Cardiac Axis Determination – Part 4

By now you should have a fairly good grasp of how the hexaxial reference system is derived from the first 6 leads of the 12 lead ECG.

Before we break down the finished diagram, let’s look at the hexaxial reference system laying on top of the patient’s anterior chest, with the arrows and leads in the position of the positive electrodes.

The first thing I would like you to notice is that lead I cuts the body in half horizontally and lead aVF cuts the body in half vertically.

The second thing I would like you to notice is that even though leads II, III, and aVF share the same positive electrode, they represent three separate vectors. This diagram should clearly demonstrate why we call them the “inferior” leads. It should also demonstrate why we call leads I and aVL the “high lateral” leads.

You will notice that leads III and aVL are on opposite sides of the hexaxial reference system. That’s why they are two of the most reciprocal leads on the 12-lead ECG. More on that later. Right now I’m just planting the seed.

You will notice that lead II cuts across the body in a “right shoulder-to-left leg” direction (white electrode to red electrode) which is the same direction as the heart’s normal axis. That’s probably why we were first taught to monitor lead II. It tends to show nice, upright P waves, QRS complexes, and T waves.

Now let’s look at the finished diagram.

If possible, click on the image above, and print it out for reference.

I’m going to do something a little bit unconventional here. Before I break down the various quadrants of the hexaxial reference system and discuss the normal ranges, we’re going to use it to calculate the hearts QRS axis on an actual ECG.

After all, this is Part 4 and we haven’t looked at a single 12-lead ECG!

axis part iv RVH wm
If possible, click on the above ECG, print it out, and lay it down next to the diagram of the hexaxial reference system.

For now, we’re only worried about the first 6 leads of the 12-lead ECG, because they are the leads that make up the frontal plane and the hexaxial reference system.

Do you remember the most important theory of ECG interpretation? If not, go back to Part 2 for a review. When the heart’s mean electrical vector (or QRS axis) moves toward a positive electrode, you get an upright complex in that lead. When it moves away from a positive electrode, you get a negative complex in that lead. When it moves perpendicular to a positive electrode, you get an equiphasic (and/or isoelectric) complex in that lead.

Now, let’s look at the first 6 leads in our sample. Can you spot the most equiphasic or isoelectric QRS complexes? If you said lead aVR, move to the head of the class!

We can deduce that this patient’s QRS axis (in the frontal plane) is moving perpendicular to the positive electrode in lead aVR. Now, look at your diagram of the hexaxial reference system and find lead aVR (-150 degrees to +30 degrees). We theorize that this patient’s QRS axis is moving perpendicular to lead aVR. So, which lead is perpendicular to lead aVR? Lead III!

Find lead III (-60 degrees to +120 degrees). The QRS axis is moving along the same vector as lead III. But is it moving toward -60 or toward +120? Go back to the sample ECG. Is the QRS complex positive or negative in lead III? It’s positive! You’ll also notice that lead III shows the tallest QRS complex in the frontal plane. Interesting! Now look at the hexaxial reference system again. You’ll see little downward arrow in front of lead III at -60 degrees and a little upward arrow in front of lead III at +120 degrees. The positive electrode for lead III is at +120.

The QRS axis should be around 120 degrees.

Let’s look at our computer measurements and see how we did. Go to the top of the sample ECG and look for P-QRS-T Axis. To the right you will see three numbers. The middle number is the QRS axis in the frontal plane. What does it say? 121 degrees.

We’re only off by 1 degree, in a 360 degree circle! That’s pretty darned good. Does it always work out that perfect? No. But you can almost always get it within 10 or 15 degrees.

Now look at the previous diagram of the hexaxial reference system laying on top of the patient’s anterior chest and find the positive electrode for lead III. You’ll notice that the vector points downward toward the patient’s right leg. Guess what? This is a right axis deviation.

Now, you may be thinking, “This is way too much work!”

Yes and no.

This may be the first time you’ve ever used the hexaxial reference system, so of course it seems complicated, but it’s really not. In fact, I no longer require the diagram. Because I’ve done this so many times, some patterns have emerged that have simplified things for me dramatically.

In other words, there are some “tricks of the trade”.

In Part 5 I’ll show you some startling relationships between the standard 12-lead ECG and the hexaxial reference system that may change the way you look at 12-lead ECGs forever!

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

18 Comments

  • Anonymous says:

    what do you do if there none of the leads are really equiphasic?

  • Tom B says:

    Anonymous – Then you take the most equiphasic, or in the event of a tie, the two most equiphasic leads in the frontal plane. Calculate both and split the difference.Tom

  • Anonymous says:

    One more thing, I have an old 12 lead that I have always thought was V-tach, but is hard to tell. The QRS axis reported by the LP12 is 259. According to your diagram, you can't have above 180 positive or negative. Is the extreme right axis quadrant still positive according to the LP12? 259 should really be -101 according to your diagram.

  • Tom B says:

    Anonymous – Are you from the United States? I have noticed that in other regions of the world, the right superior quadrant is a continuation of the right inferior quadrant.Technically, an extreme axis is an "extreme right axis deviation" since the right superior axis is on the right side of the patient.Either method is acceptable.Tom

  • Mike says:

    Yes, I am from Maryland, just thought i had to have an account or something. Name is Mike, by the way your site is great, thanks for the help.

  • Dr vithal says:

    its really useful to get the ideas clear

  • Tom B says:

    Thank you, Mike! Thank you, Dr. Vithal!Tom

  • mohammed says:

    Hi,

    It’s not clear what the ‘…little downward arrow in front of lead III at -60 degrees and a little upward arrow in front of lead III at +120 degrees…’ mean.

    also the ECG is impossible to print, it’s all fuzzy if you zoom in.

    if you could do a video of this whole process it would be really useful.
    thanks

  • Tom B says:

    Mohammed –

    Thanks for the feedback. Perhaps one day I will do a video and post it to YouTube.

    The little arrows correspond to the polarity of the QRS complex on the actual ECG.

    If it’s positive you look at the little “up” arrow and if it’s negative you look at the little “down” arrow for the answer.

    Tom

  • Hanz says:

    Hi Tom. Do you have a clearer picture for the sheet of the ECG? I am loving your discussion right here but  I find it hard to read what's in it.  can you post a clearer picture of it! thank you so much I appreciate it .:)

  • sue says:

    Dont quite understand about the 120 degree part, cuz I thought – 30 to 120 is within normal range? Beyond 120 will be right axis deviation and beyond -30 to -90 will be left axis deviation ?

  • john says:

    It would seem that AVL not AVR would be the most isoelectric. From previous discussion I thought that we would see positive amplitude followed by an equal negative amplitude. Am I missing something here?

  • john says:

    Please ignore my previous question. I was looking at the 12 lead in pre-hospital 12-lead booklet because I couldn’t clearly read your’s. I thought that they were the same but just realized that they are different. Sorry.

  • larry smith says:

    great idea for determining axis, my wife has been an ccu/icu nurse for 30 years, I just explained to her how to calculate deviation, maybe it was just me because she wasn’t impressed. Anyway how do refine the degree because your chart is +/- 30′ ?

  • darren says:

    Should be +105 degrees not +120

  • Arisia says:

    How did those little arrows come about ?i have seen hex axial figures without those arrows …and yet students get the correct answer please help

  • Marta says:

    Thank you SO MUCH!! I was beginning to think that I would never understand this! Thank you!

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