Interpreting a 12-Lead Electrocardiogram

By Carmen Corder, DNP, CCRN | Medically reviewed by Dominic Velasco, MD on August 17, 2024

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Introduction to 12-Lead Electrocardiograms
Inferior Leads
Lateral Leads
Chest Leads (Precordial Leads)
Summary

Article at a Glance

A 12-lead electrocardiogram records the heart’s electrical activity from 12 different areas.
The right coronary artery and left coronary artery are the key places affected during myocardial infarctions.
Clinicians will learn how to look for a STEMI on a 12-lead electrocardiogram.

Introduction to 12-Lead Electrocardiograms

The 12-lead electrocardiogram (ECG) is an essential diagnostic tool. Clinicians must evaluate a 12-lead ECG to look for myocardial infarction (STEMI) in each of the 12 leads.

Electrocardiogram Lead Equipment

The diagram below illustrates approximate lead placement. Each lead provides a different picture of the heart, allowing clinicians to observe heart activity from 12 angles.

The 12 leads are placed at specific locations on the body.

Clinicians check if the patient has a STEMI or elevated ST segment in each of the 12 leads.

A STEMI is an ST elevated myocardial infarction. The ST segment is elevated.

Read: An Easy Way to Interpret Arterial Blood Gases

Inferior Leads

The three inferior leads are II, III, and AVF. They’re located at the bottom of the heart. It’s where clinicians first look for a STEMI. If a STEMI is present in one or more of the inferior leads, it usually indicates a blockage somewhere in the right main coronary artery.

The coronary arteries supply blood to the heart. Blockage in the coronary arteries can cause myocardial infarction.

When a patient has a STEMI in II, III, or AVF, clinicians must rule out the possibility of a right ventricular infarction before administering vasodilators like morphine and nitroglycerin. If the patient has a right ventricular infarction, their preload is significantly reduced and administering vasodilators may lead to complications.

Lateral Leads

The next leads are I and AVL. They look at the high lateral wall. The “L” in AVL stands for “left.” If there is an ST elevation in lead I or AVL, the patient has a high lateral STEMI. In this case, there’s a blockage somewhere in the left main artery.

Chest Leads (Precordial Leads)

Clinicians last look for a STEMI via the chest leads, also called precordial leads. All of the chest leads are placed across the heart. There are six of these “V” leads: V₁, V₂, V₃, V₄, V₅, and V₆.

V₁ and V₂ look at the septum. If there is an ST elevation in V₁ and V₂, the patient is having a septal STEMI.
The next leads are V₃ and V₄. These two leads are placed in front of the heart. An ST elevation in V₃ and V₄ indicates an anterior STEMI.
V₅ and V₆ are the two leads on the lateral left side of the heart. V₅ and V₆ look at the low lateral wall.

A STEMI typically affects a combination of areas in the heart. For example, a patient with a STEMI in V₁, V₂, and V₃ has an anteroseptal myocardial infarction. The patient has STEMI in both the anterior section and the septal section.

If all the V, AVL, and I leads are involved, the patient has a massive anterior myocardial infarction.

A 12-lead electrocardiogram informs physicians of the heart’s electrical activity from 12 different angles.

Summary

A 12-lead ECG measures heart activity from 12 different angles. Clinicians begin with the inferior leads, then the lateral leads, and finally the chest leads, making sure to rule out STEMIs as they progress.

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