Mastering Axis Determination: Part 1
Updated: Mar 6
Few subjects related to 12 lead ECG interpretation provoke more controversy (or anxiety) than axis determination.
It is controversial in that not everyone agrees it is a necessary skill for prehospital providers to learn. It is anxiety-provoking in that it can be difficult to understand, especially when taught poorly.
I am of the opinion that axis determination is a critical skill to master. I would even go so far as to say that you cannot be competent at 12-lead ECG interpretation if you don’t understand the heart’s electrical axis.
In many classes, to the extent that axis is discussed at all, the instructor goes straight to providing a cheat sheet for axis determination. The most commonly taught method is the quadrant method that uses leads I and aVF. I don’t think this holds much value for the student. On the one hand, it’s perfectly true that leads I and aVF can place the QRS axis into one of four quadrants in the frontal plane. On the other hand, it leaves the student with a feeling of “so what?” The axis becomes a piece of “gee-whiz” information that doesn’t lend itself to a deeper understanding of the 12 lead ECG.
I do teach the quadrant method (and other speed methods for axis determination), but only after I teach the hexaxial reference system, and my students can place the QRS axis within 10 to 15 degrees. This is not particularly difficult, and once it is well understood, it’s a gift that keeps on giving for the rest of your career. It is similar to using the large block method for rate determination in this respect. Once you know it, you know it, and you can estimate the heart rate at a glance. And yet, most paramedics have never bothered to commit this simple method for calculating the heart rate to memory, so they are dependent on the computer, or they have to count out the QRS complexes in a 6-second strip and multiply by 10.
Before we begin looking at the hexaxial reference system, there’s a man we need to discuss, and his name was Willem Einthoven, winner of the Nobel Prize in Physiology or Medicine in 1924 for his invention of the string galvanometer, which was the first reliable electrocardiograph.
You’ll notice, in the image above, that Einthoven’s arms and his left leg are immersed in buckets of salt water. At the time, this was the only way to obtain a signal for the electrocardiograph. Even after the invention of the electrode, they continued to be placed on the subject’s arms and legs. From this configuration, leads I, II, and III were born, and they are called the “limb leads” to this day.
Leads I, II, and III have been around for a long time (over 100 years). I always laugh when I hear people suggest that using leads I, II, and III to estimate the heart’s electrical axis is somehow a new thing! It’s been happening long before any of us were born.
These first 3 leads of the 12-lead ECG form what came to be known as Einthoven’s Triangle or Einthoven’s Equilateral Triangle.
If you’re like me, you’re reading this and it sounds very confusing. After all, if you look at the image below, it’s clear that anatomically, leads I, II, and III form a scalene triangle, not an equilateral triangle. So what in the world was Einthoven talking about?
Einthoven meant that electrically speaking, leads I, II, and III form an equilateral triangle. He expressed this with Einthoven’s Law, which states:
"I + III = II which can also be written I + (-II) + III = 0"
I know what you’re thinking. This equation is scary. I’ve just lost you. Take a deep breath! Everything is going to be okay.
What is lead I? It is a dipole, with the negative electrode at the right arm (white electrode) and the positive electrode at the left arm (black electrode).
What is lead III? It is a dipole with the negative electrode at the left arm (black electrode) and the positive electrode at the left leg (red electrode). Sometimes I wonder why Einthoven didn’t call this lead II.
What is lead II? Continuing clockwise as you look at the patient, you’d think it would be a dipole with the negative electrode at the left leg (red electrode) and the positive electrode at the right arm (white electrode), but it’s not. For reasons known only to Einthoven (perhaps because he liked to view upright QRS complexes), he made lead II a dipole with the negative electrode at the right arm (while electrode) and the positive electrode at the left leg (red electrode).
Had Einthoven not switched the polarity of lead II, Einthoven’s Law would be written like this:
"I + II + III = 0"
But he did, and there’s no point in crying over spilled milk.
I still know what you’re thinking. You’re feeling anxious because you still don’t understand what the equation is referring to! That’s okay. We’re getting there. Take another deep breath and relax. Everything is still going to be okay.
Rather than explain to you why Einthoven’s Law works, I’m simply going to prove to you that it does work.
Look at the image below and come up with a numerical value for the signal recorded in lead I. The R wave is about 7 1/2 mm tall, and the S wave is about 2 1/2 mm deep. Subtract the S wave from the R wave, and you come up with 5 mm.
Let’s do the same thing for the signal in lead II. This is easier because it’s essentially a monophasic QS complex. It’s about -10 mm.
Do you see where this is going?
Now, how about lead III? There’s a little nub of an R wave that is about 1 mm high, and the S wave is about 16 mm deep. Subtract the R wave from the S wave, and you get a complex that measures approximately -15 mm.
Now let's plug these values into the equation for Einthoven’s Law.
I + (-II) + III = 0
5 + 10 -15 = 0
As you can see, when you plug in the measurements, you end up with an electrical value of zero.
You can try this trick on virtually any ECG. Because this is true, leads I, II, and III can be represented as an electrically equilateral triangle.
As you will see in Part 2, this is the key to understanding the formation of the hexaxial reference system, and understanding the heart’s electrical axis in the frontal plane.