Wednesday, 8 January 2014

Cardiac Axis

 
 

I love teaching cardiac axis…the long way. Yes, you heard it right, working out the cardiac axis with that highfalutin thingamajig (aka the hexaxial reference system). Nearly always there’s an underlying collective groan when I broach the subject in class. However, this can be done in 3 easy steps, and to the nearest 30°……

STEP 1
You need only look at the LIMB leads. Out of the 6 limb leads decide which is the most biphasic/equiphasic QRS complex. An equiphasic or biphasic complex is a waveform that is half positive and half negative.

STEP 2
Find the lead that lies at right angles to the equiphasic/biphasic lead. Well actually you don’t really need the hexaxial reference system for this because there is a pattern on the ECG that is super easy to remember.

Therefore, we can pair up each lead as follows:
I & aVF
II & aVL
III & aVR
Once you have paired up your lead FORGET about the equiphasic lead. For example if your equiphasic lead is I, this is then paired up with lead aVF. For step 3 all you are interested in now is lead aVF.
STEP 3

Still looking at the ECG, is the waveform in your new lead (the one that you paired up with in step 2) a positive or negative waveform? Only now do you need to look at your hexaxial reference system (axis chart). It is useful to have a chart that looks something like this:

Each lead runs the whole diameter of the circle, but is divided up into a solid and dotted line. Find the lead that you want, and if your waveform is positive follow the solid line, but if your waveform is negative follow the dotted line. For example, if the lead that you paired up is lead aVF, and this is a positive waveform, the answer will be +90°, but if the waveform is negative, the answer will be -90°.  Ta da!!!…let’s try some examples…….
Example 1 


Step 1
Which is the most equiphasic/biphasic lead?
In this example, lead III is the most equiphasic lead
















Step 2
Pair up your lead
Lead III is paired up with lead aVR















Step 3
Is the waveform in your paired up lead positive or negative?
The QRS in aVR is negative – find aVR on the axis chart and follow the dotted line…the axis is +30° (NB if aVR had been positive, you would’ve followed the solid line and the axis would’ve been -150°)



















Example 2




















Step 1
Which is the most equiphasic/biphasic lead?
In this example, lead II is the most equiphasic lead












 






Step 2
Pair up your lead
Lead II is paired up with lead aVL



















Step 3
Is the waveform in your paired up lead positive or negative?
The QRS in aVL is negative – find aVL on the axis chart and follow the dotted line…the axis is +150° (NB if aVL had been positive, you would’ve followed the solid line and the axis would’ve been -30°)





















Handy tips:
  • If all the six limb leads are equiphasic then the axis is indeterminate.
  • If there are two leads that are equiphasic and you can't decide, work out the axis from both leads and usually you will find that you arrive in the same quadrant i.e. left, right, normal.
  • If the QRS morphology hides the direction of the QRS then do not attempt to do the axis.
  • The plus and minus signs on the inside of the circle have nothing to do with the polarity of the QRS complexes.
  • Once you are comfortable with this method you can then move on to narrowing the axis down to the nearest 15 degrees (another post to follow shortly)

Lastly a helpful chart:


Many thanks for reading and if you have any other useful tips or comments please leave a post. Feel free to share and if you like any of the diagrams in this blog please visit our website.....


7 comments:

  1. Thank you so much! I was pulling my hair out with this until I came across your explanation. I have to sit an exam on this very shortly, and now feel confident I can get it right. :)

    ReplyDelete
    Replies
    1. Hi Heidi, thanks so much for your nice comments. Just a thought but be aware that some axis charts don't always have solid and dotted lines as a visual - not sure what you'll be given in your exam. If you find all your lines across the diameter of the chart are solid you will need to look at and follow the plus and minus signs on the OUTSIDE of the circle. Therefore you will note above that each dotted line has a minus sign above it (to represent a negative complex) and each solid line has a plus sign above it (to represent a positive complex). The method is still the same, but you might not get the solid/dotted visual representation. Hope I haven't confused you. Please do message me if you need any further help :) Mx

      Delete
  2. Thanks, Maxine! I don't think we are allowed to take any visual aids in with us - but I have made myself familiar with the axis chart and would be able to draw one up. I thought that if I can tell which area it should fall into, i.e. in example 2 above I can tell it's a right axis deviation, so it must fall into that area on the pie chart, so can't be -30 but must be 150 degrees. Hoping that I am on the right track but it seems to be working with the ones I have worked through so far. Your method is the first one that really made sense to me and which is user friendly and works a treat - thanks again, I will make sure to share it around to all other desperate souls who struggle with this! :)

    ReplyDelete
  3. Thank you very much for this awesome posting. Much more helpful than any other textbooks/YouTube videos

    ReplyDelete
  4. I will share this in my blog. Really helpful. Surely give u credit.

    ReplyDelete