- ECG Lead System Definition: The ECG lead system is defined as a method to measure heart electrical signals using surface electrodes placed at standardized positions on the body.
- Bipolar Limb Leads: These leads, part of the Einthoven lead system, record electrical potential differences between electrodes on the limbs.
- Einthoven Triangle: This triangle is formed by electrodes on the right arm, left arm, and left leg, helping to measure the heart’s electrical vector in two dimensions.
- Augmented Unipolar Leads: Introduced by Wilson, these leads use a single exploratory electrode with a central terminal to measure heart activity.
- Chest Leads: These leads are placed at specific points on the chest to capture detailed electrical activity from different parts of the heart.
Heart electrical signals are measured using surface electrodes. These electrodes detect the potential in the heart and conduct it to the body surface. Standardized positions are used to ensure accurate ECG recordings. There are three main types of electrode systems:
- Bipolar limb leads or standard leads
- Augmented unipolar limb leads
- Chest leads
Bipolar Limb leads – Standard Lead I, Lead II and Lead III
This lead system is also known as Einthoven lead system. Two electrodes record the ECG signal. As shown in figure from four body locations of our body namely Right Arm(RA), Left Arm(LA), Right Leg(RL) and Left Leg(LL) potentials are recorded.
Final output is the difference between the electrical potential generated between these two electrodes. Right leg is the ground reference electrode.
Lead I
Voltage drop occurs from left arm to right arm. Generated voltage is VI.
Lead II
Voltage drop occurs from left leg to right arm. Generated voltage is VII.
Lead III
Voltage drop occurs from left leg to left arm. Generated voltage is VIII.
Einthoven Triangle
The Einthoven triangle is the closed path formed by the right arm, left arm, and left leg electrodes. Einthoven defined that the cardiac electric vector is two-dimensional along the body’s frontal plane. According to Kirchhoff’s law, the R wave amplitude along Lead III equals the sum of the R wave amplitudes along Lead I and Lead II.
Augmented Unipolar Leads
Wilson introduced the augmented unipolar limb lead system. This system records ECG using a single exploratory electrode and a central terminal, which relates to the body’s center. Two equal and large resistors connect to a pair of limb electrodes, forming the central terminal. The rest of the limb electrode forms the exploratory electrode. This system detects small increases in ECG voltage and uses three types of connections:
Lead aVR
Two resistors are connected to left arm and left leg. The middle point of the resistor connection is connected with negative terminal. Right arm is connected with the positive terminal of the amplifier. Here also right leg acts as a reference terminal.
Lead aVL
Two resistors are connected to right arm and left leg. The middle point of the resistor connection is connected with negative terminal. Left arm is connected with the positive terminal of the amplifier. Right leg acts as a reference terminal.
Lead aVF
Two resistors connect to the right arm and left arm, with their midpoint connected to the negative terminal. The left leg connects to the amplifier’s positive terminal. The right leg serves as a reference terminal.
Chest Leads
V1 – Fourth intercostal space of right sternal margin,
V2 – Fourth intercostal space at left sternal margin,
V3 – Midpoint between V2 and V4,
V4 – Fifth intercostal space at mid – clavicular line,
V5 – Same as V4 position but on anterior auxiliary line,
V6 – Same as V4 position but on mid auxiliary line.
