negative electrode is on the right arm. Lead III
has the positive electrode on the left leg and the
negative electrode on the left arm. These three
limb leads roughly form an equilateral triangle
(with the heart at the center), called Einthoven
triangle in honor of Willem Einthoven who
developed the ECG in 1901. Whether the
limb leads are attached to the end of the limb
(wrists and ankles) or at the origin of the limbs
(shoulder and upper thigh) makes virtually no
difference in the recording because the limb
can be viewed as a wire conductor originating
from a point on the trunk of the body.
When using the ECG rules described in
the previous section, a wave of depolariza-
tion heading toward the left arm gives a posi-
tive deflection in lead I because the positive
electrode is on the left arm. Maximal positive
deflection of the tracing occurs in lead I when
a wave of depolarization travels parallel to the
axis between the right and left arms. If a wave
of depolarization heads away from the left
arm, the deflection is negative. In addition,
a wave of repolarization moving away from
the left arm is seen as a positive deflection.
Similar statements can be made for leads
II and III, with which the positive electrode
is located on the left leg. For example, a wave
of depolarization traveling toward the left
leg gives a positive deflection in both leads
II and III because the positive electrode for
both leads is on the left leg. A maximal posi-
tive deflection is obtained in lead II when the
depolarization wave travels parallel to the axis
between the right arm and left leg. Similarly,
a maximal positive deflection is obtained in
lead III when the depolarization wave travels
parallel to the axis between the left arm and
left leg.
If the three limbs of Einthoven triangle are
broken apart, collapsed, and superimposed
over the heart (Fig. 2.18), the positive elec-
trode for lead I is defined as being at zero
degrees relative to the heart (along the hori-
zontal axis; see Fig. 2.18). Similarly, the posi-
tive electrode for lead II is +60° relative to the
heart, and the positive electrode for lead III is
+120° relative to the heart, as shown in Fig-
ure 2.18. This new construction of the electri-
cal axis is called the axial reference system.
Although the designation of lead I as being 0°,
lead II as being +60°, and so forth is arbitrary,
it is the accepted convention. With this axial
reference system, a wave of depolarization
oriented at +60° produces the greatest positive
deflection in lead II. A wave of depolarization
oriented +90° relative to the heart produces
equally positive deflections in both leads II
and III. In the latter case, lead I shows no net
deflection because the wave of depolarization
is heading perpendicular to the 0°, or lead I,
axis (see ECG rules).
Three augmented limb leads exist in addi-
tion to the three bipolar limb leads described.
Each of these leads has a single positive elec-
trode that is referenced against a combination
of the other limb electrodes. The positive elec-
trodes for these augmented leads are located
on the left arm (aVL), the right arm (aVR), and
the left leg (aVF; the “F” stands for “foot”).
In practice, these are the same positive elec-
trodes used for leads I, II, and III. (The ECG
■ FIGURE 2.18 Transformation of leads I, II, and III from Einthoven triangle into the axial reference system. Leads
I, II, and III correspond to 0°, +60°, and +120° in the axial reference system.
right arm;
left arm;
left leg.
previous page 46 Cardiovascular Physiology Concepts  2nd Edition read online next page 48 Cardiovascular Physiology Concepts  2nd Edition read online Home Toggle text on/off