ECG Leads Explained

Introduction

Electrocardiography (ECG or EKG) is one of the most essential diagnostic tools in modern medicine. It provides a non-invasive method to evaluate the electrical activity of the heart, helping clinicians detect arrhythmias, conduction abnormalities, myocardial ischemia, infarction, and structural heart disease.

An accurate understanding of ECG leads—including limb leads, augmented leads, and chest (precordial) leads—is fundamental for interpreting the electrical patterns recorded on the ECG. Each lead represents the heart’s electrical activity from a unique angle, allowing a three-dimensional view of cardiac depolarization and repolarization.

This article provides a detailed, structured, and in-depth explanation of ECG leads, including their anatomical basis, electrical vectors, clinical significance, and interpretation nuances.

1. Fundamentals of ECG

Before discussing leads in detail, it is crucial to understand the basic principles of ECG:

1.1 Cardiac Electrical Activity

The heart generates electrical impulses originating from the sinoatrial (SA) node, propagating through the atria, atrioventricular (AV) node, His-Purkinje system, and ventricles.
Depolarization (activation) and repolarization (recovery) produce tiny electrical currents measurable at the skin surface.

1.2 ECG Waveforms

P wave: Atrial depolarization.
PR interval: Time from atrial depolarization to ventricular depolarization.
QRS complex: Ventricular depolarization.
ST segment: Plateau phase of ventricular action potential.
T wave: Ventricular repolarization.
U wave: Sometimes visible, late repolarization of Purkinje fibers.

1.3 Principles of Lead Placement

Electrodes on the body act as sensors detecting voltage differences.
ECG leads are not physical devices but conceptual lines between electrodes, each providing a unique vector of cardiac electrical activity.
Leads can be bipolar (measuring between two electrodes) or unipolar (measuring voltage relative to a reference point).

2. Limb Leads

Limb leads are placed on the right arm (RA), left arm (LA), and left leg (LL). Sometimes the right leg (RL) is used as a neutral ground. Limb leads provide frontal plane views of the heart.

2.1 Standard Bipolar Limb Leads (I, II, III)

Lead I

Measures the potential difference between left arm (+) and right arm (-).
Electrical vector: lateral direction (toward left lateral heart).
Clinical significance: Detects lateral wall ischemia or infarction (leads I, aVL, V5, V6).

Lead II

Measures the potential difference between left leg (+) and right arm (-).
Electrical vector: inferior direction (toward the apex of the heart).
Most commonly used for rhythm monitoring (SA node, AV node, and atrial activity).

Lead III

Measures the potential difference between left leg (+) and left arm (-).
Electrical vector: inferior direction but slightly more rightward.
Useful in detecting inferior wall myocardial infarction (MI).

2.2 Einthoven’s Triangle

Conceptual triangle formed by the three bipolar limb leads.
Law: Lead I + Lead III = Lead II.
Provides the mathematical framework to calculate the electrical axis of the heart.

3. Augmented Unipolar Limb Leads (aVR, aVL, aVF)

Unipolar limb leads measure the voltage at a single electrode relative to a combination of the other two electrodes (central terminal).

3.1 aVR (augmented Vector Right)

Positive electrode: right arm.
Looks at the heart from the right shoulder toward the base of the heart.
Clinical significance: Usually shows inverted complexes; can help identify tricuspid or right-sided abnormalities, dextrocardia, or lead misplacement.

3.2 aVL (augmented Vector Left)

Positive electrode: left arm.
Provides a high lateral view of the heart.
Clinical significance: Lateral wall ischemia (in conjunction with lead I).

3.3 aVF (augmented Vector Foot)

Positive electrode: left leg (foot).
Provides an inferior view of the heart.
Clinical significance: Detects inferior wall MI (with leads II and III).

3.4 Importance of Augmented Leads

Complement the bipolar limb leads.
Provide a more complete frontal plane perspective.
Essential for accurate diagnosis of axis deviation and localized ischemia.

4. Chest (Precordial) Leads

Chest leads provide horizontal plane views of the heart. They are unipolar leads with the positive electrode placed on the chest wall and the negative reference at the central terminal (Wilson’s central terminal).

4.1 Standard Chest Lead Placement

Lead	Placement	Cardiac View
V1	4th ICS, right sternal border	Septal
V2	4th ICS, left sternal border	Septal
V3	Between V2 and V4	Anterior
V4	5th ICS, midclavicular line	Anterior/Apical
V5	5th ICS, anterior axillary line	Lateral
V6	5th ICS, midaxillary line	Lateral

4.2 Clinical Significance of Precordial Leads

V1–V2 (Septal): Detect septal infarction, bundle branch blocks, arrhythmias.
V3–V4 (Anterior): Identify anterior wall MI, left ventricular hypertrophy (LVH).
V5–V6 (Lateral): Detect lateral wall MI, LVH.

4.3 Vector Orientation in Chest Leads

Provides horizontal plane electrical mapping.
Combined with limb leads, allows 3D reconstruction of cardiac electrical activity.

5. Heart Electrical Axis

5.1 Definition

The electrical axis represents the general direction of ventricular depolarization in the frontal plane.
Normally between -30° to +90°.

5.2 Determination Using Limb Leads

Lead I and aVF are commonly used.
Positive QRS in both: Normal axis.
Lead I positive, aVF negative: Left axis deviation.
Lead I negative, aVF positive: Right axis deviation.

5.3 Clinical Significance of Axis Deviation

Left axis deviation: LVH, left anterior fascicular block, inferior MI.
Right axis deviation: RVH, pulmonary embolism, lateral MI.

6. Correlation Between ECG Leads and Myocardial Regions

Myocardial Region	Leads Involved
Inferior	II, III, aVF
Lateral	I, aVL, V5, V6
Septal	V1, V2
Anterior	V3, V4
Posterior	Reciprocal changes in V1–V2

Understanding this correlation allows clinicians to localize myocardial ischemia, infarction, or conduction defects accurately.

7. Special Lead Considerations

7.1 Right-Sided Leads

Placing V1–V6 on the right side of the chest evaluates right ventricular infarction.
Especially important in inferior wall MI to detect RV involvement.

7.2 Posterior Leads (V7–V9)

Detect posterior wall MI, which may appear as ST depression in V1–V3 (reciprocal changes).

7.3 Esophageal Leads

Occasionally used in patients with poor precordial recording.
Provide close proximity to left atrium and ventricles.

8. Practical Tips for ECG Interpretation

Confirm electrode placement: Misplacement can mimic axis deviation or infarction.
Compare limb and precordial leads: Provides comprehensive frontal and horizontal view.
Use reciprocal changes: Infer MI localization (e.g., ST elevation in II, III, aVF → look for reciprocal ST depression in I, aVL).
Assess QRS morphology: Bundle branch blocks, hypertrophy patterns, and conduction delays.
Evaluate T wave and ST segment: Critical for ischemia and electrolyte disturbances.

9. Clinical Relevance of Lead Analysis

Acute MI localization: Guides reperfusion therapy.
Arrhythmia diagnosis: P wave orientation in limb leads identifies atrial origin.
Left ventricular hypertrophy: Tall R waves in lateral precordial leads.
Bundle branch blocks: Alter QRS in V1–V6.
Pericarditis: Diffuse ST elevation and PR depression across multiple leads.

10. Advanced Concepts

10.1 Three-Dimensional View

Combining limb (frontal) and chest (horizontal) leads allows 3D cardiac electrical mapping.
Important in electrophysiology studies and ventricular mapping.

10.2 Vectorcardiography

Leads can be converted into vectors to understand magnitude and direction of depolarization.
Provides precise evaluation of conduction abnormalities.

10.3 Lead Misplacement Recognition

Common errors: RA and LA reversal → inverted P wave in lead I.
V1–V6 swapped → misinterpret anterior vs. lateral wall patterns.
Recognizing errors prevents misdiagnosis of MI or conduction blocks.

11. Teaching the Leads

For students and trainees, a few tips:

Memorize lead orientation:
- Limb leads: frontal plane.
- Chest leads: horizontal plane.
Understand regional correlation: Inferior, lateral, anterior, septal.
Practice axis calculation: Using I and aVF.
Recognize reciprocal changes: Key for MI localization.
Simulate ECG on a mannequin or digital simulator: Reinforces spatial understanding.