Skills Laboratory: How to determine and interpret the mean electrical axis - Veterinary Medicine
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Skills Laboratory: How to determine and interpret the mean electrical axis
Don't be intimidated by this important ECG measurement. This cardiologist describes four methods for easily calculating the mean electrical axis and discusses what the abnormalities you may find signify.


VETERINARY MEDICINE



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.
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.

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.


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Source: VETERINARY MEDICINE,
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