ACR Codes: 6.7
Although numerous studies utilizing new imaging modalities such as helical CT scans have been performed to improve the diagnosis of pulmonary embolism (PE), ventilation/perfusion (V/Q) scans remain the screening tool of choice. This test is a nuclear medicine study that compares images of inhaled radionuclide material to pulmonary perfusion images of intravenously injected radiolabeled protein. These images must be closely compared to a chest X-ray taken at or near the time of the V/Q scan. This is done to ensure defects on the V/Q images are not accounted for by findings on the chest X-ray. A high probability scan, defined as two or more moderate or large segmental mismatched perfusion defects, in the presence of high clinical suspicion was found to be accurate 96% of the time in the PIOPED study. The same study also found that a low probability scan in the presence of low clinical suspicion was only falsely negative in 6% of the patients. However, this leaves a large number of cases in which there is either a moderate clinical suspicion, an intermediate scan, or a discrepancy between pre-test clinical suspicion and final test results. Therefore, more invasive tests such as pulmonary angiography must often be used for a definitive diagnosis.
The chest X-ray is also an important tool in the evaluation a patient with a suspected PE. In addition to being used in the interpretation of the V/Q scan as mentioned above, chest x-rays are often useful in determining that the cause of the patient's signs and symptoms is due to a condition other than PE. For example, a fractured rib or cavitary lung lesion may present in a manner similar to PE. In relatively uncommon situations, radiologic findings on chest X-ray such as decreased vascular markings distal to engorged arteries (Westermark's sign) or a wedge-shaped pleural based infiltrate (Hampton's hump) may lead to the diagnosis of PE.
Reference(s): 1. PIOPED Investigators. Value of the ventilation/perfusion scan in acute pulmonary embolism: results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). JAMA 1990; 263:2753-2759.
2. Maki DD, Gefter WB, Alavi A. Global theme issue: emerging technology in clinical medicine. Chest 1999; 119:1388-1401.
3. Baker WF. Diagnosis of deep venous thrombosis and pulmonary embolism. Med Clin N Am 1998; 82:459-474.
4. Tintinalli JE, Kelen GD, Stapczynski JS. Emergency medicine: a comprehensive study guide, fifth ed McGraw-Hill. 2000.
5. Murray IP, Ell PJ. Nuclear medicine in clinical diagnosis and treatment, second ed Churchill Livingstone. 1998.
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