Seeing the clot
The workhorse test is CT pulmonary angiography (CTPA). A contrast dye is timed to fill the pulmonary arteries just as the scanner sweeps through, so a clot shows up as a dark gap inside a bright vessel. CTPA is fast, widely available, and shows the clot's location directly — it can spot a saddle embolus or trace clots down into smaller branches.
CTPA needs iodine contrast, which is a problem for people with poor kidney function, contrast allergy, or pregnancy concerns. For them, the older ventilation–perfusion scan (V/Q scan) is the alternative. It maps where air goes versus where blood goes; a region that is ventilated but not perfused is the signature of a clot — the same mismatch you met in guide one, now used as a diagnostic clue.
How dangerous, right now?
Size on the scan matters less than how the heart is coping. When clots block enough of the lung's vessels, the right ventricle must suddenly push against high resistance. If it cannot keep up, the result is right ventricular strain — visible on the CT as an enlarged right ventricle, on an ultrasound as a struggling right heart, and in the blood as leaked cardiac markers. A patient whose blood pressure drops because of this is having a massive (high-risk) PE — an emergency.
- Stable patient, no strain — treat with anticoagulation alone and watch.
- Stable but with right-heart strain — anticoagulate and monitor closely, as the situation can shift.
- Unstable (low blood pressure) — this high-risk PE may need clot-dissolving drugs or clot removal, on top of anticoagulation.
The logic of anticoagulation
Anticoagulation does not dissolve the existing clot. Instead, it stops new clot from forming and lets the body's own machinery break the old one down over weeks. This is why starting it promptly matters more than its speed of effect: it prevents the next embolus, which is the one that might be fatal.
Treatment usually lasts at least three months. If the clot followed a clear, temporary trigger (like surgery), it can often stop after that. If it came out of nowhere or keeps recurring, longer or indefinite treatment is weighed against the bleeding risk it carries. Every decision balances clotting risk against bleeding risk — there is no zero-risk option, only the better trade-off.