📖 ECG (Electrocardiogram) — Complete Beginner's Guide

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PART 1 — What Is an ECG?

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PART 2 — The Cardiac Conduction System (The Heart's Electrical Wiring)

SA Node → Atria → AV Node → Bundle of His → Left + Right Bundle Branches → Purkinje Fibers → Ventricular muscle

Key concept: The AV node introduces a deliberate delay — this is what you see as the PR interval on the ECG. It lets atria push blood into ventricles before ventricles contract.

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PART 3 — The ECG Paper & How to Read It

The Grid

• Horizontal axis = TIME: Each small box = 0.04 sec; each large box (5 small boxes) = 0.2 sec
• Vertical axis = VOLTAGE: Each small box = 0.1 mV; 10 mm = 1 mV

The 12 Leads

Memory trick: Think of leads as security cameras placed around a building — each sees the same event from a different angle.

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PART 4 — The ECG Waves: What Each One Means

Normal ECG Components

Understanding Q, R, S:

• Q wave = First downward deflection before the tall spike
• R wave = Tall upward spike (largest part of QRS)
• S wave = Downward deflection after the R wave

Pathological Q wave: A Q that is > 1 small box wide OR > 1/4 the height of the R wave = sign of old myocardial infarction (dead muscle)

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PART 5 — How to Calculate Heart Rate

Rate = 300 ÷ number of large boxes between two R waves

• 1 large box between R waves → rate = 300 bpm
• 2 large boxes → 150 bpm
• 3 large boxes → 100 bpm
• 4 large boxes → 75 bpm ✓ (normal)
• 5 large boxes → 60 bpm ✓ (normal)

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PART 6 — Systematic Approach to Reading Any ECG

1. Rate — Fast, slow, or normal? (< 60 = bradycardia; > 100 = tachycardia)
2. Rhythm — Regular or irregular? Is there a P before every QRS?
3. P waves — Present? Upright in lead II? One P per QRS?
4. PR interval — Normal (0.12–0.20 sec)?
5. QRS width — Narrow (< 0.12 sec) or wide (≥ 0.12 sec)?
6. ST segment — Elevated, depressed, or flat?
7. T waves — Upright? Inverted? Peaked?

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PART 7 — Diseases With ECG Changes

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🔴 1. Myocardial Infarction (Heart Attack)

Phase 1 — Hyperacute (minutes to 1 hour)

• Tall, peaked "hyperacute" T waves — earliest sign, often missed

Phase 2 — Acute (hours)

• ST elevation — the classic STEMI pattern (convex/"tombstone" shape)
• Reciprocal ST depression in opposite leads

Phase 3 — Hours to days

• T wave inversion develops
• Pathological Q waves appear (= dead muscle)

Phase 4 — Old/chronic

• Persistent Q waves (the permanent scar)
• ST returns to normal

Anterior STEMI ECG:

Inferior STEMI ECG:

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🟠 2. Atrial Fibrillation (AF)

• ❌ No P waves — replaced by irregular "fibrillatory" baseline
• Irregularly irregular RR intervals (hallmark sign — no two spaces between beats are equal)
• Narrow QRS (unless bundle branch block also present)
• Rate usually 100–160 bpm (uncontrolled)

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🟠 3. Atrial Flutter

• "Sawtooth" flutter waves at ~300 bpm — most visible in leads II, III, aVF
• Regular ventricular response at 150 bpm (2:1 block), 100 bpm (3:1), or 75 bpm (4:1)
• Narrow QRS complexes

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🟡 4. AV Heart Blocks (Conduction Blocks)

3rd degree block is a medical emergency — the ventricles use a slow escape rhythm (20–40 bpm) to survive.

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🟡 5. Bundle Branch Blocks (BBB)

Right Bundle Branch Block (RBBB):

• RSR' pattern in V1 ("rabbit ears" — two upward bumps)
• Wide S wave in I, V5, V6
• QRS ≥ 0.12 sec

Left Bundle Branch Block (LBBB):

• Broad, notched R wave in I, aVL, V5, V6 ("W" shape in V1, "M" shape in V6)
• Deep S wave in V1
• LBBB masks myocardial infarction changes — always treat new LBBB like a possible MI

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🟢 6. Pericarditis (Inflammation Around the Heart)

Key difference from MI: Pericarditis ST elevation is diffuse (all leads), saddle-shaped, and accompanied by PR depression. MI ST elevation is regional (specific territory) and convex.

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🟢 7. Electrolyte Disturbances

Hyperkalemia (High Potassium)

1. Tall, peaked/tented T waves (earliest sign)
2. PR prolongation, P wave flattening
3. Wide QRS
4. Sine wave pattern → Ventricular fibrillation → death

Hypokalemia (Low Potassium)

• Flat T waves
• Prominent U wave (a bump after the T wave — looks like QT prolongation)
• ST depression

Hypercalcemia (High Calcium)

• Short QT interval (calcium shortens ventricular repolarization)

Hypocalcemia (Low Calcium)

• Long QT interval (calcium prolongs repolarization)

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🔵 8. Pulmonary Embolism (PE)

• S wave in lead I
• Q wave in lead III
• T wave inversion in lead III

• Sinus tachycardia (most common)
• Right bundle branch block pattern (right ventricle strain)
• T wave inversions in V1–V4

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🔵 9. Wolff-Parkinson-White (WPW) Syndrome

• Short PR interval (< 0.12 sec) — signal bypasses AV node delay
• Delta wave — a slurred upstroke at the beginning of the QRS (pre-excitation)
• Widened QRS
• Increased risk of supraventricular tachycardia

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🔵 10. Long QT Syndrome

• Congenital (channelopathy)
• Drugs (antiarrhythmics like amiodarone, antibiotics, antipsychotics)
• Hypokalemia, hypomagnesemia, hypocalcemia

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PART 8 — Quick Summary Table

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Key References

• Goldman-Cecil Medicine, International Edition — Electrocardiographic Waves & Standards
• Costanzo Physiology, 7th Edition — Electrocardiogram
• Braunwald's Heart Disease, 15th Edition — Electrocardiography
• Roberts & Hedges' Clinical Procedures in Emergency Medicine — Electrocardiography
• Harrison's Principles of Internal Medicine, 21st Edition — Cardiac Conduction System