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Electrodes and leads
 Ten electrodes are used for 12-lead ECG
the electrodes usually consist of a
conducting gel , embedded in the middle
of a self-adhesive pad. The most common
type of electrodes is silver/silver chloride
 The term “lead” in the ECG refers to the
12 different vectors along which the
heart’s depolarization is measured and
recorded.
 There are a total of six limb leads and
augmented limb leads arranged like a
spokes of wheel in the coronal plane
(vertical) and six precordial leads that lie
on the perpendicular transverse plane
(horizontal)
Electrodes and leads
Each of these leads represents the electrical potential difference
between tow points. For each lead, the positive pole is one of the
ten electrodes. In bipolar leads, the negative pole is a different one
of the electrodes, while in unipolar leads, the negative pole is a
composite pole known as Wilson's central terminal
Wilson’s central terminal VW is produced by averaging the
measurements from the electrodes RA, LA, and LL to give an
average potential across the body:
1
𝑉𝑤 = (𝑅𝐴 + 𝐿𝐴 + 𝐿𝐿)
3
In a 12-lead ECG, all leads except the limb leads are
unipolar (aVR, aVL, aVF, 𝑉1 , 𝑉2 , 𝑉3 , 𝑉4 , 𝑉5 , 𝑉6 )
Electrodes and leads
Electrode
name
Electrode placement
RA
On the right arm, avoiding thick muscle.
LA
In the same location where RA was placed, but on the left arm.
LL
In the same location where RL was placed, but on the left leg.
V1
In the fourth intercostal space (between ribs 4 and 5) just to the right
of the sternum (breastbone).
V2
In the fourth intercostal space (between ribs 4 and 5) just to the left of
the sternum.
V3
Between leads V2 and V4.
V4
In the fifth intercostal space (between ribs 5 and 6) in the midclavicular line.
V5
Horizontally even with V4, in the left anterior axillary line.
V6
Horizontally even with V4 and V5 in the midaxillary line.
Electrodes and leads
Limb leads
Leads Ι, ΙΙ, ΙΙΙ are called the limb leads. The electrodes that form these
signals are located on the limbs: one on each arm and one on the left
leg. The limb leads form the points of what is known as Einthoven’s
triangle
 Lead Ι is the voltage between the (positive) left arm (LA) electrode
and the right arm (RA) electrode:
I = LA - RA
 Lead ΙΙ is the voltage between the (positive) left leg (LL) electrode
and the right arm (RA) electrode:
II = LL - RA
 Lead III is the voltage between the (positive) left leg (LL) electrode
and the left arm (LA) electrode:
III = LL - LA
Limb leads
Augmented limb leads
Lead aVR, aVL, aVF are the augmented limb leads they are derived
from the same three electrodes as leads I, II, III, but they use
Wilson’s central terminal as their negative pole.
o Lead augmented vector right (aVR) has positive electrode on
the right arm. The negative pole is a combination of the left arm
electrode and the left leg electrode:
1
2
aVR = RA - (LA + LL) = (RA – VW)
2
3
o Lead augmented victor left (aVL) has the positive electrode on
the left arm. The negative pole is a combination of the right arm
and the left leg electrode:
1
2
aVL = LA - (RA + LL) = (LA – VW)
2
3
Augmented limb leads
o Lead augmented victor foot (aVF) has the positive electrode on the
left leg. The negative pole is a combination of the right arm electrode
and the left arm electrode:
1
2
aVF = LL - (RA + LA) = (LL – VW)
2
3
Together with leads I, II, III, augmented limb leads aVR, aVL, aVF, form
the basis of the hex-axial reference system, which is used to calculate
the heart’s electrical axis in the frontal plane
Precordial leads
The precordial leads lie in the
transverse (horizontal) plane,
perpendicular to the other six
leads. The six precordial electrodes
act as the positive poles for the six
corresponding precordial leads:
(𝑉1 , 𝑉2 , 𝑉3 , 𝑉4 , 𝑉5 , 𝑉6 ). Wilson’s
central terminal is used as the
negative pole.
Additional electrodes may rarely be placed to generate other leads for
specific diagnostic purposes. Like right sided precordial lead, posterior
leads, Lewis lead, and another leads depends on the diagnostic
purposes.
From electrode to paper
The electric waves in the heart are recorded in millivolts by the
electrocardiograph. The waves are registered by electrodes
placed on certain parts of the body. Each electrode controls an
ink needle that writes on a grid paper. The higher the intensity
of the electric wave, the higher up the needle will move on the
paper. The paper moves at a certain speed beneath the needle,
resulting in an ink curve.
A standard 12-lead ECG report shows a 2.5 second tracing of
each of the twelve leads. The tracings are most commonly
arranged in a grid of four columns and three rows. the first
column is the limb leads (I,II, and III), the second column is the
augmented limb leads (aVR, aVL, and aVF), and the last two
columns are the precordial leads (V1-V6). An ECG curve has
different characteristics depending on the location of the
electrode recording it
From electrode to paper
Contiguity of leads
Each of the 12 ECG leads records the electrical activity of the heart
from a different angle, and therefore align with different anatomical
areas of the heart. Two leads that look at neighboring anatomical areas
are said to be contiguous. In addition, any two precordial leads next to
one another are considered to be contiguous. For example, though V4
is an anterior lead and V5 is a lateral lead, they are contiguous because
they are next to one another.
Contiguity of leads
Category Leads
Activity
Inferior
leads‘
Leads II,
III and
aVF
Look at electrical activity from the vantage
point of the inferior surface (diaphragmatic
surface of heart)
Lateral
leads
I, aVL,
V5 and
V6
Look at the electrical activity from the vantage
point of the lateral wall of left ventricle
Septal
leads
V1 and
V2
Look at electrical activity from the vantage
point of the septal surface of the heart
(interventricular septum)
Anterior
leads
V3 and V4 Look at electrical activity from the vantage
point of the anterior wall of the right and left
ventricles (Sternocostal surface of heart)