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Super Axis Man SAM , Easy Way To Determine The ECG AXIS

Discussion in 'Cardiology' started by Hadeel Abdelkariem, Aug 8, 2019.

  1. Hadeel Abdelkariem

    Hadeel Abdelkariem Golden Member

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    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

    [​IMG]


    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…

    [​IMG]


    (2) Now add his arms and legs…

    [​IMG]


    (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…

    [​IMG]

    (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 0°, 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!).

    [​IMG]


    (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.)

    [​IMG]

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    ECG Axis Worked Examples

    ECG 1

    [​IMG]

    [​IMG]


    • 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…

    [​IMG]

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


    [​IMG]


    [​IMG]

    ECG 2

    [​IMG]

    [​IMG]

    • 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) .
    [​IMG]

    • 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!)

    [​IMG]

    [​IMG]

    • In Lead I, the deflection is 1 down and 6 up.
      • The NET DEFLECTION = +5 squares.
    • In aVF, the deflection is 1 square up and 6 squares down!
      • The NET deflection is = MINUS 5.
    • Let’s plot this on SAM – and be careful with aVF…

    [​IMG]

    • 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).
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    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…
    Source
     

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    Last edited: Aug 8, 2019

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