ECG Level 3 Tutorial: Axis
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Tutorial: Axis
This will teach a basic approach to determining the axis of an ECG.
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Tutorial: Axis The Isoelectric lead
Lessons
42
Times Practiced
1284
Cases Completed
1h 24m
Total Time spent
1m 24s
Average Time
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Accuracy
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Accuracy
Efficiency
1 Remember the Vector Remember the Vector
2 Quadrants Quadrants
3 Defining the Normal axis Defining the Normal axis
Current lesson 4 The Isoelectric lead The Isoelectric lead
5 Shading the QRS Shading the QRS
The Isoelectric lead
Consider this lesson to be "extra" or "advanced". If you are pressed for time, you can skip it.

Using the quadrant approach by itself provides a rough estimate of the axis. However, we need some more detailed information. Using the isoelectric lead combined with the quadrant information enables us to be more accurate.

The quadrant approach is a simplified version of the truth, because a normal axis is not 0 to +90, it is -30 to +90:
normal ECG axis

Do you still remember the isoelectric lead? This is the lead that is 90 degrees to the direction of travel:
how the ECG works

If the electrical activity is travelling at 90 degrees to the lead, then that lead will be isoelectric. Since we know the angle of the lead, we can determine 2 possible options for the axis. The axis could be travelling straight up (negative 90 degrees) or alternatively (not shown) travelling straight down (positive 90 degrees).

Both +90 and -90 are 90 degrees away from the lead I (which is zero degrees) and could both create an isoelectric QRS in lead I.


If we combine the quadrant information with the isoelectric information, then we can determine which of the 2 isoelectric options overlaps with the double shaded quadrant and thus know the axis to within 30 degrees. And that is how you can determine the axis.

Let's look at one example:
isoelectric ECG lead

Step one: what is the QRS in lead I - is it mostly positive or mostly negative?
ECG isoelectric lead
It is mostly positive.

Step two: what is the QRS in lead aVF?
ECG isoelectric lead
It is also mostly positive.

Step three: which lead is the most isoelectric?
ECG isoelectric lead
It is lead aVL.

Let's put it all together: Positive in I and positive in aVF gives us bottom right corner. The isoelectric lead aVL points at negative 30 degrees, so 90 degrees to this would be -120 or +60 degrees (the red line in the right diagram):

ECG isoelectric lead 

ECG isoelectric lead

The axis is in the lower right purple quadrant (0 to +90). The axis is either -120 or +60.  We can rule out negative 120 degrees because it is not in the purple double shaded region. The correct answer is +60 degrees, which is normal.