Wednesday, September 21, 2022

Posterior leads can give false reassurance

A 40-something woman with diabetes and peripheral vascular disease who frequently needs the ED for chronic pain called 911 for sudden severe chest pain.  The patient was very agitated and could not hold still.

I greeted medics at the door to view the prehospital ECG.

Here it is:

It is not normal, but there is no specific evidence of Occlusion MI (OMI)
There are some Q-waves in inferior and lateral leads (of uncertain age) and there is some "terminal QRS distortion" in inferior leads and V4-V6, but this finding may be present normally in these leads (it is NOT normal in lead V2/V3 in the presence of normal variant ST elevation in V2-V4)

The patient was quite agitated and needed sedation.

Because of this, it was very difficult to record an ED ECG, but eventually it became possible:

What do you think?

There is NEW very subtle ST depression in V3 and V4, and new subtle ST Elevation in V5 and V6.  

There was an old ECG available from 15 months prior:
This shows no STE and no STD.  In fact, as in most patients, there is a touch of normal STE in V3 and V4.  The presenting ECG has a touch of STD in V3 and V4.  
This is diagnostic of Occlusion MI (OMI).

I diagnosed Occlusion MI (OMI) and activated the cath lab.

Before going to cath, we recorded a posterior ECG:
Lead V4-V6 have been moved to the posterior thorax.  
Thus, they have much less voltage.  They have ZERO ST Elevation.  However, you can see that V3 still has ST Depression.  
Does this mean that the ST depression in V3 represents "anterior" subendocardial ischemia, and not posterior OMI?  
That is what many would have you believe.

This did NOT dissuade me in the least!

Does ST depression in V3 diagnose OMI of the posterior wall better than the posterior leads?  It may indeed.

She went to the cath lab and had a 100% Obtuse Marginal Occlusion (supplying the lateral and posterior wall) with TIMI-0 flow.

High Sensitivity Troponin I (Abbott Architect: URL = 16 ng/L for women, 34 for men)
Initial: 26 ng/L
Peak: greater than 50,000 ng/L.  A massive acute OMI.


The estimated left ventricular ejection fraction is 34%

Regional wall motion abnormality-lateral, akinetic.

Regional wall motion abnormality-inferior base (this is the posterior wall).

This is most consistent with ischemia/infarction in the distribution of

the left circumflex coronary artery.

Next Day ECG:
T waves in V2, V3 are larger than they were at baseline. This may be due to "posterior reperfusion T-waves", maybe not.  If it was, I would usually expect reperfusion T-wave inversion in V5 and V6 also.

Learning Points.

1. We have shown that ANY ST depression maximal in V1-V4 is 96% specific for posterior OMI.

Meyers, Bracey, Smith, et al.  Journal of the American Heart Association.  Ischemic ST depression maximal in V1-V4 (vs. V5-V6) of any amplitude, is specific for Occlusion Myocardial Infarction (vs. non-occlusive ischemia)

2. Posterior leads are unnecessary in this situation and although they could be helpful, there is a risk that they will dissuade you from making the accurate diagnosis of posterior OMI.

3.  OMI that are not STEMI can be very subtle and difficult to diagnose even though the findings are very specific.  

4.  In many such cases, it is not the ECG which is nonspecific; it is the interpreter who is nonspecific.


MY Comment, by KEN GRAUER, MD (9/21/2022):


I love cases that support my beliefs — which is why I couldn’t wait to write up my comment on this most recent post by Dr. Smith. The message conveyed by Dr. Smith is simple: — Posterior leads sometimes provide false reassurance. As occurred in today’s case — there are times when despite definitive recognition of posterior OMI by other means — no ST elevation at all will be seen in posterior leads.
  • Failure of posterior leads to consistently demonstrate ST elevation in association with subtle posterior OMI — should not be surprising. This is because posterior placement of leads V7, V8 and V9 situates these leads in a position from which electrical activity must pass through the thick musculature of the back before being recorded on the ECG. As a result — even under optimal circumstances, QRST amplitudes (and therefore the amount of ST-T wave elevation) in posterior leads is often modest.

To facilitate instant recognition of posterior OMI — I favor the MirrorTest, which I first popularized nearly 4 decades ago (circa 1983, in my 1st ECG publication).
  • In the years since I first proposed this visual aid — I have used it to identify many hundreds of posterior infarctions within seconds — and without need to delay management (or transport of the patient) in order to record additional (ie, posterior) leads. The standard 12-lead ECG is all that is needed.

What is the “Mirror” Test?
The "Mirror" Test is nothing more than a visual aid to facilitate recognition of the abnormal shape of ST depression seen in one or more of the anterior leads in association with acute posterior infarction.
  • Figure-1 — shows the rational for the Mirror Test (ie, the anterior leads provide a mirror image of electrical activity in the posterior wall). By simply inverting a standard 12-lead ECG, and then holding it up to the light — you can easily visualize the "mirror-image" of leads V1-thru-V4.
  • With a little bit of practice — it becomes EASY to recognize the "shelf-like" (flat) shape of ST depression — that looks like a stemi when this image is inverted.
  • Posterior infarctions often (though not always) result in an increase in R wave amplitude in anterior leads. The taller anterior R waves become — the more this looks like a deepening Q wave when the image is inverted.

  • KEY Point: If instead of seeing a comparable diffuse amount of ST depression in chest (and limb) leads — you see maximal ST depression in leads V2, V3 and/or V4THINK posterior infarction! Inverting the image (ie, applying the Mirror Test) then facilitates recognizing the characteristic ST-T wave depression shape indicative of posterior infarction.

  • NOTE: Because you are not having to traverse the thick back musculature to record a standard ECG (as you have to do when recording posterior leads) — the relative amplitude of ST-T wave segment deviations tends to be significantly larger than the ST-T wave amplitude seen with posterior leads. This is why I believe the Mirror Test is superior to use of posterior leads.

  • EDITORIAL Note: I do not believe I have ever seen a case in which a posterior infarction diagnosed by posterior leads was not evident by use of the Mirror Test on the standard 12-lead ECG. But as Dr. Smith illustrates in today's case — the opposite is not true (ie, Today's patient was correctly diagnosed on ECG as having posterior OMI despite the complete lack of ST elevation in posterior leads).

Figure-1: Illustration of the rational for the Mirror Test (Figure excerpted from Grauer K: ECG-2014 Pocket Brain ePub).

Applying the Mirror Test to Today's Case:
To illustrate my use of the Mirror Test in today's case — I've reproduced in Figure-2 the initial ECG obtained in the ED (which is the 2nd ECG shown above by Dr. Smith— and a prior baseline tracing on this patient (which is the 3rd ECG shown above).
  • Of NOTE  The fairly large Q waves seen in the inferolateral leads in ECG #2 are not new! Instead — comparable Q waves were seen in the prior baseline ECG, suggesting this patient had a prior infarction.

  • What is new  is lateral ST elevation, which is marked in lateral chest leads V5,V6 — and which is subtle-but-present in high-lateral leads I and aVL. This lateral ST elevation was not present on the earlier tracing.

  • What is also new — is subtle-but-real "shelf-like" ST depression that is maximal in leads V3 and V4 (and hinted at in lead V2)
  • QUESTION: Isn't it EASIER in the mirror-image (inverted) views of leads V2,V3,V4 to appreciate that this shape of anterior ST depression in this patient with new-onset chest pain is diagnostic of acute posterior OMI (ie, This is a positive Mirror Test).

  • To Emphasize — Comparison of ECG #2 with the prior baseline tracing confirms that the ST depression in ECG #2 is clearly new! But before I looked at the prior tracing — I already knew from the positive Mirror Test that the shelf-like (flattened) shape of the depressed ST segments in leads V3 and V4 was clearly abnormal — and indicative of acute posterior OMI until proven otherwise.
  • Although ST-T wave changes in lead V2 are much more subtle than what we see in leads V3 and V4 — given that the T wave is typically upright in lead V2 and the ST segment is usually sightly elevated in this lead as a normal finding — I knew that in a patient with new chest pain, the ST-T wave appearance in lead V2 was not normal.

  • Putting It All Together — As stated, the inferior Q waves (and inferior infarction) in ECG #2 is old. But the combination of lateral lead ST elevation + the positive Mirror Test in leads V2,V3,V4 — indicated acute posterolateral OMI, which strongly suggested the "culprit" artery to be a branch of the LCx (Left Circumflex). This was confirmed on cath.

Figure-2: Comparison in today's case of the initial ECG in the ED ( = ECG #2) — with a prior baseline tracing ( = ECG #3) on this patient obtained 15 months earlier. The inverted (mirror-image) views of leads V2,V3,V4 in ECG #2 constitute a positive Mirror Test — that in this patient with severe, new-onset chest pain — is diagnostic of acute posterior OMI (See text).


Selected LINKS for More on the Mirror Test:

  • ECG Blog #246 — Reviews the concept of the "Mirror Test" with a clinical example.

  • The February 10, 2022 post in Dr. Smith's ECG Blog — My Comment (at the bottom of the page) illustrates the Mirror Test in a case with posterior reperfusion waves (ie, tall anterior T waves).

The following posts in Dr. Smith's ECG Blog provide additional examples in My Comment (at the bottom of the page) that illustrate application of the Mirror Test for diagnosis of acute Posterior MI.
There are many more examples of posterior OMIs with positive Mirror Tests sprinkled throughout Dr. Smith's ECG Blog ...

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