Cardiac Axis Determination – Part 5

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 another. 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.

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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! :)

To re-enforce 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.

See also:

Cardiac Axis Determination: Part 1

Cardiac Axis Determination: Part 2

Cardiac Axis Determination: Part 3

Cardiac Axis Determination: Part 4

Cardiac Axis Determination: Part 5

Cardiac Axis Determination: Part 6

17 Comments

  • Anonymous says:

    This is great… I am so engaged in the reading and grasping the concept I almost forgot to tell you how much I am learning. Thank you!!

  • Anonymous says:

    It’s sure is darn good writing. It is put simple but not too simple. You are good at expressing and not caught up in impressing. I for one am sure I will read all of your wise words on ECG. Thanks a bunch.

  • Tom B says:

    I’m glad you guys are enjoying it! Thanks for the positive feedback.Tom

  • Kathy says:

    after 2 wks of trying to figure this stuff out on my own I finally figured know how to determine axis thanks to you. I can't believe how simple it can be! Everyone else makes it so complicated. You kept my interest even with all the theoretical stuff (which I actually found very useful). Thanks for sharing!

  • Tom B says:

    My pleasure, Kathy! Thanks so much for the feedback.Tom

  • zzyzx says:

    This is xlnt! Thank you.

  • Dr Tushar says:

    I hate cheat codes. I love learning scientifically. Its a research level presentation from u.It teaches medicine as well as physics. Thanks Tom

  • Tom B says:

    That’s a very nice compliment, Dr. Tushar. Thank you very much!

    Tom

  • naushin says:

    thanx.i hv learnt one of the most important topic. 

  • Rinav says:

    i am really amazed at the way this confusing concept is being explained with so much clarity!!!! i always wondered how the hell did HEXaxial system come from? you made it really clear!!! thanks a lot!!! loving this presentation 🙂

  • siddiq says:

    great help….needs more about p,q.s and t waves and about precordial leads..

  • FinallyIgetit says:

    I have to thank you. I read so much stuff about ECG, have seen so many videos but there was always info missing and this lead to confusion – finally here’s a guy who explains nicely.
    I am so gonna tell my friends about you!

  • FinallyIgetit says:

    One thing to add:
    Would you please in this example just add in two sentences why the ECG isn’t showing a left ventricular hypertrophy? Coz that was what I was thinking but the description on the paper says different…

  • HARI says:

    AVL + is 150 ,then how come you have mentioned positive avl as -30?

    • MIchael says:

      If I may… aVL ‘s positive pole is the left arm (LA). Looking at aVL you can see a more upward deflection of the tracing. Meaning that the electrical impulse is moving towards the positive pole (LA) … therefore, a giving axis of -30. If the tracing in aVL showed a more downward deflection ,instead of an upward deflection, then the electrical impulse is moving away from the Positive pole of the LA… giving you a axis of +150. The oposite end of the aVL lead. Hope that helps.

  • MIchael says:

    OH….. I should have indicated that the upward or downward deflection is that direction of the “QRS” complex alone.

  • Ned says:

    Hi what if there is no eqiphasic qrs tks

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