The ventilation-perfusion (V/Q) scan is performed on patients with suspected pulmonary embolism (PE). The study has a low incidence of adverse reactions, is noninvasive, and requires no advance preparation.3 However, it is performed outside of the ED, limiting its use in unstable patients.
The V/Q scan is a two-step process. First, the ventilation portion of the examination is performed with the patient breathing an aerosolized solution of technetium 99m (99mTc) diethylenetriaminepentaacetic acid (DTPA). The aerosol is deposited deep into the lungs in proportion to alveolar ventilation. Images are obtained from different projections. The second step is the perfusion study in which 99mTc-macroaggregated albumin is injected intravenously and becomes trapped within the pulmonary circulation. Again, images are obtained with the patient in multiple positions. Usually, the procedure is quite simple and the test can be finished in 1 h. Procedural problems arise when a patient is unable to lie still. The ventilation study may be limited if a patient cannot take a deep breath or follow instructions or is on a ventilator.
Diagnosis of PE on V/Q scan is based on documenting perfusion defects in an area of normal ventilation, a mismatched defect. Perfusion defects in areas of associated ventilation abnormalities are probably due to vasoconstriction secondary to hypoxia. Such matched defects may be due to pneumonia, asthma, or chronic obstructive pulmonary disease. Underlying pulmonary pathology causes abnormalities in both ventilation and perfusion, making interpretation difficult.
The V/Q scan is reported in terms of probability, correlating the findings of the V/Q scan with the chest radiograph. Clinicians should understand the definitions for high, intermediate, and low probability used by radiologists when reporting the results of a V/Q scan (Table 67-1 )4-5 The PIOPED study found that the clinical utility of a V/Q scan was enhanced when used in conjunction with clinical suspicion of embolism (Table 67-2).6 For example, if the clinical suspicion for PE is high but the
V/Q scan interpretation is low probability, the probability that a patient may still have PE is 40 percent. Many emergency department patients fall into the intermediate category for clinical suspicion, where a low probability scan does not exclude PE. In this category, 16 percent of the PIOPED study patients had angiographically proven embolism.6
TABLE 67-1 Modified Criteria of the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED)
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