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? It turns out that V2 can hold some surprises.
What’s so special about V2? Well, despite being commonly depicted as a septal lead (for purposes of STEMI localization), it would be much better described as “mid-anterior.” The true mid-anterior territory of the heart isn’t well covered by the electrocardiogram, so every bit of insight we can get there is vital.
First, though, what is the mid-anterior wall?
Most folks reading this article will be familiar with the anterior, septal, lateral, and inferior walls.
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classic teaching—well, aside from the fact that our obsession with identifying ST-elevation in “contiguous leads” is outdated and hurts patients—is that it vastly oversimplifies the way the electrocardiographic leads correspond to particular regions of the heart.
The anterior wall is actually situated quite superior; the lateral wall a bit posterior; the septal wall rather anterior; the posterior wall debated; and the apex ideally lands right around V4, pointing towards V5 (though we all know ideal situations in medicine don’t happen too often). The only lead that actually lives up to its name is the inferior wall, resting on the diaphragm.
Also, let’s clarify that for this discussion all we really care about is the left ventricle (LV)—not the entire heart. The LV myocardium constitutes the bulk of the QRS-T-complex we see on electrocardiogram in the structurally normal heart, so for now we can ignore the effects of the atria and right ventricle (RV).
It would be nice if the transverse, coronal, and sagittal views of the heart were perpendicular to its axes, similar to the animation above.
Unfortunately nature, while elegant, doesn’t like right-angles.
Instead, the LV is situated at an oblique angle, with the base pointed posterior, superior, and to the right—roughly toward the right scapula—and the apex aimed anterior, inferior, and to the left—toward 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, we can play around and theoretically re-associate the frontal leads with the regions of the heart as follows (clockwise from top) [note: these are only my made-up associations for this article—don’t actually use them in practice]:
- (-)aVF: Basal-anterior
- (-)III: Basal-anteror
- aVL: Mid-anterior
- I: Low-lateral
- (-)aVR: Low-lateral/Apical
- II: Apical/Infero-apical
- aVF: Infero-apical/Inferior
- III: Inferior
- (-)aVL: Inferior
- (-)I: 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 (plus, remember, I made them up).
Second, because the heart is a prolate spheroid (shaped like a rugby ball), each region is contiguous with its neighbor, with no clear borders. Add in the fact that each “region” is somewhat arbitrarily defined, and the results in a great deal of overlap between territories.
Third, this perfect coronal cross-section misses a lot of regions and is completely at odds with how the the field of cardiovascular imaging looks at the heart. While there are a variety of viewpoints different devices use when examining the heart, most do so using the major and minor axes of the heart as references, not the axes of the entire body. In other words, echocardiography, cardiac MRI, perfusion scans, and other cardiovascular imaging modalities examine the heart using the organ itself 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.
ECGs use the external body for all of our landmarks and points-of-reference, which can lead to confusion. For example, the “lateral” free wall of the heart (when examined with echo/MRI/etc…) is actually the electrocardiographic “posterior” wall. [more on that topic here]
Finally, the idea that there is a single electrical center of the heart (where the arrows of the axes cross in the coronal illustration) is flawed. We can approximate an electrical center, as I have, but because the entirety of the LV doesn’t contract simultaneously (see the asymmetric motion in the gif earlier in this post), the “electrical center” of the heart actually moves through the course of the cardiac cycle.
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 limb leads right with STEMI localization, they’re downright simple compared to the precordial leads. You see, the forefathers of electrocardiography had the good sense to arrange 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 look at it in the transverse.
. . . but they weren’t content with just looking at a single transverse plane, which is how an engineer would devise the system. Instead, the precordial leads were arranged 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 respect to the the “electrical center.” V1/V2 sit at one horizontal level; V4–V6 at another; and V3 in the middle (see the first image in this post).
Further confusing the matter, most illustrations of the transverse plane depict the precordial leads as below:
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 100–120 degrees; meaning the ST elevation is maximal in lead III and essentially points at lead III.
If that pretty diagram was correct, why on earth would the above EKG also display ST-elevation in V4–V6 (classically called an “infero-lateral” STEMI [flawed terminology])?
V4-V6 are located on the anterior and lateral chest, but don’t really look at the true lateral wall—doing so would actually require “posterior” leads on the back of the thorax. That question bothered me for years until I discovered the work of folks like Dr. William J. Hurst and Dr. Antonio Bayés de Luna, and figured out where I was going wrong. I don’t have time to directly address the issue here, but the simple explanation is that everything we were initially taught about leads V4–V6 is wrong. Those leads are located below the electrical center of the heart, in a certain sense making them “inferior” leads, which is one of the reasons why we commonly see a decent amount of elevation there with inferior STEMIs—even when the lateral wall is spared on perfusion imaging.
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 posted, I haven’t been able to put together a great illustration or three-dimensional model (or find someone who can), but I’ll make it happen someday. This will have to do for now: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 sticks out from the rest; it’s significantly superior compared to others.
It’s a very subtle point that hopefully the rest of the post will make it more clear, but it is vital to what makes V2 so unique.
If we imagine the LV as a football or rugby ball (or “prolate spheroid”), 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.
Consider now what would happen if we encountered an injury vector that was perpendicular to the rough plane formed by V1 and V4–V6; pointed towards the left shoulder.
We don’t have to imagine, however, because it is an electrocardiographic pattern we encounter from time to time during isolated high-lateral STEMI. Well, it used to be called “isolated high lateral STEMI,” because aVL is erroneously called a “high lateral” lead, but really it affects what we know of as the mid-anterior wall. [It can also affect the basal-anterior wall, among other neighboring regions, but we’ll simply focus on the mid-anterior wall.]
The mid-anterior wall is commonly perfused by the first diagonal (D1) off the LAD , the ramus intermediate (RI) branch off the left main, or the first obtuse marginal (OM1) off the left circumflex (LCx). As a result, nailing down the exact culprit artery is nigh-impossible on the ECG, but when there is an isolated occlusion of one of these arteries, it can present as an incredibly subtle STEMI with a prototypical pattern.
(Of note, this form of “mid-anterior” infarction should not be confused with a mid-LAD occlusion, which causes MI of the antero-apical and apical-septal walls, often along with the true apex.)
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 and maximal elevation in (-)III, which we see as pronounced ST-depression in regular-old 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 electrode switch or misplacement, but that’s not the case.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 the injury vector.
Here are some examples. Pay close attention to V2 and aVL.
Here’s the above ECG arranged “360 Degree Heart” fashion. The injury vector points directly away from lead III, so we see marked ST-depression there. aVL, being a bit off-axis, catches only a hint of the ST-elevation.
And one more super-subtle tracing…
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.
last updated 2019.01.15
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