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The Complete ECG Guide - From Basics to Advanced Interpretation

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PART 1: THE ELECTRICAL FOUNDATION

Why Does the Heart Generate Electricity?

The Cardiac Conduction System

1. SA Node (sinoatrial node) - the dominant pacemaker, located in the high lateral right atrium near the superior vena cava. It fires spontaneously and fastest (60-100 bpm at rest), setting the heart rate.
2. Bachmann Bundle - a specialized tract that rapidly conducts the impulse to the left atrium.
3. AV Node (atrioventricular node) - the only normal electrical bridge between atria and ventricles. It creates a physiologic delay (allowing atria to finish contracting before ventricles begin). The AV valve rings are electrically insulated everywhere else.
4. Bundle of His - exits the AV node; capable of rapid conduction.
5. Bundle Branches - the His bundle splits into the right bundle branch and the left bundle branch. The left further divides into the left anterior fascicle and left posterior fascicle.
6. Purkinje System - the fine distal network that rapidly distributes the impulse throughout both ventricles, triggering near-simultaneous contraction.

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PART 2: THE ECG MACHINE AND LEAD PLACEMENT

What the ECG Records

Electrode Placement

Limb Electrodes (4 electrodes)

Precordial (Chest) Electrodes (6 electrodes - V1 through V6)

Clinical tip: In female patients, place leads V3-V6 under the left breast. Never use nipples as landmarks - they vary considerably between patients.

Important: The limb electrodes can be placed on upper arms and thighs rather than wrists and ankles, but must be consistent (e.g., don't put RA on the wrist and LA on the upper arm).

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PART 3: THE 12 LEADS AND WHAT THEY "SEE"

Limb Leads - View the Frontal Plane (from in front of the body)

• Lead I = Left Arm (+) minus Right Arm (-) → looks at the lateral left side of the heart
• Lead II = Left Leg (+) minus Right Arm (-) → looks inferiorly and is the most commonly used rhythm strip lead
• Lead III = Left Leg (+) minus Left Arm (-) → looks inferiorly

• aVR = Right Arm (+) → looks at the cavity of the heart / right shoulder; P and QRS are normally inverted here
• aVL = Left Arm (+) → looks at the high lateral wall
• aVF = Left Leg (Foot) (+) → looks at the inferior wall

Precordial Leads - View the Horizontal Plane (cross-section of the chest)

• V1-V2: Right ventricular view (right precordial)
• V3-V4: Septal/anterior view
• V5-V6: Lateral left ventricular view

Which Leads Look at Which Walls?

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PART 4: THE ECG PAPER AND WAVEFORMS

ECG Paper Basics

• Paper runs at 25 mm/second
• 1 small square = 1 mm = 0.04 seconds
• 1 large square (5 small squares) = 5 mm = 0.20 seconds
• Amplitude standard: 10 mm = 1 mV

The Normal ECG Waveform

Each Component Explained

P Wave - Atrial Depolarization

• Represents electrical activation of the atria
• Normally: small, rounded, upright in leads I, II, aVF; inverted in aVR
• Duration: < 0.12 sec (3 small squares)
• Amplitude: < 2.5 mm (in II), < 1.5 mm in V1
• Every P wave should be followed by a QRS in sinus rhythm

PR Interval - AV Conduction Time

• Measured from the start of P wave to the start of QRS
• Represents the time for the impulse to travel from the SA node, through the AV node, bundle of His, and bundle branches, to the ventricles
• Normal: 0.12-0.20 seconds (3-5 small squares)
• Short PR (<0.12): pre-excitation (e.g., WPW syndrome) or junctional rhythm
• Long PR (>0.20): 1st degree AV block (delayed AV conduction)

QRS Complex - Ventricular Depolarization

• Represents simultaneous depolarization of both ventricles
• Normal duration: < 0.12 seconds (< 3 small squares)
• Wide QRS (≥0.12): bundle branch block, ventricular rhythm, or aberrant conduction
• The individual waves:
  • Q wave: Initial negative deflection. Septal q waves (small, narrow) are normal in lateral leads. Pathological Q waves are ≥0.04 sec wide or ≥25% of the R wave height - they suggest prior MI.
  • R wave: First positive deflection
  • S wave: Negative deflection after an R wave

ST Segment - Early Ventricular Repolarization

• The period between end of QRS and start of T wave
• Should be isoelectric (flat, at baseline)
• ST elevation: ischemia (STEMI), pericarditis, Brugada, early repolarization
• ST depression: ischemia, NSTEMI, digoxin effect

T Wave - Ventricular Repolarization

• "The T wave is caused by potentials generated as the ventricles recover from depolarization... The T wave is known as a repolarization wave." - Guyton & Hall
• Normally upright (concordant) in leads where QRS is upright
• Normally inverted in aVR (and sometimes V1)
• Tall peaked T waves: hyperkalemia, hyperacute MI
• Inverted T waves: ischemia, ventricular hypertrophy, PE (V1-V4), RBBB/LBBB

QT Interval

• From start of QRS to end of T wave
• Represents total ventricular depolarization + repolarization
• Must be rate-corrected (QTc): Bazett's formula: QTc = QT / √RR interval
• Normal QTc: < 440 ms in men, < 460 ms in women
• Prolonged QTc: risk of Torsades de Pointes (dangerous arrhythmia); caused by drugs, electrolyte abnormalities, congenital channelopathies

U Wave

• Small deflection after the T wave, best seen in V2-V3
• Represents late phases of ventricular repolarization
• Prominent U waves: hypokalemia, bradycardia, drugs (quinidine)

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PART 5: SYSTEMATIC ECG INTERPRETATION - THE 7-STEP APPROACH

Step 1 - Rate

• 1 box = 300 bpm
• 2 boxes = 150 bpm
• 3 boxes = 100 bpm
• 4 boxes = 75 bpm
• 5 boxes = 60 bpm
• 6 boxes = 50 bpm

Step 2 - Rhythm

• Is the rhythm regular or irregular?
• Are there P waves? Are they normal?
• Is there a P before every QRS, and a QRS after every P?
• Are all the P-P intervals and R-R intervals equal?

Step 3 - Axis (Frontal Plane)

• Both upright → Normal axis
• Lead I up, aVF down → LAD
• Lead I down, aVF up → RAD
• Both negative → ERAD

Step 4 - Intervals

Step 5 - P Wave Morphology

• Look for atrial enlargement
• Left atrial enlargement (LAE): Bifid P (P mitrale) in lead II; biphasic P with wide/deep terminal deflection in V1
• Right atrial enlargement (RAE): Tall peaked P (≥2.5mm) in leads II, III, aVF (P pulmonale)

Step 6 - QRS Morphology

• Check for Q waves (septal vs. pathological)
• Check for voltage (LVH / RVH criteria)
• Check bundle branch block pattern

Step 7 - ST Segments and T Waves

• Look for elevation or depression
• Check T wave morphology and concordance
• Look for hyperacute T waves (early MI sign)

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PART 6: COMMON ECG ABNORMALITIES

Bundle Branch Blocks

• RBBB = RSR' (R goes up, comes down, goes up again - "rabbit ears")
• LBBB = WiLLiaM and MaRRoW → in V1: W shape in LBBB, M shape in RBBB

AV Blocks

Myocardial Infarction (STEMI Patterns)

LVH and RVH

• S wave in V1 + R wave in V5 or V6 > 35 mm
• R wave in aVL > 11 mm
• Associated with strain pattern: ST depression + T wave inversion in lateral leads

• Right axis deviation (> +90°)
• Dominant R wave in V1 (R > S in V1)
• Deep S in V5/V6
• P pulmonale may be present

Common Arrhythmias at a Glance

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PART 7: QUICK CLINICAL PEARLS

1. Always check the calibration mark at the start of the ECG strip - should be 10 mm = 1 mV. If it's different, all your voltage measurements will be off.
2. aVR is the mirror of the heart - almost everything normal (P, QRS) is inverted here. ST elevation in aVR can indicate left main or proximal LAD occlusion - a "STEMI equivalent."
3. The rate-corrected QT (QTc) matters more than the raw QT because QT shortens at faster heart rates naturally.
4. Tall R wave in V1 with right axis deviation suggests either RBBB, RVH, posterior MI, or Wolff-Parkinson-White (WPW) - narrow the differential based on clinical context.
5. "Tombstone" ST elevation (massive, convex ST elevation) in anterior leads is a STEMI until proven otherwise - activate the cath lab.
6. Wellens syndrome: biphasic (Type A) or deeply inverted (Type B) T waves in V2-V3 with a pain-free presentation → critical LAD stenosis - do NOT stress test these patients.
7. De Winter T waves: ST depression with tall, symmetrical T waves in V1-V6 → LAD occlusion equivalent; treat as STEMI.
8. Brugada pattern: RSR' in V1-V2 with coved (downsloping) ST elevation → risk of sudden cardiac death from VF.
9. Right heart strain in PE: S1Q3T3 pattern (S wave in I, Q wave and T wave inversion in III) + sinus tachycardia + incomplete RBBB.
10. Hyperkalemia progression: peaked T waves → wide QRS → loss of P waves → sine wave → VF.

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PART 8: ECG READING WORKFLOW SUMMARY

Step 1: Rate ──────────────── Fast or slow?
Step 2: Rhythm ─────────────── Regular? P waves present?
Step 3: Axis ───────────────── Normal, LAD, RAD?
Step 4: Intervals ──────────── PR, QRS, QTc within range?
Step 5: P wave morphology ──── LAE, RAE?
Step 6: QRS morphology ─────── Q waves, bundle branch block, LVH?
Step 7: ST / T waves ────────── Elevation, depression, inversion?

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