2. Use the lead graphing method—Measure the net deflections of leads I and III (Figure 3). From the baseline (the P-R segment), count the number of upwardly deflected boxes in the R wave, and then subtract the
number of downwardly deflected boxes in the Q and S waves. Plot the net deflection on each lead, and then draw a perpendicular
line through each of these points. The point where the lines from leads I and III intersect is the mean electrical axis. Any
two leads can be used, but leads I and III are most often used.
Figure 3. The mean electrical axis can be identified by graphing the net deflections of leads I and III. The positive deflections
of the R waves are measured from the baseline (P-R segment). The negative deflections of the Q and S waves are similarly measured
from the baseline. Perpendicular lines are drawn at +6 for lead I and -7 for lead III. Where the two perpendicular lines intersect
is the mean electrical axis. This mean electrical axis is shifted to the left, consistent with a left anterior fascicular
block. This is a common axis deviation in cats with cardiomyopathy.
3. Find the lead with the tallest R wave (Figure 4)—The mean electrical axis is within 30 degrees of the positive pole of the tallest R wave.
Figure 4. The approximate location of the mean electrical axis can be found by identifying the lead with the tallest R wave.
The mean electrical axis will be within 30 degrees of the positive pole of this lead. In this ECG, lead II has the tallest
R wave, indicating that the mean electrical axis is normal.