LEAD
VECTORS AND HEART VECTORS
A lead can be represented as a vector (the lead vector). For simple bipolar leads, such as leads I, II, and III, the lead vectors are directed from the negative electrode toward the positive one. For unipolar leads such as the augmented limb and precordial leads, the origin of the lead vectors lies on the axis connecting the electrodes that make up the compound electrode. That is, for lead aVl, the vector points from the midpoint of the axis connecting the right arm and left leg electrodes toward the left arm. For the precordial leads, the lead vector points from the center of the torso to the precordial electrode site .
Lead vectors for the three bipolar limb leads, the three
augmented unipolar limb leads (left), and the six unipolar precordial leads
(right).
Instantaneous cardiac activity may also be approximated as a single dipole representing the vector sum of the various active wave fronts (the heart vector). Its location, orientation, and intensity vary from instant to instant because of the changing pattern of cardiac activation.
The amplitude and polarity of the
potentials sensed in a lead equal the length of the projection of the heart
vector on the lead vector multiplied by the length of the lead vector:
VL = (H)(cos Q) (L)
where L and H are the length of the lead and heart vectors, respectively, and Ø
is the angle between the two vectors.
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If the projection of the heart vector on the lead vector points toward the positive pole of the lead axis, the lead will record a positive potential. If the projection is directed away from the positive pole of the lead axis, the potential will be negative.
The lead axes of the six frontal plane leads can be overlaid to produce the hexaxial reference system,the six lead axes divide the frontal plane into 12 segments subtending 30 degrees.
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