### 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 **S**uper **A**xis **M**an

#### 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…

##### (2) Now add his arms and legs…

##### (3) Now for some labels…

**R**ight Arm (aV**R**)**L**eft Arm (aV**L**)- Then SAM’s non-gender specific
**F**iddly-bits (aV**F**) - 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…

##### (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!).

##### (5) Now for some 5^{th} 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.)

#### ECG Axis Worked Examples

##### ECG 1

- The deflection of the QRS complex in
**I**is about*7 squares up*and*3 square down*.- The NET DEFLECTION =
*+4 squares*.

- The NET DEFLECTION =
- 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**…

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

##### ECG 2

- 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**”

- NET DEFLECTION in Lead I – is
- Deflection in
**aVF**is about*17 squares up*and*2 squares down*.- NET DEFLECTION in aVF is
*+15 squares*

- NET DEFLECTION in aVF is
- Using SAM, we plot NOTHING on I, and +15 squares towards aVF (red) .

- 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!)

- In Lead
**I**, the deflection is*1 down*and*6 up*.- The NET DEFLECTION =
**+5 squares**.

- The NET DEFLECTION =
- In
**aVF**, the deflection is*1 square up*and*6 squares down*!- The NET deflection is =
**MINUS 5**.

- The NET deflection is =
- Let’s plot this on SAM – and be careful with aVF…

- 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

- ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
- ECG A to Z by diagnosis – ECG interpretation in clinical context
- ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
- 100 ECG Quiz – Self-assessment tool for examination practice
- ECG Reference SITES and BOOKS – the best of the rest

#### Advanced Reading

- Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography
- Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric
- Wagner GS. Marriott’s Practical Electrocardiography 12e
- Chan TC. ECG in Emergency Medicine and Acute Care
- Rawshani A. Clinical ECG Interpretation
- Mattu A. ECG’s for the Emergency Physician
- Hampton JR. The ECG In Practice, 6e

## ECG LIBRARY

Electrocardiogram

## Leave a Reply