ECG from the Beginning: A Complete Guide

Part 1 - Why the Heart Generates Electricity

Part 2 - The Cardiac Conduction System

Normal spread of electrical activity in the heart with corresponding ECG at each stage - Ganong's Medical Physiology

The Conduction Pathway (Step by Step)

Key point: The SA node fires fastest (~70/min), so it overrides all other pacemakers. If the SA node fails, the AV node takes over (~45/min). If the AV node fails, ventricular muscle takes over (~30/min, called idioventricular rhythm).

• Gray's Anatomy for Students, p. 247
• Ganong's Review of Medical Physiology, 26th ed., p. 524

Part 3 - ECG Waves, Segments, and Intervals

Standard ECG wave nomenclature showing P, Q, R, S, T, U waves with PR, ST, QT intervals - Ganong's Medical Physiology

Every Component Explained

• Represents atrial depolarization (both atria contracting)
• Small, rounded, upright in most leads
• Duration: < 0.12 s (3 small squares)
• Amplitude: < 2.5 mm

• From the start of P wave to start of QRS
• Represents time for impulse to travel from SA node through AV node to ventricles (AV conduction time)
• Normal: 0.12 - 0.20 s (3-5 small squares)
• Prolonged PR = heart block; Short PR = pre-excitation (e.g., WPW syndrome)

• Represents ventricular depolarization
• Q = first negative deflection (septal depolarization, left to right)
• R = tall positive deflection (ventricular depolarization, apex to base)
• S = negative deflection after R (basal depolarization)
• Normal duration: < 0.10 s (< 2.5 small squares)
• Wide QRS (> 0.12 s) = bundle branch block or ventricular rhythm

• From end of QRS (J point) to start of T wave
• Should sit on the isoelectric line
• ST elevation = acute MI, pericarditis, Brugada
• ST depression = ischemia, reciprocal changes, digoxin effect

• Represents ventricular repolarization
• Normally upright in I, II, V3-V6; inverted in aVR
• Tall peaked T = hyperkalemia
• Flat or inverted T = ischemia, LVH, electrolyte disturbance

• From start of QRS to end of T wave
• Represents total ventricular electrical activity (depolarization + repolarization)
• Normal: < 0.43 s (rate-dependent - use corrected QTc)
• Prolonged QT = risk of Torsades de Pointes (dangerous arrhythmia)

• Small wave after T wave - inconstant, not always visible
• Most prominent in leads V2-V3
• Prominent U = hypokalemia, bradycardia

Normal Interval Summary Table

Part 4 - The ECG Paper

Each small square  = 1 mm wide  = 0.04 s (40 ms)   at standard speed 25 mm/s
Each large square  = 5 mm wide  = 0.20 s (200 ms)
Each large square  = 5 mm tall  = 0.5 mV (at standard gain 10 mm/mV)

• 5 large squares across = 1 second
• Count large squares between R waves to get RR interval, then calculate rate

• Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 346

Part 5 - The 12 Leads and What They "See"

Electrode positions for unipolar leads (aVR, aVL, aVF) and precordial leads V1-V6 - Ganong's Medical Physiology

Limb Leads (Frontal Plane)

• Lead I: Left arm (+) vs Right arm (-) - looks at the heart from the left side
• Lead II: Left leg (+) vs Right arm (-) - looks at the inferior-left
• Lead III: Left leg (+) vs Left arm (-) - looks at the inferior-right

• aVR: Right arm - looks at the cavity of the heart from above-right (normally negative)
• aVL: Left arm - looks at the high lateral wall
• aVF: Left foot - looks at the inferior wall (diaphragmatic surface)

Precordial (Chest) Leads (Horizontal Plane)

Lead Groupings by Territory (Critical for MI localization)

• Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 346-350
• Miller's Anesthesia 10th ed.

Part 6 - A Systematic Approach to Reading Every ECG

Step 1 - Rate

• Normal: 60-100/min
• Bradycardia: < 60/min
• Tachycardia: > 100/min

Step 2 - Rhythm

• Is it regular? (Are all RR intervals equal?)
• Is there a P wave before every QRS?
• Is there a QRS after every P wave?
• If regular with P before QRS: Sinus rhythm

Step 3 - Axis

• Lead I +, aVF +: Normal axis (-30° to +90°)
• Lead I +, aVF -: Left axis deviation (< -30°) - seen in left bundle branch block, inferior MI
• Lead I -, aVF +: Right axis deviation (> +90°) - seen in right bundle branch block, right ventricular hypertrophy

Step 4 - P waves

• Present?
• Normal morphology (upright in II, inverted in aVR)?
• One P per QRS?

Step 5 - PR interval

• Normal (0.12-0.20 s)?
• Long = heart block
• Short = pre-excitation

Step 6 - QRS complex

• Width < 0.10 s? (Wide = bundle branch block or ventricular origin)
• Q waves present? (Pathological Q = > 0.04 s wide or > 25% of R wave height = old MI)
• R wave progression in chest leads? (R should get taller from V1 to V5, called "normal R wave progression")

Step 7 - ST segment

• Elevation (> 1 mm in limb leads, > 2 mm in chest leads)?
• Depression?
• Flat, down-sloping, or up-sloping?

Step 8 - T waves

• Upright in expected leads?
• Inverted? Peaked? Flattened?

Step 9 - QT interval

• Measure and correct for rate: QTc = QT / √RR (Bazett's formula)
• Normal QTc < 440 ms (men), < 460 ms (women)

Part 7 - Key Abnormalities at a Glance

Summary: The One-Line Takeaway for Each Part

1. The heart fires electricity in one direction: SA node → AV node → Bundle of His → Bundle branches → Purkinje fibers → myocardium
2. P = atria contract; QRS = ventricles contract; T = ventricles reset
3. The paper: 1 small box = 0.04 s and 0.1 mV
4. 12 leads = 12 views of the heart from different angles
5. Always read in order: Rate → Rhythm → Axis → P → PR → QRS → ST → T → QT