An approach to diagnostic imaging of suspected pulmonary embolism.

Risk factors for pulmonary embolism include immobilization, trauma and surgery, particularly for hip fracture. Patients may present with acute respiratory symptoms, including tachypnea, tachycardia and rales. Chest radiographs and clinical and laboratory findings alone cannot provide a firm diagnosis. A completely normal chest radiograph may be seen in up to 40 percent of patients with pulmonary embolism, and as many as 30 percent of persons with pulmonary embolism and no prior cardiopulmonary disease will have a PaO2 greater than 80 mm Hg. The ventilation/perfusion (V/Q) lung scan is central to guiding clinical decisions. V/Q scans interpreted as either normal, near normal or high probability are reasonably diagnostic. A low probability V/Q scan can exclude the diagnosis of pulmonary embolism only if the patient has a clinically low probability of pulmonary embolism. Intermediate V/Q scans are not diagnostic and call for further evaluation. Compression ultrasonography is sensitive in detecting symptomatic deep venous thrombosis in the thigh. When clinical suspicion remains high and noninvasive imaging studies are uncertain, pulmonary angiography is likely to be diagnostic.