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ECG from Zero — Complete Beginner's Guide

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1. What Is an ECG?

The ECG does not measure mechanical contraction — it measures electrical events. But because electricity drives contraction, the two are tightly coupled.

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2. The Cardiac Conduction System

SA Node (sinoatrial node)
    ↓  fires spontaneously ~60-100/min — the natural pacemaker
Atria depolarize → produce P wave
    ↓
AV Node (atrioventricular node) — DELAY (~0.12s) — protects ventricles
    ↓
Bundle of His
    ↓
Right bundle branch → Right ventricle
Left bundle branch (splits into anterior & posterior fascicles) → Left ventricle
    ↓
Purkinje fibers → Myocardium depolarizes → QRS complex
    ↓
Ventricles repolarize → T wave

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3. The ECG Waveform — Every Component Explained

The ECG Grid

• 1 small box = 0.04 seconds (time) and 0.1 mV (voltage)
• 1 large box = 0.20 seconds (5 small boxes)
• Paper runs at 25 mm/second (standard)

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4. The 12 Leads — The Heart's 12 Viewpoints

Electrode Placement

• Bipolar: Lead I (RA–LA), Lead II (RA–LL), Lead III (LA–LL)
• Augmented unipolar: aVR (right arm), aVL (left arm), aVF (left foot)

What Each Group "Sees"

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5. The Electrical Axis

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6. The Systematic Approach to Reading an ECG

Step 1 — Rate

• 1 large box = 300 bpm
• 2 large boxes = 150 bpm
• 3 large boxes = 100 bpm
• 4 large boxes = 75 bpm
• 5 large boxes = 60 bpm

Step 2 — Rhythm

1. Is it regular or irregular?
2. Is there a P wave before every QRS?
3. Is the P wave normal (upright in II, inverted in aVR)?
4. Is the PR interval constant?

Step 3 — Axis (as above)

Step 4 — P Wave

• Present? Normal shape (< 120 ms, < 2.5 mm)?
• Notched P in II → P mitrale (left atrial enlargement)
• Peaked P in II > 2.5 mm → P pulmonale (right atrial enlargement)

Step 5 — PR Interval

• Short (< 120 ms) → accessory pathway (e.g., WPW), junctional rhythm
• Long (> 200 ms) → 1st degree AV block

Step 6 — QRS Complex

• Narrow (< 120 ms) = normal ventricular conduction
• Wide (> 120 ms) = bundle branch block, pacing, hyperkalemia, accessory pathway
• Q waves (pathological if > 40 ms wide or > 25% of QRS height) → prior infarction

Step 7 — ST Segment

• Elevation (> 1 mm in limb leads, > 2 mm in precordial leads) → STEMI, pericarditis, Brugada, early repolarization
• Depression (> 1 mm) → ischemia, NSTEMI, digitalis effect, LVH strain

Step 8 — T Wave

• Flat or inverted → ischemia, LVH, RBBB, hypokalemia, PE
• Peaked/hyperacute → hyperkalemia, early STEMI ("tombstoning" precursor)

Step 9 — QT Interval

• Corrected QT (QTc) = QT ÷ √(RR interval in seconds) — Bazett's formula
• Prolonged QTc > 440 ms (men) / 460 ms (women) → risk of Torsades de Pointes
• Causes: drugs (antiarrhythmics, antipsychotics, fluoroquinolones), hypokalemia, hypomagnesemia, hypocalcemia, congenital long QT syndrome

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7. Common Arrhythmias

Sinus Tachycardia

• Rate > 100 bpm, normal P waves, regular, normal PR
• Causes: fever, pain, anxiety, dehydration, PE, thyrotoxicosis

Sinus Bradycardia

• Rate < 60 bpm, normal P waves, regular
• Causes: athletes, hypothyroidism, inferior MI, beta-blockers, increased vagal tone

Atrial Fibrillation (AFib)

• Irregularly irregular rhythm
• No P waves — replaced by chaotic fibrillatory baseline
• Narrow QRS (unless aberrant conduction)
• Rate typically 100–160 bpm (uncontrolled)

AV Blocks

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8. Bundle Branch Blocks

Right Bundle Branch Block (RBBB)

• QRS > 120 ms
• rSR' ("M shape") in V1 — classic rabbit ear pattern
• Wide S wave in I, V5, V6
• Secondary T-wave inversion in V1–V3
• Causes: PE, RVH, congenital (ASD), ischemia, or normal variant

Left Bundle Branch Block (LBBB)

• QRS > 120 ms
• Broad, notched R wave in I, V5, V6 ("M" in lateral leads)
• Deep S wave / QS in V1
• No septal Q waves in I, V6
• Secondary T-wave inversion in V5–V6
• Causes: always pathological — LVH, dilated cardiomyopathy, LAD disease, hypertension

Mnemonic: MaRRoW (M shape in V1 = RBBB, W shape in V1 = LBBB) / WiLLiaM MoRRoW

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9. Ischemia and Infarction

Evolution of STEMI

1. Hyperacute T waves (earliest, minutes) — tall, peaked, broad
2. ST elevation (hours) — convex upward ("tombstone")
3. T wave inversion (hours–days)
4. Q waves (irreversible necrosis, days–weeks) — > 40 ms wide, > 25% of R height

Localization of MI by Lead Group

STEMI Criteria (ESC 2023)

• ≥ 2 mm ST elevation in ≥ 2 contiguous precordial leads
• ≥ 1 mm ST elevation in ≥ 2 contiguous limb leads
• New LBBB with symptoms = STEMI equivalent until proven otherwise

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10. Ventricular Hypertrophy

Left Ventricular Hypertrophy (LVH)

Right Ventricular Hypertrophy (RVH)

• Right axis deviation
• Tall R in V1 (R > S)
• Deep S in V5–V6
• ST/T changes in V1–V3
• Causes: pulmonary hypertension, mitral stenosis, cor pulmonale, congenital heart disease

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11. Electrolyte Abnormalities on ECG

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12. Putting It All Together — The 9-Step Checklist

1. RATE        → Tachycardia? Bradycardia? Normal (60–100)?
2. RHYTHM      → Regular? Irregular? P before every QRS?
3. AXIS        → Lead I & aVF method
4. P WAVES     → Present, normal shape, one per QRS?
5. PR INTERVAL → 120–200 ms? Constant?
6. QRS         → < 120 ms? Pathological Q waves?
7. ST SEGMENT  → Elevation? Depression?
8. T WAVES     → Peaked? Inverted? Flat?
9. QT INTERVAL → Correct for rate, < 440/460 ms?

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Summary of Normal Values

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Sources

• Harrison's Principles of Internal Medicine, 22nd Ed. — Figures 240-1, 240-4, 240-10
• Tintinalli's Emergency Medicine — ECG interpretation chapter
• Miller's Anesthesia, 10th Ed. — Electrical events and the ECG