An electrocardiogram (ECG) is a diagnostic tool that captures the electrical activity generated by the heart muscle as it contracts. This activity involves the movement of charged particles, creating electrical potentials that travel throughout the body. Electrodes placed on the skin detect these voltage fluctuations, which are then amplified and recorded by the ECG machine. The standard 12-lead ECG utilizes ten physical electrodes to create twelve distinct views, or vectors, of the heart’s electrical activity. Each lead provides a unique perspective on the magnitude and direction of the electrical impulses.
Defining Bipolar and Unipolar Measurement
The difference between the two main types of ECG leads lies in their measurement methodology. Bipolar leads measure the voltage between two distinct, active electrode sites on the body. This setup creates a direct line of sight between a positive and a negative pole, reflecting the electrical difference between these two points.
In contrast, unipolar leads measure the electrical potential at a single active electrode (the exploring electrode) against a calculated reference point. This reference point is a theoretical zero potential, achieved by averaging the electrical signals from several other electrodes. This calculated reference, often called the Wilson Central Terminal, acts as an indifferent pole. Since all electrical measurements require two points, these leads are technically quasi-unipolar, comparing one active point to a composite zero-reference point.
The Standard Bipolar Limb Leads
Only three of the twelve standard ECG leads are bipolar: the original limb leads known as Lead I, Lead II, and Lead III. These leads were first described by Willem Einthoven and are derived from electrodes placed on the right arm (RA), left arm (LA), and left leg (LL). They capture the heart’s electrical activity within the frontal plane of the body.
Each lead is defined by the specific pairing of its positive and negative electrodes. Lead I measures the potential difference between the left arm (positive) and the right arm (negative). Lead II measures the difference between the left leg (positive) and the right arm (negative). Lead III records the electrical activity between the left leg (positive) and the left arm (negative).
These three bipolar leads create a geometric representation known as Einthoven’s Triangle, an imaginary equilateral triangle surrounding the heart. This model illustrates the electrical relationship between the limb electrodes, with the heart positioned at the center. Einthoven’s Law states that the electrical potential of Lead I plus Lead III equals the potential of Lead II. The placement of the electrodes on the limbs acts as an extension of the electrical activity where the limbs meet the torso.
Distinguishing the Unipolar Leads
The remaining nine leads in a standard 12-lead ECG are unipolar, providing viewing angles in both the frontal and horizontal planes. These nine leads are divided into the augmented limb leads and the precordial leads.
Augmented Unipolar Limb Leads
The augmented unipolar limb leads (aVR, aVL, and aVF) complete the six views of the heart in the frontal plane. The letter ‘a’ stands for augmented, ‘V’ for voltage, and R, L, and F indicate the location of the active, positive exploring electrode (right arm, left arm, and left foot/leg). These leads use the same limb electrodes as the bipolar leads, but the ECG machine calculates their reference point by averaging the electrical input from the two limbs not involved in the active lead.
This calculation, originally developed by Goldberger, increases the voltage amplitude of the waveform, which is why they are called “augmented.” For example, in lead aVR, the right arm is the positive electrode, and the negative reference is the combined average of the left arm and left leg potentials. This system provides three additional viewing angles of the heart’s electrical field in the frontal plane.
Precordial (Chest) Unipolar Leads
The final six leads are the precordial, or chest, leads (V1 through V6). These unipolar leads each use a single active electrode placed directly on the chest wall. They are referenced against the Wilson Central Terminal, which averages the potentials from the right arm, left arm, and left leg electrodes.
The precordial leads offer a horizontal, or transverse, view of the heart, showing electrical forces moving from front to back. V1 and V2 are placed on either side of the sternum in the fourth intercostal space, viewing the septum. The remaining leads (V3 through V6) are placed across the chest to view the anterior and lateral walls of the left ventricle.