The mean QRS axis in the frontal plane (limb leads) Normal limits of QRS axis in adults: - 30° to + 90°
The actual degrees of the mean QRS axis are rarely important to determine. However, the recognition of abnormal axis deviations is critical, for it leads to the presumption of disease. The mean QRS axis is derived from the net area under the QRS curves. The most efficient method of determining the mean QRS axis uses the method of Grant, which requires only leads I and II. If the net area under the QRS curves in these leads is positive, the axis falls between -30° and +90°, which is the normal range of axis in adults. (The only exception to this rule is in RBBB, in which the first 60 msec of the QRS are used. However, I recommend using the maximal amplitude of the R and S waves in leads I and II to assess the axis in RBBB.) Abnormal axes are shown in the illustrations below.

Hexaxial reference system
Arzbaecher developed the hexaxial reference system that allowed for the display of the relationships among the six frontal plane (limb) leads.

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