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A ventilation–perfusion (VQ) scan evaluates the airflow (ventilation) and blood flow (perfusion) in the lungs. They are used in several clinical settings for:
Other indications are quantifying right to left shunts and differential pulmonary blood flow (see below in Further information about VQ scan).
History of COPD, asthma, CCF.
Suspicion of a so-called ‘massive PE’, which involves marked hypoxia, hypotension or both, is usually investigated with computed tomography pulmonary angiography (CTPA), because it is faster and does not require the patient to lie supine for an extended period. CTPA also allows mapping of the extent and location of clot within the pulmonary arteries, which might be useful if embolectomy is being considered in a severely compromised patient with PE.
Breast-feeding mothers are advised to stop breast-feeding for 24 hours (see consumer section and recommendations in Nuclear Medicine item). However, the VQ scan has a lower radiation dose to the breast when compared with a CTPA.
There are no absolute contraindications.
VQ scans are nearly always preceded by a chest X-ray. The presence of air-space or interstitial infiltrates makes it more likely to get an ‘intermediate probability’ and hence an indeterminate scan (approximately 25–30% likelihood of PE). A history of asthma, COPD or cardiac failure also increases the chance of getting an ‘intermediate probability’ scan.
VQ scans can be carried out in pregnant patients; however, the injected dose of radioactivity is reduced to keep it within permissible limits for the developing foetus. If pregnancy is thought likely, it would be prudent to obtain a pregnancy test.
The patient optimally should be able to lie still for the duration of the scan; however, in the event of the patient being unable to lie supine, some gamma cameras allow scan acquisition in the sitting position. You might need to verify this with the nuclear medicine department.
There are no significant adverse effects. Allergic reactions to radiopharmaceuticals are rare.
There is no post-procedure care required by the referrer relating to the VQ scan per se.
Advice concerning stopping breast-feeding (and expressing) might be required.
CTPA has emerged as a useful modality in the diagnosis of PE with the advent of multi-detector CT scanners. See website below – Diagnostic Imaging Pathways.
CTPA is a better alternative to a VQ scan in critically ill patients, in the obese and in those with abnormal chest X-rays or significant underlying pulmonary conditions. In each situation, the chance of an indeterminate VQ scan result is higher. Sensitivity (of the VQ scan) is also reduced when pulmonary emboli involve smaller (sub-segmental) branch vessels.
1. Assessing pretest probability of PE
The interpretation of a VQ scan relies in part on clinical assessment of the pretest probability of PE. It is extremely important that referring clinicians provide an assessment of the pretest likelihood of PE (low, intermediate or high).
The assessment of pretest probability can be facilitated through use of a structured risk assessment tool, such as the revised Geneva score or those devised by Wells et al. and Kline et al. (See websites below).
2. Understanding the report for PE
VQ scans for PE have traditionally been reported as normal or in terms of probabilities (high, intermediate, low or very low), particularly when planar imaging is used. With the increasing use of VQ SPECT (volume acquisition), the reports now try to be more definitive about whether PE are present or not. If the scan is normal, the likelihood of clinically significant PE approaches zero. The degree of abnormality is based on the size and the number of mismatched or matched VQ (ventilation or perfusion) abnormalities.
To assess if the patient has a pulmonary embolus, the pre-test probability of a PE is combined with the VQ scan findings to give the post-test probability (see websites below: Diagnostic Imaging Pathways – for flow diagram and How to Use an Article about a Diagnostic Test – for a simple discussion on test probabilities and VQ scans).
3. Other indications for VQ scan: quantifying right to left shunts or calculating differential pulmonary blood flow.
Right to left cardiac shunt study: Imaging the whole body and measuring the lung counts compared with whole body counts, the shunted blood (right to left) can be quantified:
Medscape Article – emedicine.medscape.com/article/1918940-overview
MD-Calc – http://www.mdcalc.com/wells-criteria-for-pulmonary-embolism-pe/
How to Use an Article About a Diagnostic Test – casemed.case.edu/curricularaffairs/Y3Core2/readings/How%20to%20Use%20an%20Article%20About%20a%20Diagnostic%20Test.pdf
Page last modified on 24/8/2018.
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