This article is the tenth in our latest series, The 12 Leads of Christmas, where each day we examine a new finding particular to an individual electrocardiographic lead.
I love V2.
It’s probably been my favorite lead to examine and ponder this past year. The cool thing is that it doesn’t seem all that special way at first. I mean, the precordial leads form what is essentially a smooth sigmoid curve across the chest; what could one lead tell us that’s so unique compared to its neighbors? As it turns out, in the right situation, V2 can hold some surprises.
I see math everywhere. Don’t worry, this image doesn’t have any real purpose; it just looks kinda cool if you’re into logistic functions. Image source.
So what’s so special about V2? Well, despite being commonly depicted as a plain anterior or septal lead, it would be much better described as a “high-anterior” or even “high-antero-lateral” lead. The high-lateral wall of the heart isn’t really well covered by the electrocardiogram so every bit of insight we can get on this area is vital. First though, what is the high-lateral wall?
Most folks reading this article will be familiar with the anterior, septal, lateral, and inferior walls.
This chart is from an old article on our site discussing contiguous leads. I’ve left the former address at the bottom as a testament to how far we’ve come; both in our understanding of electrocardiography and in our design of graphics.
The problem with that classic teaching—well, aside from the fact that our obsession with identifying ST-elevation in “contiguous leads” is outdated and hurts patients—is that it vastly oversimplifies the way the electrocardiographic leads correspond to particular regions of the heart.
First, let’s clarify that for this discussion all we really care about is the left ventricle (LV), not the entire heart. In the structurally normal heart the LV myocardium constitutes the bulk of the QRS-T-complex we see on electrocardiogram, so for now we can ignore the effects of the atria and right ventricle (RV).
The LV is shaped like a rugby ball or American football with one end lopped off. Image source.
It would be nice if the transverse, coronal, and sagittal views of the heart were perpendicular to its axes, similar to the animation above; but nature, while elegant, doesn’t like right-angles.
Instead, the LV is situated at an oblique angle, with the base pointed at roughly the right scapula and the apex towards lead V5.
This doesn’t complicate things too much when we’re dealing with the frontal plane—made up by the limb leads.
Using the above illustration as a guide, I would theoretically re-associate the frontal leads with the regions of the heart as follows (clockwise from top) [these are only my
- (-)aVF: Basal-lateral
- (-)III: High-lateral
- aVL: High-lateral
- I: Low-lateral
- (-)aVR: Low-lateral/Apex
- II: Apex/Infero-apical
- aVF: Infero-apical
- III: Inferior
- (-)aVL: Inferior
- (-): Basal-septal
- aVR: Basal
- (-)II: Basal
Of course, there are still issues with this way of visualizing things.
First, there is the possibility for huge variation in individual anatomy—both how and where the heart sits in the chest and how the coronary arteries are distributed—so it’s not like the above associations are set in stone.
Second, because the heart is a continuous prolate spheroid (rugby ball), each region is contiguous with its neighbor. Add into this the fact that each “region” is somewhat arbitrarily defined and that results in a great deal of overlap.
Third, this perfect coronal cross-section of the heart is completely at odds with how the the field of cardiovascular imaging looks at things. While there are a variety of angles at which differing devices can examine the heart, most do so using the major and minor axes of the heart as references, not the axes of the body. In other words, echocardiography, cardiac MRI, perfusion scans, and other cardiovascular imaging modalities examine the heart using it as their point of reference, while the coronal, sagittal, and transverse planes we typically imagine (and see in non-cardiac CT scans) use the body as the reference and just happen to cut through the heart.
Consensus terminology for the regions of the heart, as correlated with cardiac MRI. These are polar maps of the heart, as if you were viewing the LV point-on from the apex and in-line with its major axis. Source.
Finally, the idea that there is a single electrical center of the heart (where the arrows of the axes cross in the coronal image) is flawed. We can approximate an electrical center, as I have, but because the entirety of the LV doesn’t contract simultaneously (see the asymmetrical motion in the gif earlier in this post), the “electrical center” of the heart actually moves through the course of the cardiac cycle.
Click image to enlarge. It took me forever to find a paper discussing this topic, but here’s the source.
But what does all of that have to do with V2?
“I told you that story to tell you this one,” (a questionable reference at the moment).
As tough as it is to get the frontal leads right in regards to infarct localization, they’re downright simple compared to the precordial leads. You see, the forefathers of electrocardiography had the good sense to give us the limb leads to form a nice, standard coronal plane. Recognizing the need to examine the heart in more than one plane, they later devised the precordial leads to examine the heart in the transverse.
…but they weren’t be content with just looking at a single transverse plane, which is how an engineer would devise the system. Instead, the precordial leads were designed to follow the flow of the heart, from the cephalad base to the more caudal apex. This makes sense and certainly has benefits, but it plays absolute havoc with the field of vector electrocardiography and our ability to visualize how the individual leads relate to different regions of the heart.
What we end up with isn’t actually a plane at all, but rather six separate views of the heart at varying angles with regards to the the “electrical center.” V1/V2 sit at one level, V4–V6 at another, and V3 in the middle (see the first image in this post).
Most illustrations of the transverse plane depict the precordial leads as below:
An incorrect representation of the electrical planes and axes. Image source.
It’s a beautiful image and gets some of the basics right, but there is also a lot wrong with it. [I don't mean to single out this one instance; almost every diagram I've seen performs the same over-simplification.]
As an example of why this format breaks down, consider for a moment inferior STEMI’s due to RCA occlusions (either proximal or distal, it doesn’t matter). They almost always present with an injury vector of 110–120 degrees; meaning the ST elevation is maximal in lead III and essentially points at lead III.
An EKG!? In this blog about EKG’s?? It’s about time. Anyway, most folks would call this an “infero-lateral” STEMI; but why would the lateral wall be involved in an inferior STEMI due to an RCA occlusion? I would term this an infero-apical-septal STEMI, but that’s a discussion for another day.
If that pretty diagram was correct, why on earth would the above EKG also display ST-elevation in V4–V6 (the classic “infero-lateral” STEMI [flawed terminology])? That question bothered me for years until I discovered the work of Dr. Antonio Bayés de Luna and figured out where I was going wrong. [Sadly, I don't have time to directly address this question here, but the simple explanation is that everything you know about leads V4–V6 is wrong.]
So what’s the right way of displaying the precordial leads?
That’s actually a pretty tough feat. A single transverse plane doesn’t do them justice, especially when you consider the moving electrical center of the heart, individual variation in cardiac anatomy and orientation, individual variation in surface landmarks, and technician variability in electrode placement (the precordials are very dependent on exact and consistent placement).
Sadly, given the time constraints of getting this post together, I haven’t been able to put together a great illustration or 3-dimensional model (or find someone who can), but I’ll make it happen someday. This will have to do for now:
All six precordial leads fanning out from the approximate electrical center (purple dot). Original image source [modified].
You can see that all six leads fan out from the heart. Try to visualize an elliptical plane whose edge touches V1 and V4–V6 . Though V3 is a little out-of-plane, V2 is the only lead that really falls out from the rest; it’s directed much more superiorly than the others in that regard.
It’s a very subtle point and hopefully the rest of the post will make it more clear, but it is vital to what makes V2 so unique.
If you were to imagine the LV as a football or rugby ball (or prolate spheroid) again, its major axis is what it spins around when it spirals. The major axis of the LV runs from roughly V4 or V5 and is directed towards the right scapula. With regard to that axis, V2 is the most superior precordial lead we have.
What would happen if we encountered an injury vector that was perpendicular the rough plane formed by V1 and V4–V6; towards the left shoulder and tilted just a bit anterior?
We don’t have to imagine this, however, because it is an electrocardiographic pattern we encounter from time to time during isolated “high-lateral” STEMI.
This area, better described as the basal-antero-lateral territory of the heart, can be perfused by with the first diagonal off the LAD (D1), the ramus intermediate branch off the left main (RI), or an obtuse marginal off the left circumflex (OM1). As a result, nailing down the exact culprit artery is nigh-impossible on the ECG, but when there an isolated occlusion of one of these arteries it can present as an incredibly subtle STEMI.
What you’re looking for is an injury vector directed high and to the left (roughly -60 degrees, but there’s a good deal of wiggle room due to the variable anatomy), which will create subtle ST-elevation in aVL with more pronounced ST-depression in lead III. There may also be elevation in lead I or depression in aVF.
What gets really interesting, and the whole reason we’re having this in-depth discussion, is that precordial leads will all show minimal changes or maybe some ST-depression…
…all except for V2!
It seems like a non-physiological pattern at first, and you might be tempted to think there was a lead switch, but that’s just not the case.
The typical injury vector in isolated “high lateral” STEMI, perpendicular to the plane formed by most of the precordial leads. V2 is the exception. Original image source [modified].
As shown above, most all of the precordial leads are perpendicular to the injury vector seen with this type of STEMI. When a lead is perpendicular to a vector it cannot see it. V2, however, is exceptional because it is just superior enough to the plane that lies perpendicular to the injury vector that it actually manages to “see” a bit of it.
Here are some examples.
Typical STEMI isolated to the “high-lateral” territory. V2 is the only precordial lead showing ST-elevation, while the marked ST-depression in lead III tells us there is ST-elevation pointing away from there—towards the left shoulder.
Here’s the above ECG arranged “360 Degree Heart” fashion.
Here’s another similar ECG. See if you can spot the pattern.
And one more, super subtle tracing…
This one is extremely subtle, even in V2, but it follows the same pattern.
The subtlety of these ECG’s is one of the reasons why this territory of the heart is often considered “electrocardiographically silent.” It’s not silent in the above tracings—it’s whispering “STEMI”—you just need to listen closely.
I hope you’re enjoying our 12 Leads of Christmas series. You can check out the rest of the posts below (updated as new posts come out):
12 Leads of Christmas: Lead I
12 Leads of Christmas: Lead II
12 Leads of Christmas: Lead III
12 Leads of Christmas: aVL
12 Leads of Christmas: aVF
12 Leads of Christmas: aVR
12 Leads of Christmas: V1
12 Leads of Christmas: V3
12 Leads of Christmas: V4
12 Leads of Christmas: V5
12 Leads of Christmas: V6
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