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CTPA has become the default for suspected pulmonary embolism, but there are four situations in which ventilation–perfusion scanning is the better test — and one of them, screening for chronic thromboembolic pulmonary hypertension, is a diagnosis that CTPA misses. A quantitative version of the same scan is also what determines whether a patient can tolerate a lobectomy, which puts it directly in the path of surgical prehabilitation.
What the scan measures
Two images are acquired and compared. Perfusion uses intravenous technetium-labelled macroaggregated albumin, which lodges in the pulmonary capillary bed in proportion to blood flow. Ventilation uses an inhaled radioactive gas or aerosol. The diagnosis of embolism rests on mismatch: a region that is ventilated but not perfused.
Modern practice increasingly uses V/Q SPECT — a tomographic acquisition — rather than planar imaging, with better sensitivity and far fewer non-diagnostic studies than the historical PIOPED-era scans that gave V/Q its reputation for equivocal reporting.1,2
When V/Q is preferred over CTPA
| Situation | Why |
|---|---|
| Pregnancy | Substantially lower radiation dose to breast tissue. A perfusion-only study is often performed first, further reducing dose1,3 |
| Renal impairment | No iodinated contrast, so no contrast-associated nephropathy risk3 |
| Contrast allergy | Avoids the agent entirely |
| Young patients, especially women | Cumulative lifetime radiation risk favours V/Q where the chest radiograph is normal3 |
| Suspected CTEPH | V/Q is the recommended screening test, not CTPA — see below4,5 |
Its main limitation is the mirror image of its strength: it requires a reasonably normal chest radiograph. Extensive parenchymal disease, large effusions or severe COPD produce matched defects and a non-diagnostic study, which is when CTPA is the better choice.
CTEPH — the diagnosis CTPA misses
This matters in respiratory physiotherapy because the patient in question is usually already in a rehabilitation programme: breathless months after a pulmonary embolism, not recovering as expected, and easily attributed to deconditioning or post-thrombotic anxiety. Persistent breathlessness three months or more after a PE warrants review, not more reassurance, and a mention of CTEPH in a referral letter is reasonable for a physiotherapist to write. Surgical pulmonary endarterectomy can be curative.
Quantitative V/Q before lung resection
The same scan, reported quantitatively by lung region, is used to predict what lung function will remain after surgery. Predicted postoperative FEV1 and transfer factor are calculated from the fraction of perfusion contributed by the segments to be removed, and those values drive operability decisions and the extent of resection.6
This is worth understanding in prehabilitation: a patient with poor spirometry may still be operable if the diseased lobe contributes little perfusion — removing it costs less function than the raw numbers suggest. It also identifies the patient whose predicted postoperative values sit near the threshold, for whom exercise testing follows and for whom prehabilitation carries the most weight. See cardiopulmonary exercise testing and complex lung function testing.
What patients should be told
- Two parts, usually within the same visit; total time around 45–60 minutes.
- The ventilation phase involves breathing through a mouthpiece or mask for a few minutes — the part most patients find awkward, and where a short coaching explanation genuinely improves image quality.
- The radiation dose is low, and lower to the breast than CTPA.
- Breastfeeding may require a short interruption — the department will advise.
What this means for physiotherapy
- Know why your patient had a V/Q rather than a CTPA — it usually tells you something about pregnancy, renal function or age.
- Persistent breathlessness months after a PE is a red flag for CTEPH, not a rehabilitation failure.
- Quantitative V/Q explains why one patient with poor spirometry is offered surgery and another is not.
- Coaching the ventilation phase is a small, real contribution to image quality in an anxious or breathless patient.
References & evidence base
- Bajc M, Neilly JB, Miniati M, et al. EANM guidelines for ventilation/perfusion scintigraphy. Eur J Nucl Med Mol Imaging. 2009;36(8):1356–1370.
- PIOPED Investigators. Value of the ventilation/perfusion scan in acute pulmonary embolism. JAMA. 1990;263(20):2753–2759.
- Konstantinides SV, Meyer G, Becattini C, et al. 2019 ESC Guidelines for the diagnosis and management of acute pulmonary embolism. Eur Heart J. 2020;41(4):543–603.
- Humbert M, Kovacs G, Hoeper MM, et al. 2022 ESC/ERS Guidelines for the diagnosis and treatment of pulmonary hypertension. Eur Heart J. 2022;43(38):3618–3731.
- Tunariu N, Gibbs SJR, Win Z, et al. Ventilation–perfusion scintigraphy is more sensitive than multidetector CTPA in detecting chronic thromboembolic pulmonary disease as a treatable cause of pulmonary hypertension. J Nucl Med. 2007;48(5):680–684.
- Brunelli A, Kim AW, Berger KI, Addrizzo-Harris DJ. Physiologic evaluation of the patient with lung cancer being considered for resectional surgery. Chest. 2013;143(5 Suppl):e166S–e190S.
References are numbered in citation order (Vancouver/BMJ style) and were current at the time of writing. Guidelines are living documents — verify against the latest version before clinical use.
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