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Introduction

Electrocardiography (ECG) is a fundamental clinical skill used to assess the electrical activity of the heart. Although ECG tracings may initially appear complex, a structured and systematic approach makes interpretation much more manageable.

This guide is designed to help beginners understand ECG basics and develop a step-by-step method for accurate interpretation.

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How an ECG Works

Each heartbeat is initiated by an electrical impulse that travels through the heart muscle, triggering contraction. The ECG machine records this electrical activity and displays it as waveforms on graph paper.

Understanding how to read these waveforms is essential for identifying normal and abnormal heart rhythms.

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ECG Paper Basics

To interpret an ECG correctly, you must understand the layout of the paper.

• Paper speed: 25 mm per second
• Horizontal axis: Time
• Vertical axis: Voltage

Time Measurements

• 1 small square = 0.04 seconds
• 1 large square (5 small squares) = 0.20 seconds
• 5 large squares = 1 second

Voltage

• 1 small square vertically = 0.1 mV

For beginners, focus primarily on time intervals rather than voltage.

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The ECG Waveform

A normal heartbeat consists of three key components:

• P wave – Atrial depolarization (atria contract)
• QRS complex – Ventricular depolarization (ventricles contract)
• T wave – Ventricular repolarization (ventricles relax)

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A Systematic 5-Step Approach

Using a consistent sequence helps prevent errors and ensures nothing is overlooked.

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Step 1: Determine the Heart Rate

Method 1 (6-second rule):

• Count QRS complexes in 6 seconds (30 large squares)
• Multiply by 10

Method 2 (300 rule):

• 300 ÷ number of large squares between two R waves (for regular rhythms)

Interpretation:

• <60 bpm: Bradycardia
• 60–100 bpm: Normal
• 100 bpm: Tachycardia

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Step 2: Assess Rhythm Regularity

Examine the spacing between QRS complexes:

• Equal spacing → Regular rhythm
• Unequal spacing → Irregular rhythm

A practical trick is to mark R–R intervals on paper and compare across the strip.

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Step 3: Evaluate P Waves

Determine whether atrial activity is normal:

• One P wave before each QRS → Sinus rhythm
• Absent or abnormal P waves → Possible arrhythmia

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Step 4: Measure QRS Duration

• Narrow QRS (<3 small squares): Impulse originates above ventricles
• Wide QRS (≥3 small squares): Ventricular origin or conduction delay

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Step 5: Analyze P–QRS Relationship

• 1:1 relationship → Normal conduction
• More P waves than QRS → Suggests heart block

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Common ECG Patterns

Normal Sinus Rhythm

• Rate: 60–100 bpm
• Regular rhythm
• P wave before every QRS
• Narrow QRS

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Sinus Bradycardia / Tachycardia

• Same pattern as normal sinus rhythm
• Rate is slower or faster

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Atrial Fibrillation

• Irregularly irregular rhythm
• No distinct P waves
• Chaotic baseline

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Atrial Flutter

• Regular rhythm
• Characteristic sawtooth pattern

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Supraventricular Tachycardia (SVT)

• Rapid, regular rhythm
• Narrow QRS
• P waves often not visible

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Ventricular Tachycardia (VT)

• Fast and regular
• Wide QRS
• Requires urgent medical attention

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Heart Blocks

• Disrupted conduction between atria and ventricles
• More P waves than QRS complexes

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The ECG Interpretation Checklist

Always follow this sequence:

1. Rate
2. Rhythm
3. P waves
4. QRS duration
5. P–QRS relationship

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Conclusion

ECG interpretation is a skill that improves with consistent practice. By applying a structured method and focusing on pattern recognition, even complex rhythms become easier to understand over time.

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