Sunday, February 21, 2016

In my words: Diagnosing a PE

I was reading about Pulmonary Embolism for USMLE Step 2 CK and started typing notes for myself to refer. I thought I'd upload it for you guys to read as well.

Disclaimer: I am talking to myself in the blog. I hope it makes sense.

I'll be talking about two scenarios -
High pretest probability scenario: Dude who sat in a 24 hour flight, smokes a lot, is taking OCP's, got his hip replaced and can't move at all.
Low pretest probability scenario: Dude who is dyspneic, hypoxic and has a normal chest x-ray.

Let's start with scenario #1 - The high pretest probability:

If there's a high index of suspicion and a treatment option is given in the options, choose the treatment one because you don't want to waste time in imaging.

If there's a high index of suspicion and a treatment option is not given in the options, they want you to choose a diagnostic modality. The "next best step" in the diagnosis. What do you choose?

CT angiography also known as spiral or helical CT (CTA)!

Why? Good question!
- It can visualize small branch artery emboli.
- Chest CT has the additional benefit of visualizing other abnormalities such as pneumonia, aortic abnormalities, or pulmonary masses that may not have been apparent on routine chest radiograph, and which may provide an alternative diagnosis for the patient's symptomatology.

Soo.. It's an awesome test. Do it first.

(Note to self: Why not d-dimer? Because it doesn't tell you shit when the pretest probability is high. Positive? Can be positive in a lot of conditions. Negative? It still can be a PE.)

If the dude in your question has a contradiction to CT angiography (Dude is allergic or has renal failure)... What do you do?

A V/Q scan. Why?
A high probability scan is sufficient to make a diagnosis.
A normal scan excludes PE.

If the CT and/or V/Q scan are nondiagnostic, but you still think it's a PE, now what?

A lower extremity venous ultrasound (LE US) demonstrating an acute DVT in a patient with signs and symptoms of PE would be sufficient to diagnose and treat PE (especially since the treatment with anticoagulation is the same). A normal ultrasound does not exclude the diagnosis since most patients with PE do not have evidence of residual DVT because the clot has already embolized.

So why didn't we get the gold standard pulmonary angiogram? Why go for a stupid LE US which wouldn't tell us anything if it was normal?

Because LE US is *ahem* non invasive. You could die during a pulmonary angiogram. kthxbye.

Anyway, so this is your diagnostic work up for a patient who you REALLY think has a PE and you wanna confirm the diagnosis (Because the exam wants you to.)
CTA --> Can't do CTA, do V/Q scan --> Negative or indeterminate --> Damn, what next? LE US! --> Negative --> Pulmonary angiogram.

Now.. Scenario #2 - Low pretest probability:

What do you do?



A negative d-dimer rules out a PE.

Now, the dude for which you had low index of  suspicion had a positive d-dimer. What do you do?


A normal scan would exclude the diagnosis of PE in a patient with low pretest probability.

To make your life difficult, you got a CT scan that is suggestive but not clearly diagnostic of embolism :/

Now what?

Test further! Go for the V/Q scan!

A normal perfusion scan excludes the diagnosis of PE with enough certainty that further diagnostic testing is unnecessary. Also, a low-probability scan with a low clinical suspicion for PE excludes the diagnosis.


Important take home points:
1. CTA is the initial test of choice for patients with a high pretest probability for PE and no contraindications.
2. V/Q scan is useful in risk-stratifying renal failure, allergic to contrast and possibly pregnant patients with suspected PE.
3. D-dimer study is useful for its negative predictive value in excluding DVT and PE.


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