Mastering Axis Determination: Part 5
Updated: Jun 21, 2021
In Part 4 I promised that I’d show you a fascinating relationship between the standard 12-lead ECG and the hexaxial reference system.
You will recall that to use the hexaxial reference system, you find the most equiphasic (or isoelectric) lead in the frontal plane (first 6 leads of the 12-lead ECG) and look for the perpendicular lead on the hexaxial reference system.
The example ECG we used was that of a 16-year-old female with a congenital heart defect. The most equiphasic lead was lead aVR. We looked at the hexaxial reference system and noticed that the lead perpendicular to lead aVR was lead III. Since the ECG showed an upright QRS complex in lead III, we knew the frontal plane QRS axis was close to 120 degrees.
So what is the most difficult part of this seemingly cumbersome process? In my opinion, it’s squinting your eyes at the hexaxial reference system to figure out which lead is perpendicular to the equiphasic (or isolectric) lead. Well, I have some good news! After performing this procedure dozens if not hundreds of times, I detected a very simple pattern.
To review, in Part IV we found out that lead III was perpendicular to lead aVR. Guess what? It works both ways. If lead III is perpendicular to lead aVR, then lead aVR is perpendicular to lead III. If you examine the hexaxial reference system, you will notice that leads I and aVF are perpendicular to each other. Likewise, leads II and aVL are perpendicular.
This diagram represents the layout of the first 6 leads of the 12 lead ECG in the standard format. You will notice that when we draw a line between the perpendicular leads, they crisscross in the center.
If you commit this pattern to memory, there’s only one reason you’ll need the hexaxial reference system, and that’s to read the answer! In fact, once you get used to the numerical values that correspond to the various leads, you won’t even need that.
Let’s look at an example:
Which lead in the frontal plane shows the most equiphasic QRS complexes? Lead II. Which lead is perpendicular to lead II in the hexaxial reference system? The lead across from lead II (according to the cheat sheet diagram we just went over) is lead aVL. If you check the hexaxial reference system it will confirm that leads II and aVL are perpendicular to each other (electrically speaking).
Now, look at the ECG. Is lead aVL positive or negative? It’s positive! Now, look at the copy of the hexaxial reference system that you printed out in Part IV. Look for the aVL with the little “up” arrow in front of it. What is the numerical value? It’s -30 degrees! We estimate the QRS axis at -30 degrees.
Let’s check our work. Go to the top of this sample ECG and look for R-QRS-T Axes. The middle number will show you the QRS axis in the frontal plane. The computer measures it at -26 degrees. We’re only off by 4 degrees!
Is this making sense? If you attempt this on every 12-lead ECG, you will be amazed how simple it is. Not only that, patterns will emerge that will deepen your understanding of the 12-lead ECG.
To the uninitiated, it seems like magic!
"But we are initiated, aren't we Tom?"
To reinforce this lesson, click here. It’s one of the coolest ECG-related things I’ve ever found on the Internet. Scroll down and click on Frontal Axis Demo. When it appears on your computer screen, click and drag the dial around the hexaxial reference system, and see what it does to the sample ECG on the screen. It’s quite fascinating! This is an incredible teaching aid and I only wish I’d thought of it!
In Part 6 we’ll go over the ranges for the QRS axis in the frontal plane.