ECG Basic:Cardiac Axis-Credit Cardio Rhythm Online Blogspot

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:

Case Study 2

Whether you are a beginner or an instructor it's useful to have these two 12-lead ECGs in your collection. The first ECG demonstrates an extreme axis deviation; this may also be referred to as "north west" or "no man's land" axis. Some instructors deem that "extreme" may wrongly imply an extreme cause of the axis deviation. This is worth noting here! The second ECG demonstrates a right axis deviation. However, both show a negative/inverted PQRST complex in lead I. What is your interpretation of each ECG?