- 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.
An ECG lead system measures heart electrical signals using surface electrodes at standardized positions. Those electrodes sense the surface potential produced by the heart; they do not carry the heart’s internal voltage out of the chest. Standardized positions are used so ECG recordings can be compared. The usual 12-lead set groups electrodes into three 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 the Einthoven lead system. Two electrodes record each bipolar ECG signal. From four body locations, 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. The right leg is the usual ground or driven-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, Einthoven’s law states that the Lead II voltage equals Lead I plus Lead III, including the R-wave amplitudes when they are measured that way.
Augmented Unipolar Leads
Wilson introduced the central terminal used in unipolar limb leads. Goldberger later formed the augmented unipolar limb leads aVR, aVL and aVF. 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 remaining limb electrode forms the exploratory electrode. Augmentation makes the ECG voltage larger than Wilson’s unipolar limb lead. The three connections are:
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 axillary line,
V6 – Same as V4 position but on mid axillary line.
