TABLE 523 Definition of Q Scan Probabilities

Combining V/Q results with the degree of clinical suspicion for PE has been shown to increase the accuracy of the test. 1° The combination of a low-probability scan with a low clinical suspicion has a 96 percent predictive value for the exclusion of PE, while a high-probability scan in the setting of high clinical suspicion has a 96 percent positive predictive value. The presence or absence of PE in this study was confirmed by pulmonary arteriography. Unfortunately, only a minority of patients

(174 out of 887) in the PIOPED study were found to have the diagnosis of PE confirmed or excluded by the combination of pretest probability and V/Q results.—

Further difficulty arises in defining "clinical suspicion." While most authors use the terms high, moderate, or low suspicion and some studies even assign a percent probability, no universally accepted diagnostic algorithm is available. The presence or absence of risk factors predisposing to the development of DVT is of prime importance, although unrecognized risk factors (e.g., occult cancer, deficiency of protein C or S) may exist in a given patient. The existence of unexplained symptoms of chest pain and dyspnea or signs such as tachypnea should raise the index of suspicion for PE. Findings of DVT, an elevated A-a gradient unexplained by exam or CXR, and the presence of a Westermark sign are all strong predictors of PE.

Spiral computed tomography (CT) compared favorably to pulmonary arteriography in one study, with an overall sensitivity of 82 to 90 percent and specificity of 93 to 96 percent. Accuracy was lowest in the setting of subsegmental PE.21 Magnetic resonance imaging (MRI) angiography has reported sensitivity of 75 to 100 percent and specificity of 95 to 100 percent in PE proven by pulmonary arteriography. 22 It is of limited utility in unstable patients, however.

Pulmonary arteriography remains the standard by which all other studies are judged. A negative study reliably excludes the diagnosis of PE. 7i0 It should be obtained when clinical suspicion for PE is high and other studies (V/Q scans, tests for leg DVT) are inconclusive or likely to produce false-positive results. It is often used prior to the initiation of thrombolytic therapy but is not necessary. Indeed, it increases the risk of bleeding complications in these patients. 18 Pulmonary arteriography has a mortality rate up to 0.5 percent and morbidity of 4 percent. These complications may be reduced by the use of selective angiography.

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