Suspected Pulmonary Embolism at 26 Weeks' Gestation: Choosing Between CTPA and V/Q
A 29-year-old woman, gravida 2 para 1, at 26 weeks' gestation presents with 2 hours of sudden dyspnoea and pleuritic chest pain. Heart rate is 118, blood pressure 108/66 mmHg, respiratory rate 26, oxygen saturation 93% on room air, temperature 36.8 degrees Celsius. Chest radiograph is normal. Compression ultrasound of both legs is negative. D-dimer is 1,900 ng/mL and she knows that pregnancy raises it. The Wells score is 6. She has no prior thrombosis, no bleeding history and normal renal function. The on-call resident proposes a CT pulmonary angiogram, but the patient's midwife has told her that CT 'will irradiate the baby' and asks for a V/Q scan instead. The department has SPECT V/Q available and can run a low-dose CTPA protocol.
1. Compare CTPA and V/Q scintigraphy in this patient with approximate dose figures and state which you would choose and why.
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Both tests are far below the 50-100 mGy threshold at which deterministic fetal effects occur, and neither should be withheld.
Fetal dose:
- CTPA: approximately 0.1-0.5 mGy, almost entirely from scatter; the fetus is outside the primary beam.
- Perfusion-only scintigraphy with technetium-99m MAA: approximately 0.1-0.2 mGy; SPECT V/Q is somewhat higher.
Maternal dose, which is often the more important consideration:
- CTPA gives a maternal breast dose in the region of 10-30 mGy, and the breast is among the most radiosensitive tissues in a woman of this age. This translates into a small but real increase in lifetime breast cancer risk, greatest in younger women.
- V/Q gives a breast dose of around 1 mGy or less.
Diagnostic performance and practicality:
- CTPA is generally more available, faster, and provides an alternative diagnosis; it has a higher rate of technically inadequate studies in pregnancy if the bolus is poor.
- V/Q has a higher non-diagnostic rate unless the chest radiograph is normal. Here it is normal, so a perfusion-only study is legitimate, but she has no prior study for comparison.
Choice: Either is defensible in a normal-chest-radiograph patient. Given the availability of a low-dose CTPA protocol and the need for a definitive answer in a haemodynamically borderline patient, CTPA with a reduced-dose protocol is a sound first-line choice; a perfusion-only or SPECT V/Q is an equally valid alternative and would be preferred if the chest radiograph were abnormal it would be the opposite - abnormal radiograph makes V/Q unreliable and CTPA the clear choice.
2. What protocol modifications would you use, and how would you counsel the patient about fetal and maternal risk?
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CTPA modifications:
- Reduce tube voltage to 80-100 kVp with automatic exposure control and iterative reconstruction; high-pitch or prospective triggering where available.
- Limit z-axis coverage to the pulmonary arteries rather than the whole chest.
- Use a compact contrast bolus (roughly 50-70 mL) with saline chaser and bolus tracking; avoid a repeat acquisition by getting the timing right first time.
- Do not apply abdominal shielding: it does not meaningfully reduce fetal dose from internal scatter and can increase tube output and effective dose by forcing the system to compensate.
- Iodinated contrast is not associated with fetal thyroid dysfunction; breastfeeding can continue normally.
Perfusion scintigraphy modifications:
- Reduce the administered activity of technetium-99m MAA to the region of 40-80 MBq and use a limited view set; combine with a low-dose ventilation phase only if needed, and omit if the chest radiograph is normal.
Counselling:
- Quote approximate figures: CTPA fetal dose under 1 mGy, equivalent to a few weeks of background radiation; both options are more than 100 times below any threshold for fetal malformation or pregnancy loss.
- Be frank about the maternal breast dose difference and that it is a small, not a zero, risk, and that untreated pulmonary embolism carries a mortality of roughly 15-30%, falling to around 1% with anticoagulation.
- Explain that the D-dimer cannot be used to rule out PE in pregnancy and that the decision is between imaging tests, not between imaging and observation.
3. The CTPA is technically adequate and negative, but she remains tachycardic at 116 with a persistently elevated D-dimer. What is your next move?
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A technically adequate negative CTPA excludes clinically significant pulmonary embolism, so the next move is to question the study's quality and then to look elsewhere, rather than to repeat imaging reflexively.
First, re-review the images:
- Confirm adequate opacification of the pulmonary arteries to the segmental and subsegmental level, no significant motion, and that the bolus was not so dense that it obscured a filling defect.
- Subsegmental emboli are the usual site of disagreement; a second read by a thoracic or general radiologist is worthwhile.
Then consider alternatives:
- Echocardiography for right ventricular strain, tricuspid regurgitation and pulmonary pressures; a raised troponin or BNP with strain would support a diagnosis of PE even with a negative CTPA and would change management.
- Bilateral leg ultrasound again if the first was early; the diagnosis may be deep vein thrombosis rather than PE.
- Consider a V/Q scan only if the CTPA is genuinely equivocal or non-diagnostic - not after a clear negative study.
- Look actively for non-thrombotic causes: pneumonia, aspiration, chorioamnionitis or endometritis, pulmonary oedema, peripartum cardiomyopathy, pneumothorax, sickle cell crisis, or anxiety with hyperventilation.
Clinically:
- Continue fetal monitoring and maternal oxygen therapy to keep saturation above 95%.
- Document the reasoning and the discussion with the obstetric team; if clinical suspicion remains high and there is right heart strain, a multidisciplinary discussion about empirical anticoagulation is reasonable, weighing bleeding risk against the consequences of untreated thromboembolism.