Super Axis Man SAM


Who is SAM?

As a routine part of ECG analysis, we need to determine the ECG AXIS. It isn’t really enough to just whimper… “Is it normal?” So, to help understand axis a tiny bit…I need to introduce you to SAM – the Super Axis Man


Building SAM – the Super Axis Man

(1) Draw a circle and put SAMs head on the top.

Note: SAM is smiling today because he is learning something…

ECG SAM Super Axis Man Drawing 001

(2) Now add his arms and legs…
ECG SAM Super Axis Man Drawing 002

(3) Now for some labels…
  • Right Arm (aVR)
  • Left Arm (aVL)
  • Then SAM’s non-gender specific Fiddly-bits (aVF)
  • He has a tickling stick (I) under his left armpit
  • and finally two supporting splints (II) and (III) – with these supports, SAM is happy and is still smiling…
ECG SAM Super Axis Man Drawing 003

(4) Then, we add SAM’s degrees
  • Note: SAM has a lot of degrees because he studied long and hard to become Super-Axis-Man
  • The degrees go in lots of 30. They are MINUS 30°+60°+90°, and +120°.
  • Start at 0° at lead I, add MINUS 30° to aVL, then +60° to II, +90° to aVF, and +120° to III.
  • If you want to you can add +210° to aVR (which may also be called -150°, just to confuse things!).
ECG SAM Super Axis Man Drawing 004

(5) Now for some 5th grade maths
  • Using a right angled triangle
  • SAM tells you that there are several right angled triangles in this diagram. But, the simplest one to use is the one that starts from 0° and ends at 90°. That is, between I and aVF
  • So, we plot the NET deflection of the QRS complex in lead I on SAM, and then we plot the NET deflection of lead aVF. We draw a line to connect the two, and BINGO, there is your axis!
  • Confused?
  • Let’s go through a couple of worked examples. (Note: I will use the term squares here to indicate small squares or millivolts, in this setting the terms are essentially interchangeable.)
Find X Here it is

ECG Axis Worked Examples

ECG 1
ECG SAM Super Axis Man 001 ECG
click to enlarge
ECG SAM Super Axis Man 001 complexes
  • The deflection of the QRS complex in I is about 7 squares up and 3 square down.
    • The NET DEFLECTION = +4 squares.
  • The deflection of the QRS complex in aVF is about 16 squares up and 1 square down.
    • The NET DEFLECTION = +15 squares.
  • Let’s plot this on SAM, using the red arrows
ECG SAM Super Axis Man Drawing 005

At the intersection of these two lines, in green, we see, the axis is about +75°.

ECG SAM Super Axis Man Drawing 006

ECG 2
ECG SAM Super Axis Man 002
click to enlarge
ECG SAM Super Axis Man 002 complexes
  • The deflection in I is 5 squares up and 5 squares down.
    • NET DEFLECTION in Lead I – is no squares!
    • Because the net deflection is NIL, this lead is called “ISOELECTRIC
  • Deflection in aVF is about 17 squares up and 2 squares down.
    • NET DEFLECTION in aVF is +15 squares
  • Using SAM, we plot NOTHING on I, and +15 squares towards aVF (red) .
ECG SAM Super Axis Man Drawing 007
  • The axis (green arrow) is 90!
  • Interestingly, this fits in with one of our shortcuts. “The axis is 90° from the isoelectric lead!”.
  • The axis IS INDEED 90° from lead I.

ECG3 (be careful!)
ECG SAM Super Axis Man 003 ECG 2
ECG SAM Super Axis Man 003 complexes
  • In Lead I, the deflection is 1 down and 8 up.
    • The NET DEFLECTION = PLUS 7 squares.
  • In aVF, the deflection is 1 square up and 11 squares down!
    • The NET deflection is = MINUS 10 squares.
  • Let’s plot this on SAM – and be careful with aVF…
ECG SAM Super Axis Man Drawing 008
  • Here, because the NET deflection in aVF is NEGATIVE, we go away from aVF, or upwards!
  • The axis is MINUS 45.
  • Interestingly, this is also an isosceles right angle triangle (the two short sides are equal length).

Notice that SAM is still smiling, despite what we threw at him! With practice, you can draw SAM in your head. And if you are ever at a hospital not far from here, and see SAM on an ECG, you know I’ve been lurking…

Why is knowing the exact axis useful?
  • Well, for one, you can show up all those wannabe cardiology registrars who think they know ECGs (this is my personal favourite reason).
  • But, secondly, you can tell if the axis has changed WITHIN NORMAL LIMITS. (e.g. if the axis goes from 0 degrees to 90 degrees, then that is a significant shift, but you would never know unless you calculated it. You may even be tempted to call it “Normal”, and walk away from a patient with right heart strain and massive PE.)

Tips and tricks…

aVL and aVR confusion is the most common ECG placement error
  • So, you can see if you swap aVR and aVL around, then I, II and III will all be miscalculated. Try it on a patient! When you swap aVR and aVL around, III becomes II, and vice versa, and I becomes anti-I
  • In this circumstance SAM doesn’t work!
  • A quick way to check is to look at aVR and aVL. aVR should have inverted P-waves, QRS-complexes and T-waves. Also, aVL and aVR are often mirror images of each other.
Another common error on SAM is plotting the wrong way with aVF
  • If the NET deflection is +3 in aVF, then we need to plot DOWN towards aVF on SAM.
  • Similarly if the NET deflection is -5 in aVF, then you need to plot UP, away from aVF!
Once you master axis using SAM then, try your luck against the computer
  • Once you start getting within 5-10 degrees of the computer, then start betting lunch money or cappuccinos against your residents. Ah, an endless supply of free coffee…

Related Topics

LITFL Further Reading

Advanced Reading

ECG LIBRARY 700

ECG LIBRARY

Electrocardiogram

Specialist Intensive Care Physician working at the Austin Hospital, Melbourne. Interests: Shoulder Dislocations, Pain Management, End-of-life care, Organ Donation and ECGs | Linkedin |

7 Comments

  1. Awesome easy to remember way of assessing access! Thanks for sharing!!

  2. Great way to calculate the axis, my only concern is how to come up with the diagonal angle on the fly.

  3. This and the whole LITFL ECG education database is awesome. Cheers from the land of thousand lakes.

  4. Rogerio Luz Coelho
    Rogerio Luz Coelho

    Ahhh Trigonometry … You can easy do it in you phone calculator.

    you will have to find the tan-1 function … Generally in scientific mode, some phones have a a sin / cos /tan and a INV (inverse) button … Just look for it.

    the angle is given by tan-1 (oposing length DIVIDED BY adjacent length)

    THE BLOG´S EXAMPLE 3 : In my phone´s calc it is easy I press INV and then tan-1(10/7) = 55,00…

    *** CAREFULL !!!! *** the tan-1 function IS NOT the the matematically inverse of a result of the tan function, so 1 / tan(10/7) = 40,1 … So you really have to find the tan-1 function on that calculator !!!!

    Cheers From Brasil – Rogerio

  5. muy interesante

  6. Thank you Gerard (and Sam)!

  7. Dr. Nazrul Islam
    Dr. Nazrul Islam

    Very much impressed the way you taught axis calculation.

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