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