ECG read in simple language

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How to Read an ECG - In Plain Language

What Is an ECG?

An ECG (electrocardiogram) is a recording of the electrical activity of your heart, picked up by electrodes placed on the skin. Every time the heart beats, a tiny electrical wave travels through it - the ECG captures that wave and draws it on paper (or a screen). It does not measure pumping force directly; it measures the electrical signals that trigger each heartbeat.

Paper speed: 25 mm/second
Small box = 1 mm = 0.04 seconds
Large box = 5 mm = 0.20 seconds
Height (amplitude): 10 mm = 1 mV

(Harriet Lane Handbook)

Step 1 - Understand the Electrical Journey of One Heartbeat

Cardiac conduction system showing SA node, AV node, Bundle of His, bundle branches, and Purkinje fibers

Every heartbeat follows this exact path:

SA node (sinoatrial node) - the natural pacemaker, sits in the right atrium. It fires first.
Electricity spreads across both atria → atria squeeze (atrial contraction).
Signal arrives at the AV node (between atria and ventricles) → it deliberately slows down (a safety pause).
Signal travels down the Bundle of His → splits into right and left bundle branches → fans out into Purkinje fibers.
Both ventricles contract together, pumping blood to the lungs and body.
The ventricles recover (repolarize) and get ready for the next beat.

(Goldman-Cecil Medicine; Miller's Anesthesia)

Step 2 - The Waves on an ECG and What They Mean

ECG cardiac cycle showing P wave, QRS complex, T wave correlated with ventricular pressure, volume, and heart sounds

Each heartbeat draws this pattern on paper:

Wave / Segment	What It Represents	Normal Duration
P wave	Atria depolarize (fire) → atria contract	< 0.12 sec (3 small boxes)
PR interval	Time from atria firing to ventricles firing (AV node delay)	0.12 - 0.20 sec (3-5 large boxes)
QRS complex	Ventricles depolarize → ventricles contract	< 0.12 sec (3 small boxes)
ST segment	Pause between ventricular contraction and recovery	Flat = normal (on the baseline)
T wave	Ventricles repolarize (recover/reset)	Smooth, upright hump
QT interval	Total ventricular electrical activity (fire + recover)	< 0.44 sec (QTc)
U wave	Late ventricular repolarization (not always seen)	Small bump after T

Simple memory trick: "P fires atria. QRS fires ventricles. T resets ventricles."

(Guyton & Hall Textbook of Medical Physiology; Goldman-Cecil Medicine)

Step 3 - The Systematic 6-Step Reading Approach

When you pick up an ECG, always go through these steps in order. Never skip around.

1. Rate - How Fast Is the Heart Beating?

Quick method: Count the large boxes between two consecutive R waves (the tallest peak), then divide 300 by that number.

Boxes between R waves	Heart Rate
1 box	300 bpm
2 boxes	150 bpm
3 boxes	100 bpm
4 boxes	75 bpm
5 boxes	60 bpm
6 boxes	50 bpm

Normal: 60-100 bpm
Bradycardia: < 60 bpm (too slow)
Tachycardia: > 100 bpm (too fast)

(Harriet Lane Handbook)

2. Rhythm - Is the Beat Regular?

Are the R-R intervals (distance between peaks) equal throughout?
Is every QRS preceded by a P wave?
Is every P wave followed by a QRS?
Sinus rhythm = P wave before every QRS, P upright in leads I and aVF. This is normal.
Irregular rhythm with no visible P waves = think atrial fibrillation.

3. Axis - Which Direction Is the Electrical Wave Going?

The QRS axis tells you whether the heart's electrical wave travels in the correct direction through the ventricles.

Look at leads I and aVF.
Both positive (pointing up) = normal axis (~0° to +90°)
Lead I positive, aVF negative = left axis deviation (can mean left bundle branch block, LVH)
Lead I negative, aVF positive = right axis deviation (can mean RVH, pulmonary embolism)

4. Intervals - Are the Timing Gaps Normal?

Interval	What to Check	Abnormal Finding
PR interval	0.12-0.20 sec	Long = heart block; Short = WPW syndrome
QRS duration	< 0.12 sec	Wide = bundle branch block or ventricular origin
QTc	≤ 0.44 sec	Long QT = risk of fatal arrhythmia (Torsades)

(Harriet Lane Handbook)

5. P Wave - Is Atrial Activity Normal?

Should be small, rounded, and upright in most leads.
Normal height < 3 mm, duration < 0.09 sec.
Tall, peaked P in lead II = right atrial enlargement ("P pulmonale").
Broad, notched P in lead II = left atrial enlargement ("P mitrale").
No P waves at all = atrial fibrillation or junctional rhythm.

6. QRS and ST-T Changes - Is the Heart Muscle in Trouble?

This is where you look for the big emergencies:

Finding	Think of
ST elevation (above baseline)	Heart attack (STEMI) - call for help immediately
ST depression	Ischemia (poor blood flow), NSTEMI
Inverted T waves	Ischemia, ventricular strain
Wide QRS (> 0.12 sec)	Bundle branch block, ventricular tachycardia
Tall R in V1, deep S in V6	Right ventricular hypertrophy
Tall R in V5/V6, deep S in V1	Left ventricular hypertrophy
Peaked tall T waves	Hyperkalemia (high potassium - dangerous)
Tiny voltages everywhere	Pericardial effusion, obesity, hypothyroidism

Step 4 - Common Rhythms Made Simple

Rhythm	What You See	Key Feature
Normal sinus rhythm	Regular, P before every QRS, rate 60-100	All steps look normal
Sinus tachycardia	Same but rate > 100	Fast but organized
Sinus bradycardia	Same but rate < 60	Slow but organized
Atrial fibrillation	No P waves, irregular R-R	Irregularly irregular
Atrial flutter	"Sawtooth" baseline, rate ~150	Regular, no true P waves
1st degree heart block	PR interval > 0.20 sec	Prolonged PR, otherwise normal
2nd degree block (Mobitz I)	PR gets longer and longer, then QRS drops	"Wenckebach" pattern
3rd degree (complete) block	P waves and QRS completely unrelated	P and QRS march independently
Ventricular tachycardia (VT)	Wide QRS, fast rate > 100, no P waves	Emergency
Ventricular fibrillation (VF)	Chaotic, no recognizable waves	Cardiac arrest

Quick Summary Checklist

When reading any ECG, ask these 6 questions in order:

Rate - Fast, slow, or normal?
Rhythm - Regular? P before every QRS?
Axis - Lead I and aVF both positive?
Intervals - PR, QRS, and QTc within normal limits?
P wave - Normal shape and size?
QRS / ST-T - Any widening, ST changes, or T wave abnormalities?

If all six are normal, the ECG is normal. If any one step is abnormal, that's your finding.

Sources: Guyton & Hall Textbook of Medical Physiology; Goldman-Cecil Medicine; Miller's Anesthesia (10th ed.); Harriet Lane Handbook (23rd ed.)

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ECG Quick Reference

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Here's what's inside the 2-page A4 guide:

Page 1

Schematic ECG waveform (P-QRS-T) with labelled intervals drawn from scratch
Normal intervals reference table (PR, QRS, QTc, P wave)
300-rule quick heart rate calculator
Section 1 - Rhythms: 12 rhythm abnormalities (sinus tachy/brady, AF, flutter, SVT, all degrees of heart block, VT, VF, Torsades) - color-coded red for life-threatening ones
Section 2 - ST/T changes: 9 patterns including STEMI, pericarditis, Brugada, Wellens', Osborn waves
Section 2a - STEMI territory localisation: 8 territories with culprit artery and reciprocal leads

Page 2

Section 3 - Conduction: LBBB, RBBB, LAFB, LPFB, bifascicular block, WPW
Section 4 - Electrolytes/metabolic: Hyperkalaemia, hypokalaemia, hypo/hypercalcaemia, hypo/hypermagnesaemia, hypothermia, digoxin toxicity
Section 5 - Chamber enlargement: LVH (Sokolow-Lyon + Cornell criteria), RVH, LAE, RAE
Section 6 - Miscellaneous: Pericarditis, tamponade, PE (S1Q3T3), low voltage, long/short QT, epsilon wave, early repolarisation, dextrocardia
Section 7 - Systematic 11-step checklist: Rate to U waves with normal vs. abnormal at a glance

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