POCUS Atlas: Pulmonary Embolism

Positive Findings (Patient Has This)

Finding	Increased Disease Probability (Positive Likelihood Ratio)
Goal-directed RV dysfunction Echo score ≥1	90
Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Negative Findings (Patient Doesn't Have This)

Finding	Decreased Disease Probability (Negative Likelihood Ratio)
Goal-directed RV dysfunction Echo score ≥1	0.0
Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Source

Weekes AJ, Thacker G, Troha D, et al. Diagnostic Accuracy of Right Ventricular Dysfunction Markers in Normotensive Emergency Department Patients With Acute Pulmonary Embolism. Ann Emerg Med. 2016;68(3):277-91.

Narrative

This was a prospective observational study (n=116) of consecutive normotensive patients with confirmed pulmonary embolism, assessing the diagnostic accuracy of biomarkers, CT, and goal-directed echocardiography for right ventricular dysfunction. Emergency physicians, blinded to clot burden and biomarkers, performed qualitative goal-directed echocardiography for right ventricular (RV) dysfunction*: RV enlargement (RV diameter greater than or equal to that of the left ventricle), severe RV systolic dysfunction (RV free wall hypokinesis or TAPSE < 1.0 cm), and/or interventricular septum flattening or bowing into the left ventricle. If any one of these were present, right ventricular dysfunction was diagnosed. Goal-directed echocardiography results were compared to comprehensive echocardiography as the gold-standard.

Caveats

Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Published in collaboration with The POCUS Atlas

Author

Matthew Riscinti, MD

Published/Updated

September 13, 2018

What are Likelihood Ratios?

LR, pretest probability and posttest (or posterior) probability are daunting terms that describe simple concepts that we all intuitively understand.

Let's start with pretest probability: that's just a fancy term for my initial impression, before we perform whatever test it is that we're going to use.

For example, a patient with prior stents comes in sweating and clutching his chest in agony, I have a pretty high suspicion that he's having an MI – let's say, 60%. That is my pretest probability.

He immediately gets an ECG (known here as the "test") showing an obvious STEMI.

Now, I know there are some STEMI mimics, so I'm not quite 100%, but based on my experience I'm 99.5% sure that he's having an MI right now. This is my posttest probability - the new impression I have that the patient has the disease after we did our test.

And likelihood ration? That's just the name for the statistical tool that converted the pretest probability to the posttest probability - it's just a mathematical description of the strength of that test.

Using an online calculator, that means the LR+ that got me from 60% to 99.5% is 145, which is about as high an LR you can get (and the actual LR for an emergency physician who thinks an ECG shows an obvious STEMI).

(Thank you to Seth Trueger, MD for this explanation!)