The 360 Degree Heart – Part I

The hexaxial reference system.

If I asked you to imagine how the limb leads “look” at the heart you would probably picture something like the image below:

Heart drawing courtesy of Dawn Altman from the ECG Guru website.

Notice those gaps in the limb leads? They don’t really exist; they’re an illusion.

This isn’t something that is commonly emphasized when the cardiac axis is being taught but it’s absolutely vital to realize that in addition to each “positive” lead seen above, there is a corresponding “negative” lead in the opposite direction. These “negative” leads, which I denote with a (-) symbol, are literally nothing more than the original lead from the standard 12-lead ECG flipped upside down.

Mapped on the above hexaxial reference system we now have the coverage shown below:

Negatives + Segments

 

Now, in theory,  we have a full 360-degree view of the heart in the frontal plane, but how does this work in real life? Consider the 12-lead ECG below, showing infero-posterior STEMI:

Infero-posterior STEMI

Infero-posterior STEMI

 

All we really care about for this discussion is the limb leads so let’s focus on them and ignore the rest of the tracing:

1202 - EKG Crop

 

This is what it would look like if we arranged the limb leads as displayed in the first diagram:

1202 - Basic + EKG

 

Notice how there’s a gap between leads I and II? Also, aVR seems way out in right field, not connected to any of the other leads. Now we’re going to include the negative versions of each limb lead:

1202 - Full + EKG

Quite literally all I did was flip each complex displayed above so that it was the exact negative of what we originally saw. Then I displayed the inverted complex 180 degrees (directly opposite) its original position.

The above representation contains no more information that we see with the six standard limb leads, but it looks like we’ve doubled the amount of leads available. This is part of the beauty of the hexaxial reference system that doesn’t get emphasized enough.

Take a few minutes to appreciate how there is now a nice, smooth progression of the QRS complexes as you travel clockwise around the circle. Really, please do this:

Starting arbitrarily at lead I (it helps that it’s the most positive QRS complex), follow the circle clockwise. Appreciate how the QRS complex decreases in size as you go from (-)aVR to II to aVF to III to (-)aVL. Then, when you reach (-)I, the QRS is now at its most negative point. Continuing clockwise, the QRS complex now begins to gain amplitude until you end up back at lead I again.

Next, investigate the T-wave. Begin this time at lead III, where it is at its tallest. Like the QRS complex, as you travel clockwise the T-wave becomes more and more inverted until reaching its most negative point at (-)III. Continuing clockwise, it begins to gain size again until we arrive back at lead III.

Finally, can you guess how the ST-segment behaves?

Since this is a classic inferior STEMI from an RCA occlusion, the ST-elevation is maximal in lead III. Since it doesn’t matter which direction we go, let’s go counter-clockwise this time, starting again at lead III. Follow as the amount of ST-elevation decreases across leads aVF, II, and (-)aVR and turns into ST-depression. This is how reciprocal ST-depression works! It’s not due to “lateral ischemia,” but rather just the mirror image of the ST-elevation centered around lead III.

While we’re at it, can you appreciate why aVL is the best lead for seeing reciprocal changes in inferior STEMI? In reality (-)III shows more ST-depression, but on the standard 12-lead aVL is the only lead that comes close to approximating (-)III, so that’s why we always look there for reciprocal ST/T-wave changes.

As a final exercise, see if you can appreciate how even the P-wave shows the same circular evolution.

I’ll be posting more on this topic over the coming weeks, but I just wanted to offer an initial understanding of what I’ll be getting into. I hope this helps, and if you have any questions please let me know in the comments.

 

 

 

 

14 Comments

  • Ken Grauer says:

    Georgeous figure you drew based on the simple (often neglected) concept of just flipping hexaxial leads and then adding them at their appropriate negative lead spot on the 360 degree continuum. That said I’m not yet sold that doing so is needed to optimize information obtained from 12-lead assessment. But I’ll look forward to next discussion on the topic! – 🙂

    • Thanks for the kind words! This example is more about introducing folks to the concepts behind why the EKG axes work and how the different leads related to one another. It’s a simple idea, but on the occasions it is actually taught it’s often done so during the early stages of a course; and thus isn’t understood, internalized, or applied in a manner that’s remembered or made practical.

      You’ve read my hand, however, and Part II does feature a nice (but rare) case where I believe “thinking upside-down” (or “thinking in reciprocals”) makes life slightly easier. I agree it’s not a necessary part of reading EKG’s, but I like to do it and few other people currently offer teaching on the subject of vectors so I’m going to attempt to make that my niche for a bit.

      Our readers keep getting smarter and smarter so it’s tough to keep ahead of them while still keeping things clinically relevant…

  • Jon Kavanagh says:

    I love the concept–too often ECG instruction and interpretation are done in vacuums, with no correlation to what is being seen; it can be hard to correlate the tracing to the physiological processes, and vice versa–this concept may help fill in the gaps. Axis and morphology are topics that could take a whole block of time to cover adequately, but too many programs gloss over the “whys” of ECGs.

    I look forward to Part II to make this make more sense from an application point of view.

  • In reality, the only leads which are recorded are leads I and II. All the others are calculated! So there have to be reciprocal changes!

  • Bryan Stottlemyer says:

    As a very graphic, visual, and logical learner, I appreciate that this is “shows” the brain how to follow (or unfollow) the vector or axis to find and/or confirm changes in ECG. Very nicely done.

  • Joseph Goodson says:

    As a new Paramedic, this information is very useful and helpful. I believe there is more to understanding paramedicine than what can be taught in a class and it takes the initiative of the medic to pursue. Thank you for providing this simple yet understandable concept. Honestly, it never dawned on me how the hexaxial reference system could be utilized. Thank you and I look forward to upcoming posts.

  • Alex says:

    Some models of ECG machines have an option to flip aVR upside down. In this case it looks at +30* and fills in the gap between I and II.

  • Keren Levi says:

    Lately, a few paramedic-students arrived at my station.
    So i tried to explain that basics at my best simpliest way.
    After readinv your perfectly coherent “article”, i couldnt stop smily for knowing we both thought of same phrases and associatives words.
    For me it is a great compliment!
    Thanks for writing!
    Cant wait for part 2!!

    B.t.w- i loved one of your presentation about AMI mimics, but unfortunately cannot find it any more.
    Why so?

  • PO J says:

    In Sweden we have used the Cabrera lead system for decades.
    https://en.m.wikipedia.org/wiki/Hexaxial_reference_system

  • Andrew Bowman says:

    Beautiful, just beautiful.

  • Dave Richley says:

    A beautiful illustration of an important concept that is not widely understood. I try to teach the concept but never thought of doing it this way – can I use your diagram, Vince?

  • yazan says:

    thanks you add a lot with these figures

6 Trackbacks

Leave a Reply

Your email address will not be published. Required fields are marked *