How 10 Electrodes Give Us a 12-lead ECG
A simple explanation of how the 12 leads of a transthoracic ECG are created.
The foundation of ECG leads
Imagine you're preparing a patient for an ECG. You place 4 electrodes on their limbs - one on each arm and one on each leg - and 6 more across their chest. These electrodes are your eyes into the electrical universe of the heart.
But here's the catch: you're aiming to get 12 different perspectives (leads) with only 10 electrodes. How does that work?
(This is a concept that I struggled with when I first started learning ECG interpretation. To be honest, I don’t think that textbooks (at least the ones I read) did their best to explain it clearly and simply. So this is what I’ll try to do here.)
Let’s start with understanding how a single lead is created first.
How a single lead is created
To form a single lead, you need two electrodes: one serves as the positive pole, and the other as the negative.
For instance, in creating Lead I, you would place electrodes on the right arm (negative pole) and the left arm (positive pole).
The ECG machine then measures the electrical activity between these two points, giving you one lead that you can look at. This principle is how we obtain the standard bipolar limb leads, and it's the foundation for understanding more complex lead configurations in ECG interpretation.
The right leg (RL) electrode
First of all, the right leg electrode acts as an earth and is not involved in any measurements.
So you’re left with 9 electrodes (3 limb electrodes and the 6 chest electrodes), and you know that for each lead, you need a pair of electrodes.
So, how’s it possible to end up with a 12-lead ECG?
The bipolar limb leads: I, II, and III
With the remaining 3 limb electrodes, we create the bipolar limb leads. As mentioned in their name, these leads actually utilise two electrodes (poles - hence, “bipolar”) to measure electrical activity between them.
Lead I: Between the right arm (RA) (negative) and left arm (LA) (positive)
Lead II: Between the right arm (RA) (negative) and left leg (LL) (positive)
Lead III: Between the left arm (LA) (negative) and left leg (LL) (positive)
***Note that the LA electrode serves as the positive pole for lead I and the negative pole for lead III.
Enter Wilson's Central Terminal
Now, let's introduce some magic into the mix.
The three bipolar leads that we mentioned above create an imaginary formation, a triangle, called Einthoven’s triangle.1
By taking an average of the electrical signals from these bipolar limb leads, we conjure up a "virtual" electrode known as Wilson's Central Terminal (WCT). It's not a physical electrode but a reference point that’s meant to be at the centre of Einthoven’s triangle.
The WCT serves as the negative pole for the rest of the leads (i.e. the augmented unipolar leads and the precordial leads).
The Augmented Unipolar Leads: aVF, aVR, and aVL
Using the WCT as a common reference point, specifically the negative pole, we then pair it with the RA, LA, and LL electrodes to create the augmented unipolar leads.
They are called “augmented” because they are augmented by 50%.
These are also called "unipolar" because they have only one pole (which is not entirely true because their other pole is always the WCT):
aVF: Between the WCT (negative) and left leg (LL) (positive)
aVR: Between the WCT (negative) and right arm (RA) (positive)
aVL: Between the WCT (negative) and left arm (LA) (positive)
And this is how you end up with all the limb leads (bipolar and augmented unipolar), which give us views of the electrical activity of the heart in the coronal plane:
The Precordial Leads: V1-V6
The same principle applies to the chest electrodes. Each one pairs with the WCT to give us the precordial leads, offering us views of the electrical activity of the heart in the transverse plane.
Bringing It All Together
And there you go; that's how we get a full view of the heart with a 12-lead ECG, using only 9 active electrodes. The secret is the Wilson's Central Terminal (WCT), a virtual electrode that helps us see the heart's electrical activity from different angles.
This clever setup lets us quickly spot not just heart problems, but also other health issues like electrolyte imbalances, intracranial hypertension, pulmonary embolism, sepsis etc.
The ECG is the fastest and easiest way to obtain the greatest number of clinical data currently available!
I hope this clears up the confusion around ECG leads. If you're still scratching your head or if you'd like to delve deeper, let's keep the conversation going. Leave a comment or send an email my way, and I'll be glad to help you out as much as I can!
Thanks for reading!
References
Lilly, L. S. (2021). Pathophysiology of Heart Disease (7th ed.). Wolters Kluwer.
Ary L Goldberger, MD. Basic principles of electrocardiographic interpretation. Post TW, ed. In: UpToDate, Post, TW (Ed), UpToDate, Waltham, MA, 2021.
Willem Einthoven was a medical doctor and physiologist, and he invented the first practical electrocardiograph. For this reason, he was awarded the Nobel Prize in Physiology or Medicine in 1924.