Wednesday, August 12, 2015

What is the culprit vessel?

A patient presented with chest pain and had this ECG:
There is obvious ischemic ST elevation with reciprocal ST depression.
What is the culprit artery?

A high lateral STEMI was diagnosed and the patient was taken to the cath lab.  

The interventionalist was surprised to find that the infarct artery was the RCA.

This was sent to me for explanation.  What is the explanation?

If you look at the QRS, you see that there are well formed R-waves in V5 and V6, but a Q-wave in aVL and very low voltage R-wave in lead I.  All are lateral leads and usually have similar QRS findings (though not always because V5 and V6 are more inferior than I and aVL).  The difference in this case is too large to be due to this normal variation.

So I stated that the leads were misplaced and to send me an ECG with the leads properly placed, if one was available.

Fortunately, there had been a second ECG recorded slighly later.   It was available and sent:
Now the R-wave is clearly positive in all lateral leads.  Thus, in this ECG, leads were properly placed.  And the ST elevation is inferior, with reciprocal ST depression in I and aVL.

Learning Point:

1. Try to assess the QRS before assessing the ST segments and T-waves.  Our eyes are always drawn to the ST segments first.

This is similar to proper technique of reading an X-Ray.  One is always drawn to the pathology, or the pathology one is seeking.  If you look elsewhere first to find any incidental or related findings, you will not forget to look for pathology.  But if you look for pathology first, you will forget to scrutinize the remainder of the exam and will miss important findings.

This behavior also leads to the misdiagnosis of PseudoSTEMI patterns as STEMI patterns: many PseudoSTEMI patterns are abnormal repolarization (ST-T abnormalities) that are due to abnormal depolarization (abnormal QRS).  If you look at the QRS first, you will not miss the LVH or RVH or bundle branch block or Brugada or hyperK.  Then when you see the ST elevation you will be prepared to know that it could be secondary to an abnormal QRS (not primary due to ischemia).


  1. Thank you for the case and the great discussion. What was the lead inversion? Was it Left Arm and Left Leg?

    1. Julio,
      I don't know for certain, but I suspect right arm/left arm because the axis inversion is right/left, not inferior/superior.
      Steve Smith

    2. Based on the 2 ECGs I would rather go for the LA/LL reversal hypothesis:
      - from first ECG to second one, lead I QRS transitions into lead II QRS and vice-versa
      - lead III QRS transitions into negative lead III, i.e -III, which means that the voltage difference between LA lead and LL was preserved at least in terms of absolute value of voltage amplitude.

  2. Left arm was switched with left leg (aVL becomes aVF and Lead I becomes Lead II).

  3. Steve,

    Excellent case. As far as the lead inversion, I believe Julio is right.

    As I go through it
    1. lead I looks different in the 2 ECG's, so one of the arm leads was moved.
    2. aVR looks the same in both ECG's so the right arm lead did not move
    3. So the left arm lead must have been swapped, but where?

    Most likely the left arm lead was swapped with left leg. In this way:
    I is actually II
    II is I
    III is -III
    aVL is aVF

    I had to draw a few pictures to work this out. Please check me to see if this makes sense.

    Thanks as always,

  4. I would also suspect left arm/left leg revearsal since that would make aVL and aVF to switch places as well as lead I and II while lead III gets inverted and aVR remains unchanged. A rather unusual limb lead switch! An LA/RA switch (which is fairly common) would not affect aVF.

  5. Great call, Steve
    Also interesting in this case is the shape of the ST segment and T wave in V2 and V3, which resembles DE WINTER T WAVES, dont you think, Smith ?
    And the rhythm seems to be atrial at the beginning, with 2 degree mobitz 1 block, then a junctional escape, and then the sinus rhythm apears

    1. Rodrigo,
      It can be difficult to distinguish posterior MI that has just reperfused (ST depression, upright T-waves) from De Winter's T-waves.
      In this case, the ST segments and T-waves in III and aVL are in opposite directions, indicating some reperfusion of the inferior wall (and thus the artery). Thus, there is probably posterior reperfusion as well, which results in some persistent ST depression (reciprocal to the posterior wall) and an upright T-wave. See my posts on "posterior reperfusion T-waves".


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