This is the discussion for 63 year old male CC: Substernal Chest Pain, brought to you by Dr. Brooks Walsh of Mill Hill Avenue Command and Doc Cottle's Desk. Be sure to review the first part!

When we last left off, our patient was a 63 year old male complaining of substernal chest pain which awoke him from sleep. We obtained standard, right-sided, and posterior ECG's which cardiology deemed non-specific. Our patient continued to have persistent chest pain even after maximal therapy.

Coronary catheterization was performed later that morning, and a total occlusion of the first obtuse marginal (OM1), a branch of the circumflex artery, was found. A drug eluting stent was placed, and his subsequent hospital course was unremarkable.

Could we have guessed this from the start? Let's take a look at his ECG's again:

Reviewing our patient's first ECG we see horizontal ST segment depression of 1 mm was seen in II and aVF, 3 mm ST depression in V2, and 1 mm ST depression in V3 and V4, all with upright T waves. An early R-wave transition was noted, with a height of 13 mV and an R/S of 1 in V2. Lead III showed T wave inversion.

The right-sided ECG featured only ST segment depression and inverted T-waves.

Reviewing the posterior leads, we see some interesting changes. Notably ST segment elevation of 0.5 mm in V7 and V8, and an unchanged appearance of the limb leads.

So, Dr. Walsh, what can we take away from this case?

Devoted readers of EMS 12-Lead will already know much about posterior MI, so I'll just do a brief review of the basics. I encourage you to search for the tag "posterior STEMI" for more information. As always, Dr. Stephen Smith's website also has plenty of examples and teaching about posterior MIs.

There is a small amount of controversy about what anatomic area of the heart a "posterior" MI actually affects. Some have suggested that the infarct is actually in the infero-basal region, while others have evidence that it should be viewed as a lateral infarct.

Regardless of the name, however, it generally is the result of an occlusion of the left circumflex coronary artery (link to cool animation). This territory is generally regarded as "electrically silent" in the standard ECG leads. As such, diagnosis relies on indirect evidence in the standard leads, as well as the use of non-standard "posterior" ECG leads.

So, how can we be Posterior STEMI Rock Stars, Doc?

For years, the standard teaching on identifying a posterior MI has emphasized some common elements. Brady summarized the most important of these:

• Horizontal ST depression in V1-V4
• Tall, broad R waves (>30ms)
• Upright T waves
• Dominant R wave (R/S ratio > 1) in V2

So, a typical posterior STEMI looks like this:

Inferioposterior STEMI courtesy of LifeInTheFastLane.

Note the ST elevations in the inferior and lateral leads; in general, a posterior MI usually shows signs of a STEMI in either of these two regions the majority of the time. It's pretty obvious that evidence of STEMI in one area (e.g. inferior) certainly strengthens suspicion for a posterior MI. 

However, here's an example of a posterior MI that does not show any inferior or lateral involvement:

Posterior STEMI without inferiolateral changes. (PubMed)

Using posterior leads (V7-V9) can show ST elevation, but the magnitude may not be as dramatic as that seen in the "usual" STEMIs. Many experts consider, for example, 0.5 mm of elevation to be significant, instead of the usual > 1 mm criteria, and that elevation in just one lead is sufficient.

So what's wrong with the conventional thinking?

There's a short-cut way to diagnose a posterior MI, that involves "flipping" the ECG. The idea is that the ST depression in the anterior leads is a "mirror" view of ST elevation in the posterior wall, and that the tall R-waves are actually deep Q-waves.

For example,  a blow-up of lead V2 from the isolated posterior MI above (the second ECG) looks like this:

It fulfills all the criteria I reviewed above, so it's a classic example of how we currently define a posterior MI. And if we "flip" it, we get this:

Yep Doc, that looks like a regular STEMI now!

For many people (MDs included!), the diagnosis of a posterior MI starts, and ends, with this flipping.

I'll tell you what has always bothered me though. Look at the R-wave in the un-flipped image. This is just the mirror image of the Q-wave, and our "flipped" image reinforces that. In fact, the Q-wave in our flipped image looks pretty darn old, like the MI has progressed far along already. 

Furthermore, the T-wave in our "flipped" image hardly looks hyper-acute. In fact, it looks like the T-waves are in the process of returning to baseline, another indication that our "classic" posterior MI is old. 

Here's an example of an subacute/old inferior MI, for comparison. Also note the resolving, partially inverted, T-waves in III and aVF:

Old Inferior MI courtesy of LifeInTheFastLane.

It looks instead like our description of posterior MI is training us to look for old, completed MIs! 

Let's look at this from another angle. Look at this typical inferior STEMI, and focus on lead aVF:

That's a classic STEMI, no doubt. Tiny Q-waves, tall T-wave – everything suggests that this is very acute.

Now, instead of being lead aVF, let's pretend it were lead V9. What would the "flipped" view of this lead be? Let's flip it and see!

This view shows a small R-wave, and a fully inverted and deep T-wave. Now, if we take a look back at the criteria listed above for a posterior MI, however, it would not meet much of the description of a posterior MI we listed before.

We're teaching people to pick up on old posterior MIs, and training them to miss the acute presentations.

Interesting, so how can we avoid this, Doc?

Well, I may have had this thought kicking around in my head, but some cardiologists actually wrote down these thoughts! Birnbaum et al. just published the article Common pitfalls in the interpretation of electrocardiograms from patients with acute coronary syndromes with narrow QRS: a consensus report. This wasn't a prospective controlled trial, but they managed to get 13 cardiologists to agree on a few things. 

One of those was that our standard description of the morphology of posterior MI on the ECG is likely wrong. Helpfully, they proceed to describe what we ought to be looking for!

After reviewing the usual evolution of LCx-occlusion MI, they conclude that the standard definition, that relies on tall R-waves and upright T-waves is: 

… the late “mirror image” of fully evolved ST-segment MI (STEMI) (Q waves with terminal T-wave inversion) and not the acute phase of STEMI. 

By contrast, they give an example of what they consider to be an acute posterior STEMI:

If we blow up lead V2…

…we see that there is a small R-wave, and fully inverted T-waves, and an R/S < 1. It meets none of Brady's criteria above. But if we flip it…

It looks like a regular ol' STEMI!

Bring it home for us, doc.

The usual description of posterior MI may be in need of revision. The posterior leads, however, remain useful to reveal acute cardiac ischemia.

Thank you again to Dr. Brooks Walsh for this case and his insight into posterior STEMI!

• Did you catch the posterior STEMI?
• Does your service allow you to activate a posterior STEMI?