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Module title = Tutorial: Axis
Lesson title = The Isoelectric lead
This is lesson 4 of 5 in this module
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:
Do you still remember the
isoelectric
lead? This is the lead that is 90 degrees to the direction of travel:
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:
Step one:
what is the QRS in lead I - is it mostly
positive
or mostly
negative
?
It is mostly positive.
Step two:
what is the QRS in lead aVF?
It is also mostly positive.
Step three:
which lead is the most
isoelect
ric?
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):
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.
Lesson 4 of 5
That was the last lesson!