The 360 Degree Heart – Part II

The first post in our “360 Degree Heart” series attempted to visualize how the different frontal plane (limb) leads relate to one another. We also introduced the concept of “negative leads,” which are just the standard leads flipped upside down. If you didn’t read that post it would probably be helpful to start there.

This week we’re going to review another cases that emphasize how the simple concepts behind the “360 Degree Heart” can improve your EKG reading.

As luck would have it, I stumbled upon the perfect addition to this series soon after writing up Part I. Many thanks to Rom Duckworth for letting me use this EKG in our little educational endeavor. He’s the editorial director at Rescue Digest, plus he’s got some great slide-sets up for browsing on SlideShare (which is where this particular tracing caught my eye).

 

This ECG shows acute postero-lateral STEMI

This ECG shows a subtle acute postero-lateral STEMI

 

One glance at this ECG tells me that this patient has an acute obstruction of their left circumflex coronary artery (LCx). Though this does not meet the formal, oft-maligned “millimeter criteria” of 1 mm or more in two contiguous leads, this is a legitimate STEMI-equivalent. There is acute occlusion or near-occlusion of a major coronary artery and this patient requires immediate intervention for the best chance at an optimal outcome.

How can I say this with such confidence? Perhaps you noticed the ST-depression from V2-V4, in this setting diagnostic of posterior MI. That’s great (and one of my favorite ECG findings), but we can further seal the diagnosis  and show a greater region of myocardium is at-risk by examining the limb leads closely.

 

Subtle ST-elevation confined to lead aVL.

Subtle ST-elevation confined to lead aVL.

 

There is significant ST-elevation in only one lead, aVL, with ST-depression in II, III, and aVF. This exposes one of the reasons why “high lateral” MI due to LCx lesions are the most “electrocardiographically silent” infarctions: Unlike the inferior and lateral walls, the EKG does not directly examine the high lateral wall. Most “high lateral” MI’s show their maximal ST-elevation in the -45 to -90 degree range, but aVL, residing at -30 degrees, is the only standard lead that comes close.
180 Degree ECG

 

Instead, let’s consider this tracing in the “360 degree” format.

360 Degree ECGIt is now evident that there is indeed ST-elevation “contiguous” to what we originally noted in aVL, we just weren’t set up to see it! The ST-elevation is actually maximal in leads (-)III and (-)aVF, extending a bit into (-)II.

Now, any time you have ST-depression on the ECG there is going to be ST-elevation in the opposite direction, it’s just that this particular pattern of ST-elevation is highly correlated with occlusion of the LCx. If the ST-elevation was more evident further “rightward,” towards aVR, then we would be dealing with a pattern of subendocardial ischemia (NSTEMI).

Which brings up one last topic I would like to touch upon: the ST-vector.

It is clear from these demonstrations that the ST-segment, when it is deviated, has a particular “direction.” It is angled towards some leads and away from others. In inferior STEMI it is directed towards leads III and aVF. In anterior STEMI it is directed towards V2-V4. These are the very basic concepts we use to “localize” a STEMI.

In this case the ST-segments are deviated towards (-)III and (-)aVF and away from (+)III and (+)aVF.

We can simplify this phenomenon and represent the general distribution of the ST-deviation as a single arrow, representing the “ST-vector,” shown below.

ST-Vector

The arrow points towards the point of maximal ST-elevation and away from the point of maximal ST-depression.

I don’t want you to dwell on this idea or worry if it’s not immediately clear, I just want you to be aware that the concept of the ST-vector exists. I will be introducing it much more thoroughly in the future.

5 Comments

  • Bryan says:

    I don’t understand why (-)III and aVL are be labeled -60 and -30 degrees instead of 300 and 330 degrees?

    • It is standard practice in electrocardiography to label the first 90 degrees counter-clockwise from “zero” that way. When you see a patient with “left axis deviation” you’ll see that their measured QRS axis is somewhere between -30 and -90 degrees.

      Imagine if you saw someone with a mean QRS axis at 5 degrees. Now imagine they changed positions slightly in the bed, shifting their cardiac axis ten degrees “leftward” (aka counter-clockwise). Now we would say their mean axis is located at (-)5 degrees, but the labeling you describe would instead call that 355 degrees. A shift from (+)5 to (-)5 is a lot easier to work with than a jump from 5 to 355.

      Also, it makes common math easier. If I said someone started with a QRS axis of (+)5 and then shifted 10 degrees leftward, you just subtract 10 from 5 and get (-)5. We do most of our work in electrocardiography in the -60 to +120 degree range, so using this standard allows us to keep “zero” where it has always been without introducing a giant jump in the numbers. It’s not mathematically pure but it makes 99% of what we do easier.

  • Steve says:

    Nice explanation, Vince. It’s also not uncommon to have the ST vector towards aVL and thus have zero ST depression in lead II, but ST elevation in I and aVL. In such a case, of course, the ST elevation is easier to see than the ST depression in III. In your case, the ST depression in III is the most visible ECG abnormality.

  • Nice case Vince (!) – as you lay the ground for your “pet concept” of the vector approach to ST elevation. That said, for those vectorially-challenged readers (like myself) – I offer the following 2 concepts:

    i) This is not diffuse subendocardial ischemia because the ST depression is not “diffuse”. Instead – there is no ST depression in V5,V6 – and if anything ST elevation in I, aVL.

    ii) One should not miss the subtle-but-real ST elevation in lead aVL. In that context – there is clearly mirror-image ST depression in each of the inferior leads (esp. III, aVF) – and that of itself is diagnostic of an acute STEMI.

    Whether you call this a “STEMI” or not (because only 1 contiguous lead has ST elevation) is academic. It acts like a STEMI – most probably due to acute occlusion of the Obtuse Marginal branch of the Circumflex artery. NICE case! I’ll look forward to more of your posts on the ST vector with acute stemi.

    • Thanks for the kind words! I agree that the diagnosis can (and should) be made here without any of my special tricks. Rather than convert the world to thinking in vectors or make it seem like they’re at all necessary to making a diagnosis, my main goal is to introduce these kinds of concepts so that folks gain a greater appreciation for how the ECG leads relate to one-another. Hopefully it’s clear that the “360 Degree Heart” contains no special information not already presented on the standard 12-lead, I’m just arranging it a bit differently.

      I’m not saying this for your benefit so much as anyone who happens to stumble upon these comments. I know you understand what I’m up to. 🙂

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