AbstractsThe diagnosis of pulmonary thromboembolism is a controversial subject. The present article outlines a practical guide for the use of ventilation-perfusion scintigraphy in patients with suspected pulmonary embolism.Fatal pulmonary embolism commonly is unrecognized before death, particularly in elderly patients and in those with congestive heart failure or pneumonia. The symptoms, physical signs, routine laboratory findings, and standard roentgenographic studies of the chest frequently are inconclusive. Dyspnea, pleuritic chest pain, and apprehension are the most common presenting symptoms, but the classic triad of pleuritic chest pain, dyspnea, and hemoptysis occurs in less than 20 per cent of affected individuals. Findings on physical examination which suggest deep venous thrombosis are present in one-third of the patients with pulmonary thromboembolism.Because of the limitations of clinical evaluation, laboratory studies, and roentgenographic studies of the chest in the evaluation of patients suspected of having this disease, the need exists for a method that provides greater sensitivity and specificity. Pulmonary ventilation-perfusion scintigraphy fulfills this role adequately. Perfusion imaging involves the intravenous injection of radiolabeled particles ranging from 10 to 60 μm in diameter. These particles are trapped in the capillaries and precapillary arterioles of the lung. The particles are distributed to the lungs in proportion to the regional pulmonary blood flow. The examination is relatively safe, although the total number of radiolabeled particles injected should be reduced in patients with severe lung disease. An adequate perfusion lung scan should consist of at least six views: anterior, posterior, left and right laterals, and left and right posterior obliques.Pulmonary embolism causes mechanical obstruction of the pulmonary artery, which results in one or more perfusion defects on the scintigrams, but is not the only cause of such defects. Other pathophysiological disturbances may lead to focal defects in pulmonary perfusion. The addition of ventilation scintigraphy to the standard pulmonary perfusion study improves specificity in the diagnosis. Combined ventilation-perfusion scintigraphy has a positive predictive value of nearly 90 per cent, as opposed to only 60 per cent for perfusion scintigraphy alone.The scintigraphic diagnosis of pulmonary embolism is based on the following: multiple regions of abnormal perfusion are present, corresponding to bronchopulmonary segments; ventilation studies are normal (ventilation-perfusion mismatch); and abnormal densities on roentgenograms of the chest are absent. Angiographically demonstrable embolization is common in patients with mismatched perfusion defects. Conversely, the condition is infrequent in patients with matching ventilation and perfusion abnormalities.Scintigrams are classified as normal or as indicating a low, intermediate, or high probability of embolus. A normal perfusion scan virtually excludes an embolus.The accurate method for the diagnosis of pulmonary embolus is pulmonary angiography. An intraluminal filling defect or an abrupt vascular “cut-off” in a large pulmonary artery is required for a specific diagnosis.When performed by experienced physicians, pulmonary angiography is an invasive, albeit relatively safe procedure, with a morbidity of approximately 1 per cent and a mortality of less than 0.3 per cent. The following general guidelines have proved useful in determining when to apply the procedure. Pulmonary angiography is not performed following a normal scan. In the case of high-probability scintigrams, angiography is reserved for those patients in whom anticoagulants are contraindicated. Most patients with low probability scans need not undergo pulmonary angiography, and they do not receive anticoagulants. Patients with intermediate probability scans are more likely to have angiography performed if the clinical setting is appropriate.