Understanding  the 12 Lead ECG!

Whether you work within a coronary care unit or a general surgical unit, we have all performed a 12 lead electrocardiogram (ECG) on one of our patients at some point or another. We’ve prepped the chest of a patient that could rival some of the sheep shearing competitions that occur in New Zealand. We’ve tried untangling the mess of leads while secretly imagining the face of the colleague that left the ECG machine in such disarray on the dart board of our local bar. We’ve all had to deal with that ONE patient who cannot comprehend what it means to “lie still and quietly”. And after all of that effort to get that perfect print out of an ECG, wouldn’t it be nice to understand what those 12 leads are actually telling you? A picture paints a thousand words, so let’s start with the placement of the leads when performing a 12 lead ECG:

When you connect a patient to an ECG machine as per the images above, it generates something that looks like this:

It is important to note that the lead II at the bottom of the ECG is the same as the lead II to the middle left of the ECG above; it is just extended for the duration of the print out. We tend to use this lead for rhythm analysis, but can ignore it for the purposes of this article and focus just on smaller stretch of lead II located to the middle left of the ECG above.

Unless I have to go back to primary school and re-learn how to add up; V1, V2, V3, V4, V5, V6, RA, LA, RL and LL is equal to 10 leads. So why is it called a 12 lead ECG; where do the additional 2 leads comes from? Let’s start with the method of elimination…

Leads V1 – V6 are easy to identify on the 12 lead ECG; they are labelled for you! So which labels on the 12 lead ECG correspond with the RA, LA, RL and LL leads? The easiest way to remember it is like this:

aVR = Right arm (RA)

aVL = Left arm (LA)

aVF = Foot (this gives you a 50% chance of guessing whether this “foot” lead refers to the left or the right leg; the heart is on the left side of the heart, so that makes the left leg (LL) the important one that comes up on the 12 lead ECG)

That leaves the right leg (RL) lead which does not actually come up on the 12 lead ECG; it merely acts as a grounding lead to minimise any interference.

So we’ve accounted for the 10 leads that we have connected to the patient, understanding that only 9 of these leads actually show up on the 12 lead ECG because RL is merely a grounding lead. So that leaves the burning question; where do leads I, II and III come from?

To answer this question, we have to go back to 1901 to when a Dutch doctor named Willem Einthoven invented the first practical ECG. Like us, he was able to figure out which leads on the patient corresponded to V1 – V6, aVR, aVL and aVF. But it took him years before he figured out where leads I, II and III came from (he pretty much ignored these leads for a period of time). When he figured out the theory behind where these leads came from, he named it Einthoven’s triangle. Brilliant man, just not very original! 

Einthoven’s triangle works on the principle that leads RA, LA and LL not only record the electrical activity of the heart in relation to themselves via aVR, aVL and aVF respectively; they also correspond with each other to form leads I, II and III.

The RA lead speaks to the LA lead to form lead I, the RA lead also speaks to the LL lead to form lead II, and the LL speaks to the LA lead to form lead III. Notice in the image to the left below, the lines that form a triangle from these relationships do not actually intersect the heart. But the heart is the most important thing when we are doing an ECG. Therefore, it makes sense for these lines to intersect the heart! Notice in the image to the right below, the lines have maintained the same angles as the image to the left, but have moved to intersect the middle of the heart. This intersection point is known as the central terminal of the heart.

The central terminal of the heart is a very important concept; it is where all the leads on an ECG look towards. What do I mean by that? Consider the positions of leads V1 – V6 on the chest wall. From each of their positions, they are trying to “look” at the central terminal of the heart. But these leads are not like superman, they do not have x-ray vision. Therefore, they can only “see” the surface of the heart that their line of vision first comes in contact with. So imagine lead aVR sitting on your right shoulder trying to “look” at the central terminal of your heart, and the first surface area of your heart that it is going to “see” in that line of vision. Now any electrical activity within the heart is going to be recorded on the aVR lead from that viewing position on the surface of the heart. This makes it easy to understand where V1 – V6, aVR, aVL and aVF record electrical activity from. If you are confused, just hang in there through a little bit more confusion as there is a brilliant image coming up that will make it all crystal clear!

Leads I, II and III are a little trickier because they are each derived from two leads, creating what we know as a positive and negative pole. Because we should all have a POSITIVE outlook on life, we view the heart from the POSITIVE pole. The image above to the right clearly shows how each of these leads are trying to “look” at the central terminal, but “see” a particular surface of the heart in their line of sight first.

It can get confusing when we use 2 dimensional diagrams to explain Einthoven’s triangle, because the heart is a 3 dimensional structure. So something like aVF is actually looking from the left foot directly up to the heart, meaning that it sees the under side (or inferior) portion of the heart. And something like aVL is looking from the left arm to the left side (or lateral) portion of the heart. While something like V2 that sits on the chest wall, looks directly at the front side (or anterior) portion of the heart. The image below by Associate Professor Tor Ercleve depicts how the various leads look at various portions of the heart perfectly:

When teaching this in person, I usually bruise myself from energetically pointing at various points on my body to try and explain that the lead is looking “THROUGH here to see whatever surface of the heart it first comes in contact with trying to find the central terminal of the heart”. Unfortunately, I don’t have the luxury of doing that in this article so I hope that this brilliant image has helped you visualise which part of the heart each of the 12 leads on an ECG are actually looking at.

Looking at all the colourful shaded areas that correspond with a viewing lead, it is clear to see that we have great view of a lot of different areas of the heart. So now if there are any deviations from normal in a particular lead (let’s say ischaemic changes from insufficient blood flow secondary to a blocked coronary artery), it allows us to narrow it down to the specific area of the heart that is being affected. All you really need to do is superimpose one image over the other image below to narrow down which coronary artery is the culprit and get in there and fix it! So for instance, if you have ischaemic changes in leads V2, V3, V4…it is most likely the left anterior descending (LAD) artery that has been affected.

It’s all well and good to understand the theory behind how 10 leads can give us a 12 lead ECG, and where all of those 12 leads are looking in relation to the various areas of the heart. But it needs to relate to practice. So how do we pick which lead is which when we see a 12 lead ECG on paper, without randomly pointing at yourself while standing at the nurses station trying to remember which lead is looking where…

aVR tends to look at the right top part of the heart where the right atrium and superior vena cava resides. It does have it’s diagnostic purposes, but we tend to leave it out of lead groupings. Lead grouping V1 to V6 are easy because they sit directly on the front of the chest and therefore look directly at the front of the heart, therefore being the anterior leads. As V5 and V6 start to curve around to the side of the body, they also give us a bit of a lateral view mixed with an anterior view. Therefore, these two are known as our antero-lateral leads. Notice how leads II, III and aVF are at the bottom of your ECG and form the shape of a boot? Seeing as your boot goes on your foot and that is the most inferior portion of your body, the leads that form the boot are your inferior leads! Then the two leads left over that are lateral to each other are leads I and aVL, simple method of elimination!

What I suggest to people that want to try to commit this to memory is this: start grouping all of the leads on every 12 lead ECG you come into contact with. Anterior, inferior, lateral…anterior, inferior, lateral…anterior, inferior, lateral! There is a reason why we remember the lyrics to songs we haven’t heard for years from the 80’s and 90’s, repetition from singing it over and over again at some point in our lives! I hope this has helped demystify the 12 lead ECG for you a little bit more and tune in next time to learn about the ECG changes associated with myocardial ischaemia and infarction.