1. 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)

2. 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:
𝑉𝑤= 1 3 (𝑅𝐴+𝐿𝐴+𝐿𝐿)
In a 12-lead ECG, all leads except the limb leads are unipolar (aVR, aVL, aVF, 𝑉 1 , 𝑉 2 , 𝑉 3 , 𝑉 4 , 𝑉 5 , 𝑉 6 )

3. Electrodes and leads Electrode placement Electrode name RA LA LL V1 V2
On the right arm, avoiding thick muscle. RA
In the same location where RA was placed, but on the left arm. LA
In the same location where RL was placed, but on the left leg. LL
In the fourth intercostal space (between ribs 4 and 5) just to the right of the sternum (breastbone). V1
In the fourth intercostal space (between ribs 4 and 5) just to the left of the sternum. V2
Between leads V2 and V4. V3
In the fifth intercostal space (between ribs 5 and 6) in the mid-clavicular line. V4
Horizontally even with V4, in the left anterior axillary line. V5
Horizontally even with V4 and V5 in the midaxillary line. V6

4. Electrodes and leads

5. 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

6. Limb leads

7. 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.
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:
aVR = RA (LA + LL) = 2 3 (RA – VW)
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:
aVL = LA (RA + LL) = 2 3 (LA – VW)

8. Augmented limb leads
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:
aVF = LL (RA + LA) = 2 3 (LL – VW)
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

9. 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: ( 𝑉 1 , 𝑉 2 , 𝑉 3 , 𝑉 4 , 𝑉 5 , 𝑉 6 ). 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.

10. 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

11. From electrode to paper

12. 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.

13. Contiguity of leads Activity Leads Category Leads II, III and aVF
Look at electrical activity from the vantage point of the inferior surface (diaphragmatic surface of heart)
Leads II, III and aVF
Inferior leads‘
Look at the electrical activity from the vantage point of the lateral wall of left ventricle
I, aVL, V5 and V6
Lateral leads
Look at electrical activity from the vantage point of the septal surface of the heart (interventricular septum)
V1 and V2
Septal leads
Look at electrical activity from the vantage point of the anterior wall of the right and left ventricles (Sternocostal surface of heart)
V3 and V4
Anterior leads