30
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
ventricles, because the impulses generated by
the ectopic site are not conducted through
normal pathways.
Volume Conductor Principles and
ECG Rules of Interpretation
The previous section defined the components
of the ECG trace and what they represent in
terms of electrical events within the heart.
This section examines in more detail how the
appearance of the recorded ECG waveform
depends on (1) location of recording electrodes
on the body surface; (2) conduction pathways
and speed of conduction; and (3) changes in
muscle mass. To interpret the significance of
changes in the appearance of the ECG, we
must first understand the basic principles of
how the ECG is generated and recorded.
VECTORS AND MEAN ELECTRICAL AXIS
The ECG records time-dependent changes in
electrical activity within the heart. At a given
instant in time, the recording electrodes “see”
a summation of all the regions of the heart
that are undergoing depolarization or repo-
larization. To help understand this concept,
Figure 2.15 illustrates waves of depolarization
originating within the SA node and then
spreading into the atrial muscle. When the SA
node fires, many separate depolarization waves
emerge from the SA node and travel throughout
the atria. These separate waves can be depicted
as arrows representing individual electrical
SA Node
■ FIGURE 2.15 Electrical vectors. Individual
instantaneous vectors of depolarization
(black
arrows)
spread across the atria after the sinoatrial
(SA) node fires. The mean electrical vector
(red
arrow)
represents the sum of the individual vec-
tors at a given instant in time.
vectors. At any given instant, many individual
instantaneous electrical vectors exist;
each
one represents action potential conduction in
a different direction. An instantaneous mean
electrical vector can be derived by summing
the individual instantaneous vectors.
In the heart, the mean electrical vector
changes its orientation as different regions of
the heart undergo depolarization or repolariza-
tion. The direction of the mean electrical vec-
tor relative to the axis between positive and
negative recording electrodes determines the
polarity and influences the magnitude of the
recorded voltage as illustrated in Figure 2.16.
This
illustration
depicts
the
sequence
of
depolarization within the ventricles by show-
ing four different mean vectors representing
different times during depolarization. In this
model, the septum and free walls of the left
and right ventricles are shown, and each of
the four vectors is depicted as originating from
the top of the septum where the left and right
bundle branches divide. The size of the vector
arrow is related to the mass of tissue undergo-
ing depolarization; the larger the arrow (and
tissue mass), the greater the measured voltage.
The placement of the positive recording elec-
trodes represents leads II and aVL
(described
later in this chapter). Before the ventricles
undergo depolarization (Panel A), there are
no electrical vectors so the voltage recording
in either lead will be zero. Early during ven-
tricular activation (Panel B), the first region
to depolarize is the interventricular septum,
which normally depolarizes from left to right
as depicted by the mean electrical vector. The
vector is small because the tissue mass is small.
Because the vector is heading away from the
aVL
positive electrode, this results in a negative
voltage in that lead (Q wave of the QRS). The
same mean vector, however, when recorded
using lead II will not show a change in volt-
age (no Q wave) because the mean vector is
oriented perpendicular to the lead II axis.
About 20 milliseconds later (Panel C), the sep-
tum is completely depolarized and the apex
of the heart begins to depolarize. At this time,
the mean electrical vector points downward
toward the apex and is heading roughly per-
pendicular to the aVL
lead axis, thereby gener-
ating only a very small positive voltage in aVL
.
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