4. Use the quadrant graphing method4 —In this method, the hexaxial lead system is divided by leads I and aVF into four equal quadrants. Arrows are drawn on leads I and aVF in the direction of the predominant QRS complex deflection. For example, in Figure 5 the QRS complex in lead I is predominantly negative, so an arrow will be drawn toward the negative pole of lead I. Lead aVF is also predominantly negative, so an arrow will be drawn away from its positive pole. This would place the mean electrical
axis in the upper right quadrant, indicating a right axis shift. The mean electrical axis can be further estimated by examining
the relative size of the QRS complexes in leads I and aVF. The mean electrical axis will be closer to the larger of the two. Since lead aVF is more negative than lead I, the mean electrical axis will be closer to –90 degrees than to –180 degrees.
Figure 5. The mean electrical axis can be approximated by determining the quadrant that it lies within. In this ECG, lead
I is negative, so the mean electrical axis must lie either in the upper or lower right quadrant. Lead aVF is also negative, so the mean electrical axis is in the upper right quadrant. This is a marked right axis shift in a cat
with pulmonic stenosis and right ventricular hypertrophy.
If an estimate of the mean electrical axis is ambiguous with the latter two methods, then one of the more precise methods
should be used to verify the results.
Abnormalities of the mean electrical axis
Table 1 summarizes the causes of various mean electrical axis shifts and their ECG characteristics. The mean electrical axis normally
points toward the left ventricle because this is the largest mass of myocardium, so the surface ECG predominantly records
its activity.4 Severe hypertrophy of the right ventricle will cause the electrical activity of the right heart to dominate over that of
the left on the ECG, resulting in a right axis shift of the mean electrical axis. The ECG changes associated with left ventricular
hypertrophy include increased R wave amplitude and a mild increase in QRS complex duration. Left ventricular hypertrophy does
not usually cause a shift in the mean electrical axis because the left ventricle tends to enlarge in the direction of lead
Table 1 Abnormalities of the Mean Electrical Axis and Potential Underlying Causes
Intraventricular conduction defects cause a shift in the mean electrical axis toward the ventricle that is depolarized by
the defective bundle branch or fascicle. The mean electrical axis shifts toward the defect because the myocardium that is
normally depolarized by that portion of the conduction system must now be depolarized by slow myocyte-to-myocyte conduction.
The increased time needed for depolarization results in the electrical activity of that region of myocardium predominating
over that of the normal myocardium. The QRS complex that results from a bundle branch block can be quite abnormal in appearance,
but it can often be distinguished from a ventricular premature complex by the appearance of a normal P wave preceding the