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Electrocardiogram (ECG) - A Complete Guide

• Guyton and Hall Textbook of Medical Physiology, p. 142
• Harrison's Principles of Internal Medicine 22E, Chapter 247

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1. The Cardiac Conduction System - The Basis of the ECG

1. SA Node fires → depolarizes both atria (generates the P wave)
2. AV Node - the impulse slows here (physiologic delay = the PR interval)
3. Bundle of His - a single trunk that bifurcates into:
   • Right bundle branch (to right ventricle)
   • Left bundle branch (divides into left anterior and left posterior fascicles)
4. Purkinje fibers - rapidly conduct impulse to ventricular myocardium (endocardium to epicardium)
5. Ventricular myocardium contracts (generates the QRS complex)
6. Repolarization of ventricles (generates the T wave)

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2. The Normal ECG Waveform

P Wave

• Origin: Atrial depolarization
• Shape: Small, rounded, upright in leads I, II, aVF
• Duration: < 0.12 sec (120 ms)
• Amplitude: < 2.5 mm in limb leads, < 1.5 mm in precordial leads
• Represents the spread of the electrical impulse from the SA node across both atria
• Clinical note: Absent in atrial fibrillation; inverted in junctional rhythms

PR Interval

• Origin: Measured from onset of P wave to onset of QRS
• Normal duration: 120-200 ms (0.12-0.20 sec)
• Represents the time of AV conduction including the physiologic delay at the AV node
• Prolonged PR = first-degree AV block
• Shortened PR = pre-excitation (e.g., Wolff-Parkinson-White syndrome)

QRS Complex

• Origin: Ventricular depolarization
• Normal duration: 100-110 ms or less (< 0.12 sec)
• Three components:
  • Q wave: First downward deflection before R; small normal Q waves from septal depolarization
  • R wave: First upward deflection; largest component
  • S wave: Downward deflection after R wave
• Widened QRS (> 120 ms): Bundle branch block, ventricular pacemaker, hyperkalemia, preexcitation
• Pathological Q waves: Wide (> 40 ms) or deep (> 25% of R wave height) Q waves suggest prior myocardial infarction

ST Segment

• Origin: Isoelectric period between end of QRS and start of T wave (corresponds to action potential plateau, Phase 2)
• J point: Junction between QRS end and ST segment start
• Normal: Isoelectric (flat at baseline)
• ST elevation: Transmural ischemia/infarction, pericarditis, Brugada syndrome, early repolarization
• ST depression: Subendocardial ischemia, digoxin effect, reciprocal changes

T Wave

• Origin: Ventricular repolarization (Phase 3 of action potential)
• Shape: Asymmetric, gradually ascending then sharply descending
• Polarity: Normally upright in leads I, II, V3-V6; inverted in aVR
• Duration: 0.25-0.35 sec after ventricular depolarization
• The T wave is a repolarization wave
• T wave inversions: Ischemia, bundle branch block, ventricular hypertrophy, PE (V1-V4), raised intracranial pressure

U Wave

• Origin: Possibly repolarization of Purkinje fibers or "after-depolarizations"
• Small, positive deflection after T wave (best seen in V2-V3)
• Prominent U waves: Hypokalemia, bradycardia, drugs (quinidine, sotalol)

QT Interval

• Origin: Measured from onset of QRS to end of T wave; represents total ventricular electrical systole (depolarization + repolarization)
• Normal: Varies with heart rate; QTc (corrected) < 460 ms in women, < 450 ms in men
• QTc formula (Bazett): QTc = QT / √RR (in seconds)
• Prolonged QT: Drugs (amiodarone, sotalol, macrolides, antipsychotics), electrolyte disturbances (hypokalemia, hypomagnesemia, hypocalcemia), congenital long QT syndromes - risk of torsades de pointes
• Short QT: Hypercalcemia, digoxin toxicity, congenital short QT syndrome

RR Interval

• Distance between two consecutive R waves; used to calculate heart rate
• Heart rate = 300 / number of large squares between R waves (at 25 mm/s)
• Or: Heart rate = 1500 / number of small squares between R waves

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3. The 12-Lead ECG System

Limb Leads (Frontal Plane - 6 leads)

• aVR normally shows a negative (inverted) QRS because the mean cardiac vector points away from the right arm
• Leads II, III, aVF: "Inferior leads" - look at the inferior wall of left ventricle (RCA territory)
• Leads I, aVL: "Lateral leads" - lateral wall

Precordial (Chest) Leads (Horizontal Plane - 6 leads)

• V1-V2: Right heart and septum; QRS mainly negative (depolarization wave moves away from base)
• V4-V6: Left heart; QRS mainly positive (depolarization moves toward apex)
• R-wave progression: R wave gradually increases in amplitude from V1 to V5/V6. The transition zone (where R = S) is normally at V3-V4

Additional Leads

• Right-sided leads (V3R, V4R): Used to detect right ventricular infarction (especially in inferior STEMI)
• Posterior leads (V7-V9): Used to detect posterior MI (tall R in V1-V2 is reciprocal change)

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4. ECG Paper and Calibration

• Horizontal axis = Time: 1 small box = 1 mm = 40 ms (0.04 sec); 1 large box = 5 mm = 200 ms (0.20 sec) at standard paper speed of 25 mm/s
• Vertical axis = Voltage (amplitude): 1 mm = 0.1 mV; standard calibration = 10 mm = 1 mV
• Always check the calibration marker (1 mV = 10 mm box at start of trace)

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5. Calculating Heart Rate

• Regular rhythm: 300 ÷ (number of large squares between R-R) = BPM
  • 1 large box between R waves = 300 bpm
  • 2 large boxes = 150 bpm
  • 3 = 100, 4 = 75, 5 = 60, 6 = 50 bpm
• Irregular rhythm: Count QRS complexes in a 10-second strip × 6

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6. The Electrical Axis

• Both positive → Normal axis
• Lead I positive, aVF negative → LAD (check lead II; if negative, LAD confirmed)
• Lead I negative, aVF positive → RAD
• Both negative → Extreme axis deviation ("no man's land")

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7. Systematic ECG Interpretation (The Step-by-Step Approach)

1. Rate - Bradycardia (< 60) vs tachycardia (> 100)
2. Rhythm - Regular or irregular? Sinus vs non-sinus
3. PR interval - Normal, short, or long?
4. QRS duration - Narrow (< 120 ms) or wide (> 120 ms)?
5. QT/QTc - Prolonged or short?
6. Mean QRS electrical axis - Normal, LAD, or RAD?
7. P waves - Present, morphology, relation to QRS?
8. QRS voltages - Low (< 5 mm in limb leads) or high?
9. Precordial R-wave progression - Normal transition zone?
10. Abnormal Q waves - Pathological?
11. ST segments - Elevation or depression?
12. T waves - Normal, inverted, peaked?
13. U waves - Prominent?
14. Compare with previous ECGs whenever available

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8. Hypertrophy Patterns

Left Ventricular Hypertrophy (LVH)

• Associated with: ST depression, T wave inversion in lateral leads ("strain" pattern)
• Causes: Hypertension, aortic stenosis, hypertrophic cardiomyopathy

Right Ventricular Hypertrophy (RVH)

• Dominant R wave in V1 (R > S in V1)
• Right axis deviation
• T wave inversion V1-V3
• Causes: Pulmonary hypertension, pulmonary stenosis, cor pulmonale

Left Atrial Enlargement (P mitrale)

• Broad, bifid (notched) P wave in lead II (> 120 ms)
• Biphasic P wave in V1 with prominent negative terminal component

Right Atrial Enlargement (P pulmonale)

• Tall, peaked P wave in lead II (> 2.5 mm)
• Causes: COPD, tricuspid valve disease, pulmonary hypertension

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9. Bundle Branch Blocks

Right Bundle Branch Block (RBBB)

• QRS > 120 ms
• rSR' ("M" or "rabbit ears") pattern in V1-V2
• Wide S wave in leads I, V5, V6
• T wave inversion in V1-V2 (secondary repolarization change)
• Causes: Normal variant, right heart strain, congenital heart disease, anterior MI

Left Bundle Branch Block (LBBB)

• QRS > 120 ms
• Broad, notched R wave in lateral leads (I, aVL, V5-V6) - "M" shape
• rS or QS pattern in V1-V2
• T wave opposite to the main QRS deflection (discordant)
• In LBBB, standard ischemia criteria are unreliable (Sgarbossa criteria used instead)
• Causes: Ischemic heart disease, hypertension, cardiomyopathy, aortic stenosis

Fascicular Blocks (Hemiblocks)

• Left anterior fascicular block (LAFB): QRS < 120 ms, marked LAD (axis more negative than -45°), small Q in I/aVL, small R in II/III/aVF - most common cause of marked LAD in adults
• Left posterior fascicular block (LPFB): RAD (axis > +110°), rare as isolated finding

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10. Ischemia and Infarction

Evolution of STEMI (ST-Elevation Myocardial Infarction)

Localizing the Infarct

NSTEMI / Unstable Angina

• ST depression (subendocardial ischemia - ST vector points toward the cavity)
• T wave inversions without ST elevation
• No Q waves

Wellens Syndrome

• Deep, symmetric T wave inversions in V1-V4 (often with preserved R waves)
• Indicates critical stenosis of the proximal LAD
• Called the "Wellens T wave sign"

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11. Common Arrhythmias and their ECG Patterns

Sinus Arrhythmias

• Sinus tachycardia: Rate > 100, P before every QRS, normal P morphology
• Sinus bradycardia: Rate < 60, otherwise normal
• Sinus arrhythmia: Irregular variation in R-R interval with breathing (normal in young people)

Atrial Arrhythmias

• Atrial fibrillation (AF): No discrete P waves, irregularly irregular R-R intervals, fibrillatory baseline
• Atrial flutter: Sawtooth flutter waves at 300/min (usually 2:1, 3:1 or 4:1 block), regular or regularly irregular rate
• SVT (AVNRT/AVRT): Narrow QRS tachycardia, rate 150-250/min, P waves often buried in or just after QRS

AV Blocks

Ventricular Arrhythmias

• PVCs (premature ventricular contractions): Wide QRS, no preceding P wave, compensatory pause
• Ventricular tachycardia (VT): Wide QRS tachycardia (> 120 ms), rate > 100, AV dissociation (independent P waves)
• Ventricular fibrillation (VF): Chaotic, irregular waveforms with no identifiable QRS - cardiac emergency
• Torsades de pointes: Polymorphic VT with "twisting" QRS axis around the isoelectric line; associated with prolonged QT

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12. ECG Changes in Electrolyte Disturbances

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13. ECG in Special Conditions

Pericarditis

• Diffuse (saddle-shaped) ST elevation in multiple leads (not just one territory)
• PR depression (especially in lead II)
• No reciprocal ST changes (except aVR and V1)

Pulmonary Embolism (PE)

• Sinus tachycardia (most common)
• Classic but rare: S1Q3T3 (deep S in lead I, Q wave and T inversion in lead III)
• New RBBB
• T wave inversions V1-V4
• Right axis deviation

Digoxin Effect

• "Reverse tick" or "Salvador Dali moustache" ST depression (scooped ST)
• Shortened QT interval
• Flattened/inverted T waves

Digoxin Toxicity

• Any arrhythmia, especially: PAT with block, bidirectional VT, junctional tachycardia

Hypothermia

• Osborn wave (J wave): Distinctive convex hump at the J point
• QRS and QT prolongation
• Sinus bradycardia

WPW (Wolff-Parkinson-White)

• Short PR interval (< 120 ms)
• Delta wave (slurred upstroke of QRS)
• Wide QRS
• Secondary ST-T changes
• Mechanism: Pre-excitation via accessory pathway (Bundle of Kent)

Brugada Pattern

• Right bundle branch block morphology
• ST elevation in V1-V2 (coved or saddle-back pattern)
• Associated with sudden cardiac death risk, especially in young Asian males

Long QT Syndrome

• QTc > 460 ms (women) or > 450 ms (men)
• Risk of torsades de pointes and sudden cardiac death
• Congenital (LQT1, LQT2, LQT3) or acquired (drugs, electrolytes)

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14. Ambulatory ECG Monitoring

• Holter monitor: Continuous 24-48 hour recording during normal daily activities
• Event recorder: Patient-activated, worn for weeks; records when symptoms occur
• Implantable loop recorder (ILR): Subcutaneous device, can monitor for up to 3 years
• Indications: Unexplained syncope, palpitations, cryptogenic stroke (to detect paroxysmal AF), monitoring after antiarrhythmic therapy

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15. Limitations of the ECG

• A normal ECG does not exclude ischemia or even acute infarction
• LBBB makes ischemia diagnosis unreliable on standard criteria
• A single ECG is a snapshot - serial ECGs are important
• Sensitivity and specificity limitations in diagnosing both acute and chronic ischemic heart disease
• ECG cannot directly measure mechanical function (echocardiography needed for that)

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Summary Table: Key Normal Values

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• Guyton and Hall Textbook of Medical Physiology, Chapters 11-12 (Fundamentals of Electrocardiography, Cardiac Arrhythmias)
• Harrison's Principles of Internal Medicine 22E (2025), Chapter 247 (Electrocardiography)
• Goldman-Cecil Medicine, Chapter on Conduction Abnormalities and Axis Deviation
• Medical Physiology (Boron & Boulpaep), Chapter on Cardiac Electrophysiology

• Heart rate = 300 ÷ number of large squares between R-R waves
• QTc (Bazett) = QT ÷ √RR interval (in seconds)
• Critical alert: QTc >500 ms warrants urgent review - high risk of torsades de pointes