CHAPTER 2 • ELECTRICAL ACTIVITY OF THE HEART
31
aV|_
1
LV free
wall
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*
II
aVL
aVL
C
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II
aVL
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e
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II
-A
aVL
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A
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II
■ FIGURE 2.16 Generation of QRS complex from tw o different recording electrodes. A. Ventricles prior to
depolarization; isoelectric (zero) voltage recorded by electrodes aVL and II. B. Septal depolarization; vo lt-
age aVL < II. C. Apical depolarization; voltage aVL < II. D. Left ventricular depolarization (prim arily); voltage
aVL > II. E. Left ventricular depolarization; voltage in aVL > II. F. Ventricles depolarized; isoelectric voltage
in aVL and II;
red arrow
represents mean electrical axis.
In contrast, the mean vector is heading almost
directly towards the lead II positive electrode,
which results in a very tall, positive deflection
(R wave of the QRS). After another 20 milli-
seconds (Panel D), the apex and most of the
right ventricular free wall are completely depo-
larized. At this time, the left ventricular free
wall depolarizes from the endocardial (inside)
to epicardial ( outside) surface. The resulting
mean vector is mostly heading toward the aVL
electrode and is almost perpendicular to the
lead II axis. Therefore, this vector produces a
large positive voltage in lead aVL
and a rela-
tively small positive voltage in lead II. The last
regions of the left ventricle to depolarize (Panel
E) result in a mean vector that is heading some-
what toward lead aVL, and away from lead II.
Therefore, aVL
will still record a small positive
voltage, whereas lead II will record a small neg-
ative voltage (S wave of the QRS). When the
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