34
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
I FIGURE 2.19 The axial reference system show-
ing the location within the axis of the positive
electrode for each of the six limb leads.
machine does the actual switching and rear-
ranging of the electrode designations.) The
axial reference system in Figure 2.19 shows
that the aVL
lead is at -30° relative to the lead
I axis; aV„ is at -150°, and aV„ is at +90°. It is
critical to learn which lead is associated with
each axis.
The three augmented leads, coupled with
the three standard limb leads, constitute the
six limb leads of the ECG. These leads record
electrical activity along a single plane, the
frontal plane relative to the heart. The direc-
tion of an electrical vector can be determined
at any given instant using the axial reference
system and these six leads. If a wave of depo-
larization is spreading from right to left along
the 0° axis (heading toward 0°), lead I shows
the greatest positive amplitude.
Likewise,
if the direction of the electrical vector for
depolarization is directed downward (+90°),
aVp shows the greatest positive deflection.
DETERMINING THE MEAN ELECTRICAL
AXIS FROM THE SIX LIMB LEADS
The mean electrical axis for the ventricle can
be
estimated
by using the six limb leads and the
axial reference system. The mean electrical axis
corresponds to the axis that is perpendicular to
the lead axis with the smallest net QRS ampli-
tude (net amplitude = positive minus negative
deflection voltages of the QRS complex). If, for
example, lead III has the smallest net amplitude
(a biphasic QRS with equal positive and nega-
tive deflections) and leads I and II are equally
positive, the mean electrical axis is perpendic-
ular to lead III, which is 120° - 90°, or +30°
(see Fig. 2.19). In this example, lead aVR
has
the greatest net negative deflection of the QRS.
It is often important to determine if there
is a significant deviation in the mean electrical
axis from a normal range, which is between
-30° and +90°. Less than -30° is considered a
left axis deviation, and > +90° is considered a
right axis deviation. Axis deviations can occur
because of the physical position of the heart
within the chest or changes in the sequence
of ventricular activation (e.g., conduction
defects). Axis deviations also can occur if
ventricular regions are incapable of being
activated (e.g., infarcted tissue). Ventricular
hypertrophy can display axis deviation (a left
shift for left ventricular hypertrophy and a
right shift for right ventricular hypertrophy).
CASE 2-3
A patient’s ECG recording shows that
the net QRS voltage is zero (equally
positive and negative voltages) in lead I,
and that leads II and III are equally
positive. What is the mean electrical
axis? How would leads aVL
and aVR
appear in terms of net negative or net
positive voltages?
ECG CHEST LEADS
The last ECG leads to consider are the unipo-
lar, precoidial chest leads. These six positive
electrodes are placed on the surface of the chest
over the heart to record electrical activity in a
horizontal plane perpendicular to the frontal
plane (Fig. 2.20). The right arm, left arm, and
left leg electrodes are used as a combined nega-
tive electrode. The six leads are named
V1
to V6.
V1
is located to the right of the sternum over the
fourth intercostal space, whereas V6 is located
laterally (midaxillary line) on the chest over the
fifth intercostal space. With this electrode place-
ment,
V1
overlies the right ventricular free wall,
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