

ECG Primer: The Mean Electrical Axis 

Created by Diane R. Karius, Ph.D.  
The mean electrical axis tells us the net direction the depolarization or repolarization is heading. An axis can be determined for each wave of the ECG (P, QRS, and T) and important information can be derived from each one (for example, the P wave axis can tell us whether or not the SA node is driving the ventricles). When used without specification of the wave, the term mean electrical axis denotes the ventricular mean electrical axis. From here on in, I will focus only on the ventricular mean electrical axis.
Calculation of the mean electrical axis requires some additional background associated with the leads of the electrocardiogram. If you remember from lecture, we can take the three bipolar leads and create a triangle out of them (Einthoven's triangle), as illustrated below:
Note that we moved the left leg electrode (which had appeared on the left hip) into the center, becoming the apex of the Einthoven's triangle. We can do this because the other leg (the right, for anatomically impaired physiology types :) is the ground. The zero on each of these leads is in the middle. The ventricles are at the center of the triangle.
For the sake of ease, we often take the zero points and superimpose them, creating the reference axes:
When we do this, we keep the same orientation (i.e. the + and  poles are still in the same place). In the figure above, the lead names are labeled at their positive pole, and the negative poles are indicated by the the () sign. Note that the heart is located at the point of intersection.
At this point, you are probably wondering why this is being brought up  beyond the physiologist's desire to confuse you. The big point of all of this is: The reference axes (or Einthoven's triangle) are used to calculate the Mean Electrical Axis of the ventricles! We will now run through a sample calculation of the mean electrical axis using the reference axes.
The following Flash animation shows you how to use leads 1 and aVF to calculate the mean electrical axis  this is how Dr. Johnston usually shows the class.
The reality is that you can use any two leads of the six limb leads (I, II, III, aVR, aVL, or aVF) to calculate the mean electrical axis. In the following set of illustrations, we'll use a different pair of leads to make the calculation. In order to calculate the mean electrical axis, you will need an ECG with at least two of the bipolar leads on it and the reference axes. Below is leads I and II from a standard 12lead ECG:
To calculate the mean electrical axis of the ventricle, we are going to focus on the QRS complex (since that is the wave that represents ventricular depolarization). Thus, we need to identify the QRS complexes in each lead:
In lead I (shown above, the leads have been separated for ease of work), we have an initial positive going wave (R) followed by small S wave. In lead II (below), we have an R wave, with no significant negative deflections on either side of it. In each of these figures, the green line indicates the isoelectric (zero current) line.
In order to calculate the mean electrical axis, our next step is to estimate the area of each of the wave (Q wave, R wave and S wave get considered separately...). DON'T PANIC!!!! With a normal ECG, you do NOT have to calculate the area under the curve!!!! A quickie shortcut is to count how tall the wave is (i.e. how many squares from the isoelectric line to the peak of the wave. For lead I:
For the R wave: count from the isoelectric line (green) to the peak (red): 3.5 squares = 3.5 mm (each square is a millimeter on a side).
For the S wave: Again, from the isoelectric line (green) to the peak (red  but now that's down): 1 square = 1 mm.
Our next step for this lead is to subtract the heights of the negative going leads from the height of the positive going leads:
(height of the R wave)  (height of the Q wave)  (height of the S wave)
since we have no Q wave in this example, this works out to :
(height of the R wave)  (height of the S wave)
3.5 mm  1 mm = 2.5 mm
We're not quite finished with this lead yet: We need to plot this number (2.5 mm) on the appropriate axis:
Remembering from above (and relabeled here), Lead I is represented by the horizontal line. Zero is the intersection of all these leads, while positive and negative are as marked (it's as if you are face up, lying down on the paper  your left arm would be over the + sign). To plot the +2.5 mm, just move 2.5 mm from the zero point (intersection of the lines) towards the + end (as indicated on top of the picture). You then draw a line perpendicular to the axis through this point (the purple/black line  since lead I is horizontal, the perpendicular is a simple vertical line).
We then repeat this process with the next lead:
Height of the R wave: 14 squares = 14 mm.
There is no Q or S wave to speak of.
(height of the R wave)  (height of the Q wave)  (height of the S wave)
(height of the R wave)  (height of the Q wave)  (height of the S wave)
14 mm  0 mm  0 mm = 14 mm
Which is then plotted on the reference axis:
Identifying Lead II on this plot is probably the hardest part of the process: If you start with lead I and go clockwise, then next is Lead II (as labeled), then lead III (not labeled). We count off 14 mm and draw a line perpendicular to the orientation of the axis as shown.
Extend both lines (the lines we drew for lead I and lead II) until they intersect (as shown  the fact that it runs off the graph paper doesn't matter...)
We are finally ready to actually see what the mean electrical axis is. To make this final step, we start at the zero point (the center, where the heart is) and draw a line to the point at which our two perpendiculars (the magenta and the purple/black) intersect:
The mean electrical axis is expressed in degrees (as in degrees of a circle), starting with 0^{o} at the postive end of lead I and going clockwise around:
A vertical line straight down would be 90^{o}, but the line we drew isn't quite vertical. The mean electrical axis indicated here is in the vicinity of 85^{o} or so. Since the normal range (using even the tightest standard) is between 0 to 90^{o}, this is perfectly normal. Note: on a test, you will not be expected to make a terribly precise call on the numerical value of the axis. You would be expected to know if this is normal or not.
What it means: The mean electrical axis tells us the net direction the depolarization of the ventricles (since we used the QRS complex) is heading: in the case we calculated about 85 degrees (rough estimate)  meaning that the net depolarization is heading almost straight down towards the apex of the heart.