The Breathless 34-Week Pregnant Woman with Suspected PE

This is a haemodynamically stable pregnant patient with a non-diagnostic D-dimer and a normal chest radiograph, so the question is which test gives a definitive answer with the least maternal and fetal risk.

• First-line: bilateral lower-limb compression ultrasound. It is radiation-free, quick, and if it shows a proximal DVT, treatment is started and no chest imaging is needed. A negative result does not exclude PE, so it must not be the end of the pathway.
• If the leg ultrasound is negative, proceed to imaging. In most centres, CT pulmonary angiography (CTPA) is preferred in the second and third trimester because it is more often diagnostic, has fewer non-diagnostic studies than ventilation-perfusion (V/Q) scanning, and is more readily available out of hours. V/Q SPECT (or planar V/Q) is a reasonable alternative where available, particularly if there is a history of contrast allergy or renal impairment, and it delivers a lower maternal breast dose.
• Do not wait 3 hours for a "convenient" slot. The patient is symptomatic with tachycardia and borderline saturations; delay is a risk. Escalate to the on-call consultant and radiology superintendent to expedite.
• Do not withhold the scan because of pregnancy. The maternal mortality of untreated PE far outweighs the theoretical fetal risk from a single CTPA.
• Avoid D-dimer as a rule-out in the third trimester - it is often raised physiologically and here is above the gestational cut-off anyway, so it is non-discriminatory.

Technique:

• Use a pregnancy-adapted CTPA protocol: narrow collimation, high pitch, automated exposure control, and a reduced z-axis coverage from the lung apices to the costophrenic angles (not a routine chest/abdomen).
• Use a reduced contrast volume (e.g. 50–70 mL of a low-osmolar agent) with a saline chaser and bolus tracking on the pulmonary trunk.
• Shield the abdomen with a lead apron if local protocol permits (the evidence is weak but it is commonly done and reduces scatter).
• Aim for a fetal dose of well under 1 mGy; a well-optimised CTPA typically delivers <0.5 mGy to the fetus, comparable to V/Q.

Counselling:

• Explain that the fetal absorbed dose is very low and well below the 100 mGy threshold at which deterministic effects occur; the risk is stochastic and theoretical.
• Explain the maternal risk of missing a PE: untreated PE carries a mortality of around 15–30%, falling to <1% with anticoagulation.
• Document the discussion, including that the decision was made jointly with the obstetric and emergency teams, and that the patient agreed.
• Note that the iodine contrast is safe in pregnancy; there is no need for thyroid blocking in the fetus, though neonatal thyroid function can be checked if there is concern.

A negative CTPA in a pregnant patient with a good-quality study effectively excludes PE, and further imaging is not routinely required. However, several caveats apply here:

• Check study quality. If the pulmonary arteries are not opacified to the subsegmental level, or there is motion artefact, the study is non-diagnostic and a V/Q scan or repeat CTPA should be considered.
• Consider alternative diagnoses. Normal CXR, tachycardia and hypoxia in a pregnant woman also raise the possibility of pneumonia, pulmonary oedema, or, less commonly, amniotic fluid embolism or peripartum cardiomyopathy. A bedside echocardiogram may be useful.
• Reassess clinically. If symptoms persist or worsen, escalate to a V/Q scan or repeat imaging rather than simply discharging.
• Anticoagulation: if PE is excluded, stop therapeutic anticoagulation but consider prophylactic low-molecular-weight heparin given the ongoing risk factors (pregnancy, immobility, recent admission) until delivery, in consultation with obstetrics.
• Document the negative result and the plan in the notes, and ensure the patient is reviewed by the obstetric team before discharge.