An electrocardiogram (ECG or EKG) is a non-invasive medical test that measures the electrical activity of the heart. Sensors placed on the skin detect the electrical changes produced by the cardiac muscle as it depolarizes and repolarizes with each heartbeat. The resulting recording maps the heart’s rate and rhythm. The primary purpose of this test is to monitor cardiac function, helping to diagnose conditions such as irregular heartbeats (arrhythmias) and signs of a previous or current heart attack.
The Conceptual Framework of the 12-Lead System
The heart’s electrical system generates impulses that move in a three-dimensional path through the chest, much like a complex electrical wave. To capture this activity fully, a system of multiple views is necessary. The 12-lead ECG system provides twelve distinct perspectives of the heart’s electrical field, allowing for a comprehensive, quasi-three-dimensional analysis. This is achieved by combining the signals from just ten physical sensors placed on the patient’s body.
It is important to distinguish between an electrode and a lead, as the terms are often confused. An electrode is the physical, conductive pad or sticker placed on the skin that detects the electrical signal. A lead is the actual electrical view or signal recorded by the ECG machine, derived by measuring the voltage difference between two or more electrodes.
The twelve leads are organized into two primary electrical planes that dissect the heart’s activity. The six limb leads provide a view of the frontal plane, which is the vertical slice of the heart’s electrical activity. The other six leads, known as the precordial or chest leads, capture the horizontal plane, which is the electrical activity viewed as a transverse slice across the chest. The pattern of electrical current flow is often described using vectors, which show the direction and magnitude of the electrical wave.
The Six Frontal Plane Leads
The six frontal plane leads are derived from four electrodes placed on the limbs: the right arm (RA), left arm (LA), and left leg (LL). A fourth electrode on the right leg (RL) serves as a ground and does not contribute to the tracing. These leads are divided into two groups based on how the voltage is measured.
Standard Limb Leads
The first group consists of the three standard limb leads: Lead I, Lead II, and Lead III. These are bipolar leads, meaning they measure the electrical potential difference between two specific limb electrodes. Lead I measures the voltage between the left arm and the right arm. Lead II measures the difference between the left leg and the right arm, and Lead III measures the voltage between the left leg and the left arm. This relationship forms Einthoven’s triangle, which helps visualize these electrical axes.
Augmented Unipolar Leads
The second group includes the three augmented unipolar limb leads: aVR, aVL, and aVF. These leads are unipolar because they measure the electrical potential at a single limb electrode relative to a calculated central reference point. Lead aVR views the electrical activity toward the right arm, while aVL looks toward the left arm. Lead aVF is oriented toward the left foot, or the inferior aspect of the heart. Leads II, III, and aVF are grouped as the inferior leads, viewing the bottom surface of the heart.
The Six Horizontal Plane Leads
The six horizontal plane leads, also called precordial or chest leads, provide a transverse view of the heart’s electrical activity. Unlike the limb leads, these are all unipolar leads, with each of the six chest electrodes acting as a positive pole that records the electrical potential relative to the central reference point. These leads are labeled V1 through V6 and are placed across the chest wall to provide a sequential look at the anterior and lateral surfaces of the ventricles.
V1 and V2 Placement
V1 and V2 are placed in the fourth intercostal space (the space between the fourth and fifth ribs). V1 is positioned immediately to the right of the sternum, and V2 is placed immediately to the left of the sternum. These leads view the anteroseptal region, which includes the wall dividing the two ventricles.
V3 and V4 Placement
V4 is located in the fifth intercostal space along the midclavicular line, which runs vertically down from the middle of the collarbone. V3 is then placed midway in a straight line between the V2 and V4 positions. This specific arrangement allows V3 and V4 to provide a view of the heart’s anterior surface.
V5 and V6 Placement
V5 and V6 complete the horizontal plane view by looking at the lateral wall of the heart. V5 is placed at the same horizontal level as V4, but along the anterior axillary line (running down from the front fold of the armpit). V6 is placed at the same horizontal level as V4 and V5, but along the midaxillary line. Leads V5 and V6 are known as the lateral leads, providing a view of the side of the left ventricle.