Pulmonary embolism: answer these three key questions with chest CT

In this video, you'll learn how to use CT findings to answer three critical questions that determine the next clinical steps for a patient suspected of having PE.

Brian F. Mullan, MD MMEd MS FACR FCCP
Brian F. Mullan, MD MMEd MS FACR FCCP
9th Aug 2021 • 3m read

Do you know what to look for in a CT report if you suspect PE? Many clinicians don't. In this video, you'll learn how to use CT findings to answer three critical questions that determine the next clinical steps for a patient suspected of having PE.

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Video transcript

A lot has been written about the CT evaluation of pulmonary emboli, or PE. In this Medmastery lesson, I have chosen to focus more on what the clinician should look for in a CT report, if there's a suspicion of PE. Working with my clinical colleagues, I found that there are three basic questions that a radiologist needs to answer when looking at the CT image.

Are emboli present? Or is something else present that could explain the patient's symptoms? If emboli are present, are they sufficient to explain the patient's symptoms? Or, are they incidental with something else probably causing the symptoms? If they are present, and are the probable cause of the symptoms, will anticoagulation be sufficient, or a more aggressive therapy be needed to remove the clot?

The answer to these questions comes from a combination of CT and clinical findings. Let's start with the first question. Are emboli present? CT can answer this yes, no question with high accuracy. That's the beauty of CT. We simply look for a filling defect surrounded by white contrast. On to the second question, are they observed emboli sufficient to explain the patient's symptoms?

On CT, we often find incidental PE's in people who are scanned for non PE related reasons, such as a follow up scan after a bout of pneumonia. But, do these small PE's matter? As CT technology improves, we were able to see more than we used to. When multislice CT came out. It was better than the single slice CT.

It was able to see the small vessels better, there was greater interobserver agreement and it found more small PE's. But despite finding and treating the smaller PE's, clinical outcomes didn't change. This makes physiologic sense. We know that our bodies routinely produce small clots and response to the normal wear and tear of daily life.

The lungs act as a filter for the small clots, preventing them from circulating to the brain and elsewhere. That we can now see the small emboli may be more a result of improved CT technology than anything else. It is my opinion, and that of many thoracic radiologists and pulmonologist, that these incidental PE's are probably not clinically relevant.

So when we see a small PE in a symptomatic patient, it strikes me as unlikely to be the cause of the symptoms, even when there's no other cause evident. Furthermore, when another pathology is present, along with a small PE, such as pneumonia, or pulmonary edema, it is more likely that the other pathology accounts for the symptoms.

The small PE is probably incidental to the symptoms rather than the cause. Finally, the third question, will anticoagulation suffice, or will a more aggressive therapy be needed? To answer this, CT provides three pieces of information. First, it localizes the emboli to the central or peripheral pulmonary arteries.

This is important in that large central emboli, such as this one can break off, migrate distally and cause sudden occlusions. Second, CT can evaluate the morphology of the right ventricle, or RV, relative to the left ventricle, or LV, as an indication of right heart strain due to the PE. In a normal sized RV, the ratio of RV diameter to LV diameter is less than 0.9.

This enlarged RV, the RV to LV diameter ratio is clearly greater than 0.9. This is a sign of right heart strain. Third, CT can evaluate for other pathology that may limit the patient to physiologic reserve, even in the face of fairly minor embolic disease. This can include diseases affecting lung parenchyma, such as emphysema, heart failure, pleural disease, and others.

So, if a person has a large central embolus, or signs of RV strain, or pathology that limits physiologic reserve, the clinician might want to consider more aggressive therapy. Lastly, if the patient has the added complexity of pregnancy in the mix, there are additional considerations. We will discuss those in the handbook.