Rule of Porportionality

Check out this 12-lead.

What do you think?

Okay, fair enough, you need a story.  Let’s say the best story possible: pale, clammy diaphoretic, crushing chest pain of sudden onset, history of coronary artery disease.

What do you do if you are the medic?

Call in a STEMI alert?

Call in a possible STEMI alert?

Don’t call in a STEMI alert, because it is not a STEMI?

I have showed this strip to many doctors and paramedics  and the general consensus is it is a nondiagnostic 12-lead (insufficient ST elevation in V1, V2 and V3), and absent a recent 12-lead, they would only send the patient to the cath lab with a really good story.

What was the bottom line?

The hero medic (damn, not me) called it a STEMI and the patient who went to the cath lab based on story, not so much on the 12-Lead, had a 100% occlusion of the LAD.

Look again at the 12-lead.  How can you reconcile the 12-lead with the final diagnosis?

If you are a reader of the EMS 12-Lead blog you are likely jumping up and down, waving your arm in the air, trying to get me to call on you.  If you have read that blog, you likely have the answer.  It is called the law of porportionality.

I read about it recently on the site and it came to mind in the later discussions of this 12-lead.

Here is what Tom Bothhillet, the guru behind the EMS 12-Lead blog, has to say about the law of porportionaility when discussing similar ECGs:

“Our threshold for ST/T wave abnormalities is lower when the QRS complex is smaller. In other words, we don’t necessarily need 1 (or 2) mm of ST-elevation to be significant, especially when the QRS complex is < 5 mm in amplitude….We…need to be suspicious when the T-waves appear disproportionately large for the size of the QRS complex….The T-waves …don’t look particularly large, but they are disproportionately large when one considers the relatively small size of the QRS complexes. This is referred to as the “rule of proportionality” which states that repolarization (the ST/T-waves) should be proportional to depolarization (the QRS complex).
 – Tom Bouthillet, EMS-12 Lead

So with that in mind,  let’s look at V3 in particular.

 Using a powerpoint to stretch the ECG vertically, we get a better sense of the porportion between the QRS and the ST segment.

The ST segment is disporpotionately large compared to the QRS.

This is something valuable to keep in mind, when the QRS is small in leads V2 and V3.  You may be able to see an anterior STEMI that others may miss.

This tip and many others are available to readers of the EMS 12-lead blog.

If you really want to improve your 12-lead understanding, EMS 12-Lead is must reading.  Bouthillet also offers an excellent web cast called STEMI Recognition: Beyond the Basics, which is available on his site.


  • Many years ago one of the my favorite medical directors told me that a lot of what we would be learning about MIs in the future would come from pre hospital ECGs. His theory was that most of the time it is the paramedics that see the patient much earlier in the evolution of the MI and that we’d see things on the ECG that normally weren’t seen in the ED.

    I think that this is probably a good example of that. I don’t know that I’d have called a STEMI in this case, but I would certainly call the hospital to tell them that it was ACS. STEMIs often evolve rapidly and a subsequent 12 Lead, either in the field or in the ED could show changes not seen earlier.

    All too often, it’s easy to forget that medical problems evolve, often rapidly. Which is why we’re taught to reassess patients often.

  • medicscribe says:

    Great comment. Very true. Thanks.