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ECG Made Easy - A Complete Guide
What is an ECG?
An electrocardiogram (ECG) is a recording of the electrical activity of the heart at the body surface. When a cardiac impulse passes through the heart, electrical current spreads into surrounding tissues and all the way to the skin. Electrodes placed on opposite sides of the heart detect these potential differences and trace them as waveforms on graph paper. - Guyton and Hall Textbook of Medical Physiology
Step 1 - The Conduction System (What generates the ECG)
Before reading the tracing, understand what produces it:
The normal heartbeat starts at the sinoatrial (SA) node in the right atrium - the natural pacemaker. The impulse travels:
- SA node → both atria (produces the P wave)
- AV node → slows the impulse (produces the PR interval delay)
- Bundle of His → splits into right and left bundle branches
- Purkinje fibers → rapidly spreads to ventricular myocardium (produces the QRS complex)
- Ventricles then repolarize (produces the T wave)
Step 2 - The ECG Paper
The paper is a grid of 1 mm squares:
| Measurement | Value |
|---|---|
| 1 small box (horizontal) | 0.04 seconds (40 ms) |
| 1 large box (5 small boxes) | 0.20 seconds (200 ms) |
| 1 small box (vertical) | 0.1 mV |
| Standard calibration | 1 mV = 10 mm |
| Paper speed | 25 mm/second |
Heart rate calculation: Count large boxes between two R waves, divide 300 by that number. (e.g., 4 large boxes = 300 ÷ 4 = 75 bpm)
Step 3 - The Waveforms
P Wave
- Represents atrial depolarization
- Caused by the spread of the depolarization wave from the SA node through both atria
- Normal: upright in lead II, inverted in aVR
- Duration: 0.06 - 0.12 sec (up to 3 small boxes)
- Amplitude: 2-3 mm
QRS Complex
- Represents ventricular depolarization
- Q wave: first downward deflection
- R wave: first upward deflection
- S wave: downward deflection after the R wave
- Normal duration: up to 0.10-0.11 sec (2.5 small boxes)
- A wide QRS (>0.12 sec) suggests bundle branch block or abnormal conduction
T Wave
- Represents ventricular repolarization
- Normally concordant with QRS (upright where QRS is predominantly positive)
- Occurs 0.25-0.35 seconds after depolarization
- Inverted T waves can indicate ischaemia, LVH, or electrolyte abnormality
U Wave
- Small wave sometimes seen after the T wave
- Represents late phases of ventricular repolarization
- Prominent U waves: seen in hypokalaemia
ST Segment and J Point
- The J point is where QRS ends and ST segment begins
- Normally isoelectric (flat, at baseline)
- ST elevation = acute injury/STEMI
- ST depression = ischaemia or reciprocal change
Step 4 - Key Intervals (Normal Values)
| Interval | What it measures | Normal value |
|---|---|---|
| PR interval | Atrial → ventricular conduction (AV node delay) | 120-200 ms (3-5 small boxes) |
| QRS interval | Ventricular depolarization | ≤ 100-110 ms (<2.5 small boxes) |
| QT interval | Total ventricular depolarization + repolarization | Varies with rate; QTc ≤ 460 ms (women), ≤ 450 ms (men) |
| RR interval | Time between beats | Used to calculate heart rate |
Prolonged PR (>200 ms) = 1st degree AV block. Prolonged QRS (>120 ms) = bundle branch block. Prolonged QT = risk of Torsades de Pointes.
Step 5 - The 12 Leads
The 12-lead ECG views the heart from 12 different angles - like 12 camera positions:
Limb leads (frontal plane):
| Lead | View of the heart |
|---|---|
| I | Lateral (left side) |
| II | Inferior |
| III | Inferior |
| aVR | Right side (often negative in normal ECG) |
| aVL | Lateral (high left) |
| aVF | Inferior (foot) |
Precordial (chest) leads (horizontal plane):
| Lead | Position | What it sees |
|---|---|---|
| V1 | Right sternal border, 4th ICS | Right ventricle |
| V2 | Left sternal border, 4th ICS | Septum |
| V3-V4 | Anterior chest | Anterior wall |
| V5-V6 | Lateral chest | Lateral wall |
A positive deflection is recorded when a depolarization wavefront moves toward the positive pole of that lead. - Harrison's Principles of Internal Medicine, 22E
Step 6 - Electrical Axis
The QRS axis is the mean direction of ventricular depolarization in the frontal plane:
| Axis | Range | Clinical meaning |
|---|---|---|
| Normal | -30° to +100° | Normal |
| Left axis deviation (LAD) | More negative than -30° | LVH, left anterior fascicular block, inferior MI |
| Right axis deviation (RAD) | More positive than +100° | RVH, left posterior fascicular block, lateral MI, pulmonary embolism |
| Extreme axis | Beyond +180° | Electrode reversal, severe RVH |
Quick trick: If QRS is positive in lead I and positive in aVF → normal axis. Positive in I, negative in aVF → LAD. Negative in I, positive in aVF → RAD.
Step 7 - The Cardiac Cycle in One Picture
This diagram shows how the ECG waveforms map to mechanical cardiac events:
Key relationships:
- P wave = atrial systole begins (the "atrial kick")
- QRS = ventricular systole begins; mitral and tricuspid valves close (S1)
- T wave = ventricular repolarization; aortic and pulmonic valves close (S2)
Step 8 - Systematic Approach (The 6-Step Method)
Always read an ECG in order to avoid missing findings:
- Rate - fast (>100), normal (60-100), or slow (<60)?
- Rhythm - regular or irregular? Is there a P before every QRS?
- Axis - normal, left, or right deviation?
- PR interval - normal, prolonged (AV block), or short (WPW)?
- QRS - narrow (<0.12 s) or wide (BBB, ventricular rhythm)?
- ST-T changes - elevation, depression, T wave inversion?
Quick Reference Cheat Sheet
| Finding | Think of |
|---|---|
| No P waves + irregular rhythm | Atrial fibrillation |
| Sawtooth baseline, regular | Atrial flutter (rate ~300 bpm, ventricular ~150 bpm) |
| Wide QRS, RBBB pattern (RSR' in V1) | Right bundle branch block |
| Wide QRS, LBBB pattern (broad notched R in V5/V6) | Left bundle branch block |
| ST elevation (convex) | STEMI - acute MI |
| ST depression + T inversion | Ischaemia / NSTEMI |
| Tall peaked T waves | Hyperkalaemia (early sign) |
| Prolonged QT | Hypokalaemia, hypocalcaemia, drugs (e.g. amiodarone) |
| Short PR + delta wave | Wolff-Parkinson-White (WPW) |
| Electrical alternans | Cardiac tamponade |
Sources: Guyton and Hall Textbook of Medical Physiology; Harrison's Principles of Internal Medicine, 22E; Miller's Anesthesia, 10e
For a video walkthrough, this 13-minute overview by Rhesus Medicine is excellent:
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