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How to Read an ECG - A Complete Systematic Guide

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Part 1: Understanding What an ECG Measures

The Cardiac Conduction System

• SA node fires first → produces the P wave
• Signal crosses the AV node (physiologic delay) → produces the PR interval
• Bundle branches + Purkinje fibers depolarize the ventricles → produces the QRS complex
• Ventricular repolarization → produces the T wave

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Part 2: The Basic ECG Waveforms and Intervals

• 1 small box = 1 mm = 40 ms (0.04 s)
• 1 large box = 5 mm = 200 ms (0.20 s)
• Voltage: 1 mV = 10 mm vertically

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Part 3: The 12 ECG Leads - What They "Look At"

Limb leads (frontal plane):

Chest (precordial) leads (horizontal plane):

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Part 4: A Systematic 14-Step Approach to Every ECG

1. Standardization/calibration - Is the ECG calibrated? Is the paper speed correct? Check for lead reversal artifacts.
2. Rhythm - Regular or irregular? Is every beat the same?
3. Heart rate - Count the R-R intervals. Formula: 300 ÷ number of large boxes between two R waves. Or: 1500 ÷ number of small boxes.
4. PR interval - Normal 120-200 ms. Short = pre-excitation (WPW). Prolonged = heart block.
5. QRS interval - Normal ≤110 ms. Wide QRS = bundle branch block, hyperkalemia, pre-excitation.
6. QT/QTc interval - Prolonged = risk of torsades de pointes (TdP).
7. Mean QRS axis - Normal is 0° to +90°.
8. P waves - Present? Normal morphology? One before every QRS?
9. QRS voltages - High = hypertrophy. Low = pericardial effusion, obesity, COPD.
10. Precordial R-wave progression - R waves should grow from V1 to V5/V6.
11. Abnormal Q waves - Q waves >40 ms wide or >25% of R-wave height = pathological (infarction).
12. ST segments - Elevated or depressed?
13. T waves - Inverted, peaked, or flattened?
14. U waves - Prominent? Inverted?

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Part 5: Determining Heart Rate

• Count large boxes between two consecutive R waves
• Divide 300 by that number
• 300, 150, 100, 75, 60, 50 (memorize this for 1, 2, 3, 4, 5, 6 large boxes)

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Part 6: Axis Determination

• Normal axis: 0° to +90° (leads I and aVF both positive)
• Left axis deviation (LAD): more negative than −30° to −45° (lead I positive, aVF negative)
• Right axis deviation (RAD): more positive than +90° to +110° (lead I negative, aVF positive)
• Extreme axis deviation: −90° to ±180° (leads I and aVF both negative)

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Part 7: ECG of a Normal Healthy Adult

• Upright P wave in lead II (sinus rhythm), inverted in aVR
• PR interval 120-200 ms
• Narrow QRS <110 ms
• Small r wave in V1, growing progressively to V4/V5 (R-wave progression)
• T waves concordant (same direction) with QRS

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Part 8: Hypertrophy Patterns

Left Ventricular Hypertrophy (LVH)

• S in V1 + R in V5 or V6 >35 mm (Sokolow-Lyon criterion)
• R in aVL >20 mm (women) or >28 mm (men)
• ST depression + T-wave inversion in leads with tall R waves (formerly "strain" pattern)
• Left atrial abnormality often coexists

Right Ventricular Hypertrophy (RVH)

• Tall R wave in V1 (R ≥ S in V1)
• Right axis deviation (>+90°)
• ST depression + T-wave inversion in right precordial leads (V1-V3)
• Causes: pulmonary hypertension, pulmonic stenosis

P-wave abnormalities

• Right atrial overload: Tall, peaked P waves ≥2.5 mm in lead II ("P-pulmonale")
• Left atrial abnormality: Broad (≥120 ms), notched P in limb leads; biphasic P in V1 with broad negative component ("P-mitrale")

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Part 9: Bundle Branch Blocks

Right Bundle Branch Block (RBBB)

• QRS ≥120 ms
• rSR' in V1 ("rabbit ears" or "M" pattern)
• Wide S waves in leads I, V5, V6
• T-wave inversion in V1-V3 (secondary change)
• Can be normal variant; also seen with atrial septal defect, PE, anterior MI

Left Bundle Branch Block (LBBB)

• QRS ≥120 ms
• Broad monophasic R in V5/V6 (no septal q wave)
• QS or rS in V1
• T-wave inversion in lateral leads (secondary, discordant)
• Usually indicates significant heart disease (ischemia, hypertension, cardiomyopathy, valvular disease)
• LBBB makes ischemia interpretation unreliable - use Sgarbossa criteria if MI suspected

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Part 10: Ischemia and Infarction Patterns

ST-Segment Elevation MI (STEMI)

• Definition: New ST elevation ≥1 mm in 2+ contiguous leads (≥2 mm in V1-V3)
• Associated with: tall, peaked "hyperacute" T waves early, then Q waves and T-wave inversions develop over hours-days
• Localization by leads:

• Reciprocal changes: ST depression in leads opposite the area of infarction (e.g., inferior STEMI with ST depression in I, aVL) strongly supports true ischemia.

Non-ST Elevation MI (NSTEMI) / Unstable Angina

• ST depression (especially in V1-V4 with horizontal/downsloping morphology)
• T-wave inversions (deep, symmetric inversions in anterior leads = LAD territory "Wellens pattern")
• No Q-wave formation

Q Waves (Pathological)

• Duration >40 ms (1 small box) wide
• Depth >25% of the R-wave in the same lead
• Present in 2+ contiguous leads
• Represent electrically silent (infarcted) myocardium
• Normal: small q waves in I, aVL, V5, V6 (septal q waves) are acceptable

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Part 11: Common Arrhythmia Recognition

Sinus Rhythms

• Normal sinus rhythm: Regular P-P and R-R intervals, P upright in II, inverted in aVR, rate 60-100 bpm
• Sinus tachycardia: Same as above, rate >100 bpm (pain, fever, hypovolemia, PE)
• Sinus bradycardia: Rate <60 bpm (athletes, vagal tone, hypothyroidism, beta-blockers)

Atrial Arrhythmias

• Atrial fibrillation (AF): Irregularly irregular rhythm, no distinct P waves, fibrillatory baseline (most common sustained arrhythmia)
• Atrial flutter: Regular sawtooth flutter waves at ~300 bpm, typically 2:1 or 4:1 ventricular rate (~150 or 75 bpm); best seen in II, III, aVF, V1
• SVT (AVNRT): Narrow complex tachycardia 150-250 bpm, P waves buried in or just after QRS

AV Blocks

Ventricular Arrhythmias

• PVCs: Wide QRS (>120 ms) without preceding P wave, bizarre morphology, followed by compensatory pause
• VT: ≥3 consecutive PVCs at rate >100 bpm; wide complex, AV dissociation
• VF: Chaotic irregular waveforms, no organized QRS - requires immediate defibrillation

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Part 12: Other Important ECG Patterns

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Summary: The Quick Systematic Read (Memory Trick: "Rate Rhythm Axis Intervals Morphology")

1. Rate - Fast, slow, or normal?
2. Rhythm - Regular? P before every QRS? QRS after every P?
3. Axis - Normal, left, right?
4. Intervals - PR, QRS, QT within limits?
5. Morphology - P wave shape, QRS morphology, ST-T changes, Q waves, R-wave progression, U waves

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