Electrocardiographers use a special axis convention. A horizontal line from the heart to the patient's left side is 0°, as shown in Fig. 5. The angle increases in a clockwise manner as viewed from the front of the patient. Thus, an orientation pointing directly down from the heart would be +90° and horizontal to the right of the heart +180°. Moving counterclockwise from 0°, the numbers become negative. An orientation pointing vertically above the heart would be —90°. This continues to —180°, which is the same as +180°.
It is interesting to note the peculiar way in which the polarities of the attachments for leads I, II, and III are arranged. This convention was adopted early in the development of electrocardiography simply because it caused a positive QRS deflection to occur in all three leads in most people. It is easy to appreciate that a mean electrical axis down and to the left, which is the normal direction for most individuals, will point toward the positive electrode in all three leads. The top panel of Fig. 5 shows an example of a mean electrical axis of 30o and what the QRS complexes in leads I, II, and III would look like. Note that lead III is parallel to the isoelectric plane.
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