32
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
ventricles are completely depolarized (Panel
F), the voltage in all recording leads will be
zero. It is important to note that the placement
of the recording electrode determines the shape
of the QRS complex that is recorded.
If the four mean vectors in Figure 2.16 (Pan-
els B-E) are summed, the resultant vector (Panel
F large red arrow) is the mean electrical axis.
The mean electrical axis represents the aver-
age of all of the instantaneous mean electrical
vectors occurring sequentially during ven-
tricular
depolarization.
The
determination
of mean electrical axis is of particular signifi-
cance for the ventricles and is used diagnosti-
cally to identify left and right axis deviations,
which can be caused by a number of factors
including conduction blocks in a bundle
branch and ventricular hypertrophy.
Based on the previous discussion, the follow-
ing rules can be used in interpreting the ECG:
1. A wave of depolarization (instantaneous
mean electrical vector) traveling toward
a positive electrode results in a posi-
tive deflection in the ECG trace. (Corol-
lary: A wave of depolarization traveling
away from a positive electrode results in a
negative deflection.)
2. A wave of repolarization traveling toward
a positive electrode results in a negative
deflection. (Corollary: A wave of repolari-
zation traveling away from a positive elec-
trode results in a positive deflection.)
3. A wave of depolarization or repolariza-
tion oriented perpendicular to an elec-
trode axis produces no
net
deflection.
4. The
instantaneous
amplitude
of
the
measured potentials depends upon the ori-
entation of the positive electrode relative
to the mean electrical vector.
5. Voltage amplitude (positive or negative) is
directly related to the mass of tissue under-
going depolarization or repolarization.
ECG Leads: Placement of
Recording Electrodes
The ECG is recorded by placing an array of
electrodes at specific locations on the body sur-
face. Conventionally, electrodes are placed on
each arm and leg, and six electrodes are placed
at defined locations on the chest. Three basic
types of ECG leads are recorded by these elec-
trodes: standard limb leads, augmented limb
leads, and chest leads. These electrode leads are
connected to a device that measures potential
differences between selected electrodes to pro-
duce the characteristic ECG tracings. The limb
leads are sometimes referred to as bipolar leads
because each lead uses a single pair of positive
and negative electrodes. The augmented leads
and chest leads are unipolar leads because
they have a single positive electrode with the
other electrodes coupled together electrically
to serve as a common negative electrode.
ECG LIMB LEADS
Standard limb leads are shown in Figure 2.17.
Lead I has the positive electrode on the left arm
and the negative electrode on the right arm,
therefore measuring the potential difference
across the chest between the two arms. In this
and the other two limb leads, an electrode on
the right leg is a reference electrode for record-
ing purposes. In the lead II configuration, the
positive electrode is on the left leg and the
■ FIGURE 2.17 Placement of the standard ECG
lim b leads (leads I, II, and III) and the location of
the positive and negative recording electrodes for
each of the three leads.
RA,
right arm;
LA,
left arm;
RL,
right leg;
LL,
left leg.
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