A comprehensive guide to 12-lead ECG interpretation. Learn the mechanics of bipolar and unipolar leads, Einthoven’s Law, and how to map ECG leads to specific coronary artery territories during myocardial infarction.
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| Normal ECG Waveforms, 12-Lead Placement Guide, Einthoven’s Law, and Heart-Coronary Summary Diagram |
ECG (Electrocardiogram):
An ECG (Electrocardiogram) consists of 12 leads. These are divided into two main types: bipolar leads and unipolar leads. Bipolar leads include Lead I, Lead II, and Lead III, while unipolar leads include augmented limb leads (aVR, aVL, aVF) and chest leads (V1–V6).
Bipolar leads are also called Einthoven leads. In these leads, both ends of the lead are active. Einthoven considered the human body as an equilateral triangle formed by the right arm (RA), left arm (LA), and left foot (LF). This is known as Einthoven’s triangle. Current always flows from the negative end to the positive end.
In Lead I, the right arm acts as the negative end and the left arm acts as the positive end. In Lead II, the right arm is the negative end and the left foot is the positive end. In Lead III, the left arm is the negative end and the left foot is the positive end. Bipolar leads measure the potential difference between two active ends.
According to Einthoven’s law:
Lead I + Lead III = Lead II
Augmented leads are unipolar limb leads and were developed by Goldberger. Their voltage amplitude is increased by 1.5 times compared to bipolar leads. These leads are aVR, aVL, and aVF.
In aVR, the right arm is the positive end, while the left arm and left foot together form the negative end.
In aVL, the left arm is the positive end, while the right arm and left foot form the negative end.
In aVF, the left foot is the positive end, while the right arm and left arm form the negative end.
In unipolar leads, only one end is active while the other end becomes inactive, called the central terminal or inactive terminal. Therefore, unipolar leads measure the potential difference between one active end and one inactive end.
The six limb leads provide a frontal plane view of the heart, while the six chest leads provide a horizontal plane view. These 12 leads observe the electrical activity of the heart from different angles, similar to CCTV cameras viewing the heart from multiple directions.
Different ECG leads represent different surfaces of the heart:
Lead II, Lead III, and aVF view the inferior surface of the heart and are called inferior leads. These leads are important in diagnosing inferior myocardial infarction (MI).
V1 and V2 lie over the interventricular septum and are called septal leads.
V3 and V4 observe the anterior surface of the heart and are called anterior leads.
V1 to V4 together are called anteroseptal leads.
V5, V6, Lead I, and aVL observe the lateral surface of the heart and are called lateral leads.
ECG is also called poor man’s echocardiography because it provides significant information about cardiac electrical activity in a simple and inexpensive way.
The coronary artery supply corresponding to ECG leads includes:
LAD (Left Anterior Descending artery) commonly associated with V1–V4.
LCX (Left Circumflex artery) commonly associated with V5, V6, Lead I, and aVL.
RCA (Right Coronary Artery) commonly associated with Leads II, III, and aVF.
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