Pulmonary embolism ventilation/perfusion scan
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Associate Editors-in-Chief: Ujjwal Rastogi, MBBS [2]
Synonyms and keywords: PE
Ventilation/perfusion scan (or V/Q scan or lung scintigraphy), which shows that some areas of the lung are being ventilated but not perfused with blood (due to obstruction by a clot). This type of examination is used less often because of the more widespread availability of CT technology, however, it may be useful in patients who have an allergy to iodinated contrast or in pregnancy due to lower radiation exposure than CT. * The ventilation/perfusion ratio (V/Q) Scan: The PIOPED data suggests that normal perfusion scans are almost never associated with recurrent pulmonary embolism, even if anticoagulation is withheld.
- High-prob scans, however only identified 41% of patients with PE.
- In the setting of a high pre-test probability, a high-prob scan revealed PE in 95% of cases.
- Unfortunately, 41% of all scans in PIOPED were interpreted as intermediate, and an additional 16% were interpreted as low-prob.
- Upon angiography, however, 30 and 14% of these patients respectively were found to have PE.
- Based on these numbers, there has been a huge movement to abolish the low-prob, and intermediate-prob categories, and have readings either be high-prob, normal, or non-diagnostic.
- It should also be realized that the false-positive rate for high-prob scans was 14%, and that 72% of patients in PIOPED had a clinical – scan combination that required further investigation.
- It has been suggested by some authors that patients with an intermediate pre-test probability of PE a + venous ultrasound would provide the same justification for anticoagulation as would a confirmed PE.
- Spiral CT scanning is now a standard modality to non-invasively diagnose PE.
- Initial studies reported sensitivities for diagnosing emboli to the segmental level (4th order branch) as high as 98%, however subsequent studies have found sensitivities to be lower.
- Obviously, the sensitivity is higher with more proximal clot.
- Although smaller clot, in the subsegmental arteries, is certainly not as physiologically important as the larger, more proximal clot, they may be important predictors of future, larger clots.
- The study by Mayo et.al. concludes that the sensitivity and specificity of CT angio are higher than that of V/Q scans, as is the inter-observer agreement.
- They recommend getting a CT angio as the next test following an indeterminate V/Q scan, however caution that if the pre-test probability is ‘sufficiently high’ a standard angiogram should still be obtained after a negative CT angio.
- van Erkel et.al. performed a cost-effective analysis using spiral CT angio for the diagnosis of PE.
- The use of CT angio in a diagnostic algorithm was by far and away a more cost-effective strategy.
- If the sensitivity of CT angio was < 85%, conventional angiography was associated with a lower mortality, but still remained a more expensive strategy.