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📋 ECG (Electrocardiogram) - Complete Beginner's Guide

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PART 1 - What Is an ECG?

• Is the heart beating at the right rate and rhythm?
• Is electricity traveling through the right pathway?
• Is any part of the heart muscle damaged or under strain?

"When a cardiac impulse passes through the heart, electrical current spreads from the heart into adjacent tissues... a small portion spreads all the way to the surface of the body. If electrodes are placed on the skin on opposite sides of the heart, electrical potentials generated by the current can be recorded." - Guyton & Hall Textbook of Medical Physiology

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PART 2 - The Electrical Conduction System (How the Heart Generates the Signal)

• Harrison's Principles of Internal Medicine 22E, p. 1911

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PART 3 - The ECG Waveforms Explained

The Waves

Q, R, and S - Breaking Down the QRS

• Q wave - first downward deflection before the R
• R wave - first upward deflection
• S wave - downward deflection after the R

"The P wave is caused by electrical potentials generated when the atria depolarize before atrial contraction begins. The QRS complex is caused by potentials generated when the ventricles depolarize before contraction... The T wave is caused by potentials generated as the ventricles recover from depolarization." - Guyton & Hall

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PART 4 - The ECG Paper and Measurements

Horizontal axis = TIME
  - 1 small box = 1 mm = 40 ms (0.04 s)
  - 5 small boxes = 1 large box = 200 ms (0.20 s)
  - Paper speed = 25 mm/s (standard)

Vertical axis = VOLTAGE (amplitude)
  - 1 mV = 10 mm (with standard calibration)

Key Intervals (with Normal Values)

• Harrison's Principles of Internal Medicine 22E, p. 1912

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PART 5 - The 12 Leads (12 Camera Angles of the Heart)

Limb Leads (Frontal Plane - front-to-back view)

Chest (Precordial) Leads (Horizontal Plane - side cross-section)

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PART 6 - Systematic Approach to Reading an ECG

Step 1 - Rate

• Quick method: Count the number of large boxes between two R waves, then divide 300 by that number
  • 1 large box = 300 bpm, 2 = 150, 3 = 100, 4 = 75, 5 = 60, 6 = 50
• Normal: 60-100 bpm
• Bradycardia: <60 bpm
• Tachycardia: >100 bpm

Step 2 - Rhythm

• Is the rhythm regular (RR intervals equal) or irregular?
• Are there P waves?
• Does every P wave have a QRS after it?
• Does every QRS have a P wave before it?
• A regular rhythm with P before every QRS = sinus rhythm (normal)

Step 3 - PR Interval

• Measure from start of P to start of QRS
• Short PR (<120 ms) → pre-excitation (e.g., WPW syndrome)
• Long PR (>200 ms) → first-degree AV block

Step 4 - QRS Complex

• Width: Narrow (<110 ms) = normal. Wide (≥120 ms) = bundle branch block or ventricular rhythm
• Height: Tall voltages → hypertrophy; low voltages → effusion, obesity, COPD
• Q waves: Pathological Q waves (>1 mm wide, >¼ of the QRS height) = old MI

Step 5 - ST Segment

• ST elevation (>1 mm above baseline) = acute MI (STEMI) or pericarditis
• ST depression (below baseline) = ischemia or NSTEMI
• The J point is where QRS ends and ST begins

Step 6 - T Waves

• Normal: upright in most leads, inverted in aVR
• T-wave inversion = ischemia, strain, bundle branch block
• Peaked/tall T waves = early MI (hyperacute T waves) or hyperkalemia

Step 7 - QT Interval

• Measure from start of QRS to end of T wave
• Correct for rate with the Bazett formula: QTc = QT ÷ √RR (both in seconds)
• Prolonged QTc → risk of torsades de pointes (dangerous arrhythmia)

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PART 7 - Common Abnormal Patterns to Know

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PART 8 - Bundle Branch Blocks (Quick Summary)

• Harrison's Principles of Internal Medicine 22E, p. 1913

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PART 9 - Reading Your First ECG (Checklist)

□ 1. Rate:          _____ bpm  (normal 60-100)
□ 2. Rhythm:        Regular / Irregular   P waves present? Y/N
□ 3. PR interval:   _____ ms   (normal 120-200)
□ 4. QRS width:     _____ ms   (normal <110)
□ 5. ST segment:    Normal / Elevated / Depressed
□ 6. T waves:       Upright / Inverted / Peaked
□ 7. QTc:          _____ ms   (normal <450/460 ms)
□ 8. Axis:          Normal / LAD / RAD
□ 9. Overall:       _______________________

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Summary Table - Normal ECG Values

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• Guyton and Hall Textbook of Medical Physiology - Fundamentals of Electrocardiography
• Harrison's Principles of Internal Medicine, 22E (2025) - Basic ECG Waveforms and Intervals, ECG Leads

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📋 ECG Deep Dive - Advanced Interpretation

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MODULE 1 - Arrhythmias

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1.1 Normal Sinus Rhythm (the reference)

• Rate: 60-100 bpm
• Regular rhythm
• P wave before every QRS, every P wave followed by a QRS
• PR interval 120-200 ms
• Narrow QRS (<110 ms)

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1.2 Rate Abnormalities

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1.3 Atrial Arrhythmias

Atrial Fibrillation (AF)

• Irregularly irregular RR intervals (no two RR intervals the same)
• No discrete P waves - replaced by a fibrillatory baseline (chaotic wiggles)
• Narrow QRS (unless bundle branch block or pre-excitation is present)
• Rate: ventricular rate usually 100-160 bpm (uncontrolled)

"Performing an ECG during an episode of palpitations is extremely useful in making a definitive diagnosis." - Goldman-Cecil Medicine

• AF causes loss of the atrial "kick" (atrial contraction) - reduces cardiac output by ~20%
• Blood pools in the atria (especially the left atrial appendage) - risk of clot and stroke
• Treatment goals: rate control, rhythm control, anticoagulation

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Atrial Flutter

• Sawtooth (flutter) waves at ~300/min, best seen in leads II, III, aVF
• Regular ventricular rate, classically at 150 bpm (2:1 block), or 100 (3:1) or 75 (4:1)
• Narrow QRS (unless BBB)
• No distinct P waves - replaced by the flutter waves

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Supraventricular Tachycardia (SVT)

• Rate: typically 150-250 bpm
• Narrow QRS (regular)
• P waves may be hidden within or just after QRS (retrograde conduction)
• Sudden onset and offset ("paroxysmal")

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1.4 Ventricular Arrhythmias

Premature Ventricular Complex (PVC)

• Wide, bizarre QRS not preceded by a P wave
• Followed by a compensatory pause
• T wave is opposite in direction to the QRS
• Isolated PVCs are common and often benign; frequent (>10/hour), multiform, or on-T ("R-on-T") PVCs raise concern

Ventricular Tachycardia (VT)

• Wide QRS tachycardia (QRS ≥120 ms) at rate >100 bpm
• Rate typically 100-250 bpm
• AV dissociation (P waves and QRS complexes firing independently) is diagnostic of VT
• Life-threatening - causes hemodynamic collapse

Ventricular Fibrillation (VF)

• Completely chaotic, irregular, no recognizable QRS
• Cardiac arrest - requires immediate defibrillation

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1.5 AV Blocks (Conduction Problems)

• Mobitz I (Wenckebach) = "The PR interval lengthens until a beat drops" - like a Wenckebach pattern (easier to remember: "longer, longer, longer, DROP")
• Mobitz II = No warning, just sudden dropped beats - much more dangerous

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MODULE 2 - ST Changes and Ischemia

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2.1 Why the ST Changes in Ischemia

"Severe, acute ischemia lowers the resting membrane potential and shortens the duration of the action potential. Such changes cause a voltage gradient between normal and ischemic zones. These currents of injury are represented on the surface ECG by deviation of the ST segment." - Harrison's Principles of Internal Medicine, 22E

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2.2 Transmural vs Subendocardial Ischemia

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2.3 STEMI - ST-Elevation MI

• New ST elevation at the J point in ≥2 contiguous leads
• ≥2 mm in V2-V3 (men) or ≥1.5 mm (women)
• ≥1 mm in all other contiguous leads

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2.4 Evolutionary Changes of MI

Minutes → Hours:
  Hyperacute T waves (tall, peaked, broad)
  ↓
Hours:
  ST elevation
  ↓
Hours to Days:
  T wave inversion (in leads with ST elevation)
  Q waves develop
  ↓
Days to Weeks:
  ST returns to baseline
  T-wave inversions persist
  Q waves persist (permanent marker of old MI)

• Width ≥40 ms (1 small box)
• Depth ≥25% of the overall QRS height
• Must be present in ≥2 contiguous leads

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2.5 Important ST Mimics (Not Always MI)

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MODULE 3 - Electrical Axis

3.1 Normal Axis

3.2 Axis Summary Table

3.3 Quick Bedside Axis Method (using Leads I and aVF)

1. Look at the QRS in Lead I - is it mostly upright (+) or mostly downward (-)?
2. Look at the QRS in Lead aVF - is it mostly upright (+) or mostly downward (-)?

Lead I (+), aVF (+) → Normal axis
Lead I (+), aVF (-) → Left axis deviation
Lead I (-), aVF (+) → Right axis deviation
Lead I (-), aVF (-) → Extreme (northwest) axis

3.4 Causes of Axis Deviation

"Left axis deviation results from hypertension causing left ventricular hypertrophy... Right axis deviation may result from congenital pulmonary valve stenosis, tetralogy of Fallot, or other conditions causing right ventricular hypertrophy." - Guyton & Hall Textbook of Medical Physiology

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MODULE 4 - Hypertrophy Patterns

4.1 Left Ventricular Hypertrophy (LVH)

• SV1 + RV5 or RV6 >35 mm (Cornell: RaVL >20 mm women, >28 mm men)

• Left axis deviation
• Left atrial enlargement (bifid P wave in II, biphasic P in V1)
• "Strain" pattern: ST depression + T-wave inversion in I, aVL, V5, V6 (leads with tall R waves)

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4.2 Right Ventricular Hypertrophy (RVH)

• Right axis deviation (>+90°)
• Tall R wave in V1 (R>S in V1, or R>7 mm)
• Deep S waves in V5, V6
• T-wave inversion V1-V3
• Right atrial enlargement: peaked P wave in II >2.5 mm ("P pulmonale")

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MODULE 5 - Special Patterns to Know

5.1 Wolff-Parkinson-White (WPW)

• Short PR interval (<120 ms) - signal bypasses AV node delay
• Delta wave - slurred upstroke at the start of QRS (slow conduction through myocardium before normal conduction catches up)
• Wide QRS (total from delta to end)
• Risk: rapid conduction down the accessory pathway in AF can trigger VF

5.2 Long QT Syndrome

• QTc >450 ms (men) or >460 ms (women)
• Causes: drugs (amiodarone, quinidine, haloperidol, erythromycin), hypokalemia, hypomagnesemia, congenital
• Risk: Torsades de pointes - a polymorphic VT that looks like a twisting ribbon, which can degenerate to VF and cause sudden death

5.3 Brugada Syndrome

• Coved-type ST elevation in V1-V2 with RBBB-like morphology
• Can be unmasked by fever, sodium channel blockers
• Risk of sudden cardiac death from VF - ICD implantation needed

5.4 Hyperkalemia (Potassium Effect on ECG)

K+ 5.5-6.5: Peaked (tall, narrow, symmetric) T waves
K+ 6.5-7.5: Wide QRS, prolonged PR, flattened P waves
K+ >7.5:   Loss of P waves → "sine wave" pattern
K+ >8-9:   VF or asystole

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Summary - The Full Approach in One Place

Step 1  Rate:           _____ bpm (normal 60-100)
Step 2  Rhythm:         Regular/Irregular | P before every QRS?
Step 3  PR interval:    _____ ms (normal 120-200)
Step 4  QRS width:      _____ ms (<110 normal)
Step 5  Axis:           Lead I ___ aVF ___ → Normal/LAD/RAD
Step 6  ST segment:     Normal/Elevated/Depressed
           If elevated → Which leads? → Localize territory
           Reciprocal changes? → Confirms STEMI
Step 7  T waves:        Upright/Inverted/Hyperacute/Peaked
Step 8  QTc:            _____ ms (men <450, women <460)
Step 9  Hypertrophy:    Voltage criteria? Strain pattern?
Step 10 Pattern:        BBB? WPW? Brugada? Ischemia?

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• Harrison's Principles of Internal Medicine, 22E (2025) - ECG Waveforms, Myocardial Ischemia and Infarction
• Guyton and Hall Textbook of Medical Physiology - Axis Deviation, ECG Fundamentals
• Tintinalli's Emergency Medicine - STEMI Criteria, ST-Segment Based Diagnosis
• Goldman-Cecil Medicine - Electrocardiography, Arrhythmia Diagnosis

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