diffuse pulmonary edema or reticulonodular disease may not cause the perfusion lung scan to be abnormal.
The CXR is abnormal in the majority of cases of PE. The PIOPED study showed that among patients with angiographically proven pulmonary embolism, only 12% had chest X-rays interpreted as normal. (24% of patients with PE in another study had normal CXR's ). Atelectasis and other focal pulmonary parenchymal abnormalities are the most common CXR findings in pulmonary embolism, occurring in up to 68% of patients with PE. Pleural effusions are also common but usually small and unilateral. Other palin film findings indicative of PE include regional oligemia (Westermark sign), a pleural-based wedge shaped area of increased opacity (Hampton's hump), and prominence of the central pulmonary artery with abrupt tapering (Knuckle sign).
In pulmonary angiography,an embolus classiclally produces a filling defect within the affected pulmonary artery. Non-occlusive emboli have a "tram-track" appearance. Although considered the gold standard, angiography may not always detect the presence of emboli. Some indirect angiographic evidence for the presence of emboli such as vascular pruning and delayed capillary blush are non-specific. Additionally, agreement among angiographers regarding the presence of subsegmental emboli is poor and can be as low as 15%. V/Q scans can provide a road map to angiography, but if the abnormally perfused segment on the V/Q scan appears normal at angiography, complete evaluation the remainder of the lungs for the presence of pulmonary emboli is warranted. One important point to remember is that a negative angiogram has been shown to be an excellent indicator of a good prognosis.
Helical CT is able to identify main, lobar, and segmental emboli with a reported sensitivity over 90%. Although the detection of subsegmental emboli is worse, the clinical significance of these small emboli has not yet been establishished. Additionally, on angiography there is poor interobserver agreement for the presence of subsegmental emboli and the true incidence of isolated subsegmental emboli is difficult to determine. Helical CT has also been shown to have a significantly better sensitivity, specificity, positive, and negative predictive values compared to V/Q scanning . Helical CT permits a more confident diagnosis to be made in a greater number of cases when compared with V/Q scanning .
It has been suggested that helical or electron beam CT should be the initial imaging modality to screen patients suspected of having PE- particularly in patients with abnormal CXR's in whom there is a greater likelihood of inconclusive V/Q scan results. If emboli are detected, no further work-up is required. If this study is negative, a lower extremity US or CT DVT exam can be performed to assess for the presence of DVT. Again, a positive exam would lead to patient treatment. If both studies were negative, then a decision would be required regarding whether the patient should proceed to angiography or not. Another benefit of CT is the ability to suggest an alternative diagnosis.
Reference(s): Pulmonary embolism revealed on helical CT angiography: comparison with ventilation-perfusion radionuclide lung scanning. Journal of Vascular Surgery. 33(1):206, January 2001.
AJR 2000; Blachere H, et al
Henteleff, Harry J.A.. Drover, John W.. Members of the CAGS Evidence Based Reviews in Surgery Group *. Canadian Association of General Surgeons Evidence Based Reviews in Surgery. 3. Helical computed tomography versus pulmonary arteriography in pulmonary embolism. Canadian Journal of Surgery. 45(5):373-375, October 2002.
Worsley et al. Comprehensive Analysis of the Results of the PIOPED Study. Journal of Nuclear Medicine, vol 36, No 12 Dec 95
Scott C. Williams, www.auntminnie.com(reference section, thoracic imaging)
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