Figure 12-6

Determination of a projected vector B along the axis of lead I when vector A represents the instantaneous potential in the ventricles.

Determination of projected vectors in leads I, II, and III when vector A represents the instantaneous potential in the ventricles.

instantaneous electrical potential of a partially depolarized heart. To determine the potential recorded at this instant in the electrocardiogram for each one of the three standard bipolar limb leads, perpendicular lines (the dashed lines) are drawn from the tip of vector A to the three lines representing the axes of the three different standard leads, as shown in the figure. The projected vector B depicts the potential recorded at that instant in lead I, projected vector C depicts the potential in lead II, and projected vector D depicts the potential in lead III. In each of these, the record in the electrocardiogram is positive—that is, above the zero line—because the projected vectors point in the positive directions along the axes of all the leads. The potential in lead I (vector B) is about one half that of the actual potential in the heart (vector A); in lead II (vector C), it is almost equal to that in the heart; and in lead III (vector D), it is about one third that in the heart.

An identical analysis can be used to determine potentials recorded in augmented limb leads, except that the respective axes of the augmented leads (see Figure 12-3) are used in place of the standard bipolar limb lead axes used for Figure 12-6.

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Essentials of Human Physiology

Essentials of Human Physiology

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