Pulmonary embolism ventilation/perfusion scan
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Editor(s)-In-Chief: The APEX Trial Investigators, C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:
Overview
A ventilation/perfusion scan (otherwise known as V/Q scan or lung scintigraphy) is a study which shows whether an area of the lung is being ventilated with oxygen and perfused with blood. In the setting of a PE, perfusion can be obstructed due to the formation of a clot. The V/Q scan is less commonly used due to the more widespread availability of CT technology, however it may be useful in patients who have an allergy to iodinated contrast. It may also be useful in pregnant patients in an attempt to minimize radiation exposure. The diagnostic value of the results of the ventilation/perfusion scan is improved when combined with the clinical pretest probability of PE. A high probability scan coupled with a high clinical pretest probability of PE is diagnostic for PE, while a normal scan regardless of the clinical pretest probability excludes PE. For the majority of the cases of suspected PE, the ventilation/perfusion scan does not exclude PE and further tests are required.[1]
Ventilation/Perfusion Scan
Principle
Technetium (Tc)-99m labeled macro-aggregated albumin particles, when injected, fill small fractions of the pulmonary capillaries. This helps in the assessment of lung perfusion at the tissue level via scintigraphy. When any branch of the pulmonary artery is occluded, the particles will not reach the capillaries rendering the area "cold" on imaging.
Indications
The utilization of V/Q scanning has declined since the advent of more widespread availability of CT technology, however it may be useful in particular subgroups of patients, such as:
- Patients who have a known allergy to iodinated contrast. To read more about contrast allergy, click here.
- In pregnant patients to minimize exposure to radiation.
- For patients who are in a hospital lacking CT technology.
Interpretation
A normal or near normal V/Q scan excludes the diagnosis of PE.[1][2] When the result of the V/Q scan is not normal or near normal, the results of the V/Q scan should be interpreted in combination with the pretest probability of PE. Shown below is a table that summarizes the possible outcomes of a V/Q scan.[2]
| V/Q Scan | Clinical Probability | Interpretation |
| Normal | Any probability | PE excluded |
| Low probability scan | Low | PE excluded |
| High probability scan | High | PE confirmed |
| Any other combination of V/Q scan and clinical probability | Further tests are needed | |
Any defect that is located pleurally, has a triangular/concave shape, and is in the anatomical distribution of a lung segment should be considered segmental in nature. A defect of the right lower lobe involving the medial basal segment may be undetectable in any view.[3]
Supportive Trial Data
- Data regarding the use of the ventilation/perfusion scan for the evaluation of patients with suspected PE is provided by the findings of the PIOPED study. The rates of confirmed PE by angiography for each of the categories of the ventilation/perfusion scan are:[1]
- High probability scan: 88%
- Intermediate probability scan: 33%
- Low probability scan: 16%
- Normal or near normal probability scan: 9%
- Shown below is the sensitivity and specificity of ventilation/perfusion scan for detecting PE:[1]
| Category of ventilation/perfusion scan | Sensitivity | Specificity |
| High probability scan | 41% | 97% |
| High or intermediate probability scan | 82% | 52% |
| High, intermediate, or low probability scan | 98% | 10% |
- For a prevalence of PE of 33% (confirmed by angiography), the positive predicitve value (PPV) and negative predictive value (NPV) of the ventilation/perfusion scan for PE are as follows:[1]
- PIOPED demonstrated that the value of the ventilation/perfusion scan is improved when combined with the clinical pretest probability of PE. For example, among patients with suspected PE and high probability scan, the rates of confirmed PE by angiography were 56%, 88%, and 96% in cases of low, intermediate, and high clinical pre-test proability of PE.[1]
Comparison with CT Pulmonary Angiography
Benefits of V/Q Scan over CTPA
- Less radiation exposure
- Diagnostic test of choice in an institution lacking a CT facility, or with inexperienced staff
Benefits of CTPA over V/Q Scan
- Cost effectiveness
- CT may also identify right heart dysfunction, or provide an alternative diagnosis.
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 PIOPED Investigators (1990). "Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED)". JAMA. 263 (20): 2753–9. PMID 2332918.
- ↑ 2.0 2.1 Torbicki A, Perrier A, Konstantinides S, Agnelli G, Galiè N, Pruszczyk P; et al. (2008). "Guidelines on the diagnosis and management of acute pulmonary embolism: the Task Force for the Diagnosis and Management of Acute Pulmonary Embolism of the European Society of Cardiology (ESC)". Eur Heart J. 29 (18): 2276–315. doi:10.1093/eurheartj/ehn310. PMID 18757870.
- ↑ Morrell NW, Roberts CM, Jones BE, Nijran KS, Biggs T, Seed WA (1992). "The anatomy of radioisotope lung scanning". J. Nucl. Med. 33 (5): 676–83. PMID 1569475. Retrieved 2012-01-12. Unknown parameter
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