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Tuesday, May 13, 2014

New thoughts on posterior MI for EMS

Many savvy medics will check the "extra" ECG leads V7 - V9 to look for a posterior MI. However, this isn't always necessary, since the appearance of leads V1 - V3 will often show sufficient evidence of an acute MI. The only two problems here are
  1. Many medics don't know the classic criteria for posterior AMI.
  2. The classic criteria may need to be changed somewhat!
I wrote an article about this topic a few years ago in a post at EMS 12-Lead.  A newly published case report got me thinking about this again, though! 

(Feel free to check out that link for a longer discussion.)

The Case Report
In the article "Acute Coronary Ischemia Identified by EMS Providers in a Standing Middle-aged Male with Atypical Symptoms," the authors describe the case of apatient who had syncope, followed by cardiac arrest. After the patient had ROSC, they obtained an ECG:

SOURCE
They described this pattern of anterior ST depression only as "anterolateral ischemia," but could this really be a posterior STEMI?

ECG findings - The old thinking
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 classic posterior MI should look something like:

Problems with the old thinking?
The short-cut way to diagnose a posterior MI 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,  when we take the ECG above, and "flip" leads V1 - V3, it now looks like a standard STEMI.

LEFT: Unflipped - just boring ST depression
RIGHT: Flipping reveals an exciting STEMI. It's Magic!

So, our "classic" posterior, when it is flipped, looks like a STEMI. 

There is one problem though. The flipped ECG shows a big Q-wave, and the T-wave has started to invert. Usually, these findings aren't found in the early , acute stages of a STEMI.

Evolution of ECG in STEMI (source)
Instead, this pattern of Q-waves and T-wave inversion suggests an AMI that has been progressing for a few hours.

ECG findings - The new thinking
This problem - that a classic posterior STEMI looks like a subacute or old MI - was described by the authors of Common pitfalls in the interpretation of electrocardiograms from patients with acute coronary syndromes with narrow QRS: a consensus report. These 13 cardiologists agreed that the old definition of posterior MI, that relies on tall R-waves and upright T-waves in leads V1 - V4, describes
"... 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."
They do not propose a better definition of a posterior STEMI ECG pattern. They do, however, offer this example of an ECG that better illustrates a truly acute posterior STEMI, resulting from a left circumflex occlusion. Note the ST depression in V1 - V3, and no significant R-waves or T-waves.



Back to the case report!
The ECG from the case at the start of this post showed ST depression in V1 - V4, but only small R-waves, and only a hint of an upright T-wave:


This ECG does not fulfill the "Brady" criteria listed above, but if we "flip" the ECG, we see that...

LEFT: Zoom on V1-V3
RIGHT: Flipped!
... we indeed have a classic, acute-looking STEMI! And in line with this interpretation, the patient was found to have a complete occlusion of the circumflex.

The Bottom Line
We don't have a good "new" definition of posterior STEMI that is based on interpretation of the anterior leads, but it appears that the "old" definition has shortcomings. Hopefully, future research will clarify the best ways to discern a posterior MI on the standard 12-lead ECG.





4 comments:

  1. Given the normal V6 and no ST-E in aVR in the initial ECG, I find it hard to believe that ECG shows "antero-lateral ischemia". Actually, I always find it hard to believe when someone purports to localize ischemia (I was once on that fool's errand).

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    1. Right! Common, and persistent, misunderstanding.

      By contrast, I recall from somewhere in Marriott's Practical Electrocardiography that you can use T wave inversion to localize ischemia, e.g. during stress tests. This subject, however, is not in my bailiwick!

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  2. Nice tracings and good concepts Brooks! That said - "plus ça change, plus c'est la même chose" (= the more things change - the more they remain the same). Tall anterior R waves were never intended to be a criterion for "acute" posterior MI. The biphasic T waves in your 2nd tracing (of "classic" post MI) - is not what I conceive of as classic "acute" posterior MI for precisely the reasons you state (the terminal positive portion of the T wave implies you are no longer in the "earliest stage" because this corresponds to the stage of T inversion while the ST segment starts coming down when flipped in a mirror). Instead - your last ECG (showing just V1,V2,V3 upright and then flipped) is a "classic" picture of acute posterior MI - with the "mirror test" being all that is needed for those not yet able to instantly recognize this pattern with leads upright. Posterior leads are NOT needed (in my opinion) when the picture looks like this. My impression is that this describes things as they have always been for as Iong as I've been interpreting.

    As to "localizing ischemia" - stress testing does NOT do this. First - the patient is upright during a treadmill test - which changes orientation to some extent. My impression is that we pick up subendocardial ischemia - with the BEST lead on simple ETT being lead V5. Victor Froelicher I believe was among the first to show that isolated inferior ST depression on ETT during exercise had very poor specificity for a positive test .... (but specificity goes way up if you develop both inferior AND lateral precordial ST depression). Anterior ST changes on ETT are far less often seen .... The "money" is in leads V4,V5,V6 ... with Bottom Line that ETT can't be used to localize ischemia. On the other hand - I remain under the deception that moderate-to-deep symmetric T inversion that is localized to a specific lead area on a resting 12-lead ECG does indeed correlate with area of ischemia.

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    1. Ken, I very much appreciate your perspective here!

      Unfortunately, the requirement for tall R waves and biphasic/upright T waves is enshrined in many sources that EM and EMS use for ECG education.

      For example, Life in the Fast Lane, Academic Life in Emergency Medicine, and Amal Mattu all repeat the same definition as Brady does. My "classic posterior" ECG example was taken from a cardiology paper on STEMI-equivalents.

      So, where did the erroneous definition of acute posterior MI come from then? I suspect that the older literature, based in the antiquated language of "Q-wave MIs" is to blame.

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