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ECG (Electrocardiogram) — Complete Easy Guide

🫀 What Is an ECG?

"Electrical events of the cardiac pacemaker and the specialized conduction system are represented by the ECG at the body surface." — Miller's Anesthesia

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1. The ECG Paper

• A standard ECG records 10 seconds of cardiac activity
• It usually records all 12 leads simultaneously — each group recorded for 2.5 seconds

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2. The ECG Waveform — One Heartbeat

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🔵 P Wave — Atrial Depolarization

• The SA node fires → electrical impulse spreads across both atria
• Atria contract (atrial systole = "atrial kick")
• On ECG: small rounded upward bump
• Normal duration: < 0.12 sec (120 ms)
• Broad P wave = atrial enlargement or conduction delay

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🟡 PR Interval — AV Node Delay

• Measured from start of P wave → start of QRS
• Represents the pause at the AV node (gateway between atria and ventricles)
• This delay allows the ventricles to fill before contracting
• Normal: 0.09–0.20 sec (90–200 ms)

• 0.20 sec = 1st degree AV block (prolonged conduction)

• Short PR = Wolff-Parkinson-White (pre-excitation via accessory pathway)

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🔴 QRS Complex — Ventricular Depolarization

• Impulse travels: His bundle → left & right bundle branches → Purkinje fibers → ventricular muscle
• Both ventricles depolarize and contract simultaneously
• On ECG: the tallest, sharpest deflection
• Normal duration: 0.075–0.11 sec (75–110 ms)

Pathological Q waves (deep, wide) indicate prior myocardial infarction

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🟢 ST Segment — Ventricular "Plateau"

• Flat line between the end of QRS (J-point) and start of T wave
• Represents early ventricular repolarization
• Normally isoelectric (flat)
• ST elevation = acute injury/infarction (STEMI)
• ST depression = ischemia or subendocardial infarction

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🟠 T Wave — Ventricular Repolarization

• Ventricles reset electrically
• Normally upright (positive) in most leads
• Inverted T wave = ischemia, ventricular strain, or electrolyte abnormality
• Tall, peaked T waves = hyperkalemia or early MI (hyperacute T wave)

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⚪ U Wave — Small wave after T

• Small, often invisible upward deflection after the T wave
• Represents late repolarization of Purkinje fibers
• Prominent U waves → hypokalemia

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🔵 QT Interval — Full Ventricular Cycle

• Measured from start of QRS → end of T wave
• Includes both depolarization + repolarization
• Must be corrected for heart rate using Bazett's formula:

QTc = QT ÷ √(RR interval)

• Males: 390–450 ms
• Females: 390–460 ms

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3. Normal ECG Intervals at a Glance

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4. Heart Rate Calculation

HR = 60,000 ÷ RR interval (in ms)

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5. The 12 Leads — What They "See"

Limb Leads (Frontal Plane)

Precordial Leads (Horizontal Plane — chest)

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6. The Cardiac Conduction Pathway (Simplified)

SA Node → Atria (P wave)
    ↓
AV Node (PR interval delay)
    ↓
Bundle of His
    ↓
Right Bundle Branch   Left Bundle Branch
    ↓                        ↓
Purkinje Fibers → Ventricular Muscle (QRS complex)
    ↓
Ventricular Repolarization (T wave)

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7. Key ECG Patterns to Recognize

✅ Normal Sinus Rhythm

• Regular P wave before every QRS
• Rate 60–100 bpm
• PR interval 120–200 ms
• QRS < 110 ms

🔴 STEMI (Heart Attack — Acute MI)

• ST elevation ≥ 1 mm in ≥ 2 contiguous leads
• Tombstone morphology (ST merges with T wave)
• Pathological Q waves develop over hours
• Reciprocal ST depression in opposite leads

⚡ Ventricular Tachycardia (VT)

• Wide QRS complexes (> 120 ms)
• No visible P waves
• Rate > 100 bpm
• Life-threatening

💗 Atrial Fibrillation (AF)

• No P waves — replaced by chaotic fibrillatory baseline
• Irregularly irregular RR intervals
• Narrow QRS (unless aberrant conduction)

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8. Systematic Approach to Reading Any ECG

1. Rate — Count beats (fast/normal/slow?)
2. Rhythm — Regular or irregular? P before every QRS?
3. P wave — Present? Shape? Duration < 120 ms?
4. PR interval — Normal 120–200 ms? Prolonged = AV block?
5. QRS complex — Width < 110 ms? Pathological Q waves?
6. ST segment — Elevation or depression?
7. T wave — Upright or inverted? Peaked?

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Quick Memory Trick for Waves

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