TABLE 672 Prevalence of Pulmonary Embolism as Compared with Angiographic Results or Outcome in the Prospective Investigation of Pulmonary Embolism Diagnosis Pioped Trial Percent

The V/Q scan ¡s most useful when it is either normal or high probability. A normal lung scan has a 4 percent overall probability of a PE; 96 percent of the PIOPED

study patients did not have the condition. - Unfortunately, normal findings on a V/Q scan are unusual; only 14 percent of patients in the PIOPED study had normal scan findings. A high-probability scan had an 87 percent overall probability of a PE; the 13 percent of high-probability scans that were later found to have normal pulmonary angiograms were false positive, possibly due to vasculitis or neoplasm. «6 Again, a high-probability V/Q scan ¡s uncommon; only 13 percent of the PIOPED

patients had a high-probability scan. ® The majority of patients undergoing V/Q scanning will have intermediate- and low-probability scans. The major difficulty in interpreting a^Q scan occurs in those with underlying pulmonary disease, such as asthma and emphysema, which alters both ventilation and perfusion. Pretest chest radiographs aid radiologists in such cases. For example, patients with matched defects on V/Q Scan and a corresponding infiltrate on plain chest film may have pneumonia or pulmonary infarct. There is some literature comparing the size of matched defects with the size of the infiltrate on chest radiograph to classify the likelihood of embolism.7

Although V/Q scans remain a common initial test for PE, many patients require further testing, especially those with intermediate or high clinical suspicion and intermediate- or low-probability V/Q scans.

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