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Electrocardiogram (ECG) - Complete Reference Guide

What is an ECG?

An ECG records the electrical activity of the heart from electrodes placed on the skin. The standard 12-lead ECG uses 10 electrodes to produce 12 views (leads) of cardiac electrical activity. It is the most important non-invasive cardiac diagnostic tool.

The 12 Leads

Limb leads (frontal plane):

Bipolar: I, II, III
Augmented unipolar: aVR, aVL, aVF

Precordial (chest) leads (horizontal plane):

V1-V6

Normal ECG with lead placement shown on chest anatomy

Normal ECG tracings from all leads, with electrode positions shown - Ganong's Review of Medical Physiology

ECG Paper & Measurements

Grid	Time	Voltage
Small square	0.04 sec	0.1 mV
Large square (5 small)	0.20 sec	0.5 mV
Standard paper speed	25 mm/sec	-

Heart rate calculation:

Count R-R intervals: HR = 300 ÷ (number of large squares between R waves)
Or: HR = 1500 ÷ (number of small squares between R waves)

The ECG Waveforms

P Wave

Represents atrial depolarization (SA node fires → both atria depolarize)
Normal: positive in lead II, negative in aVR
Duration: < 0.12 sec (3 small squares)
Amplitude: < 2.5 mm

PR Interval

From start of P wave to start of QRS complex
Represents AV nodal conduction time
Normal: 0.12-0.20 sec (3-5 small squares)
Prolonged PR > 0.20 sec = 1st degree AV block

QRS Complex

Represents ventricular depolarization
Normal duration: 0.06-0.10 sec (< 3 small squares)
Wide QRS (>0.12 sec) = bundle branch block or ventricular origin
Q wave: initial negative deflection (small septal q waves normal in V5, V6)
R wave: first positive deflection
S wave: negative deflection after R wave

ST Segment

From end of QRS (J-point) to start of T wave
Should be isoelectric (flat, on the baseline)
Elevation suggests: STEMI, pericarditis, Brugada syndrome
Depression suggests: ischemia, posterior MI, digoxin effect

T Wave

Represents ventricular repolarization
Should be in same direction as QRS in most leads
Normal: upright in I, II, V3-V6; inverted in aVR
Peaked T waves: hyperkalemia
Flat/inverted: ischemia, hypertrophy, electrolyte disturbances

QT Interval

From start of QRS to end of T wave
Represents total ventricular depolarization + repolarization
Must be corrected for heart rate (QTc using Bazett formula: QTc = QT / √R-R)
Normal QTc: < 440 ms (men), < 460 ms (women)
Prolonged QT: risk of Torsades de Pointes

U Wave

Small wave after T wave, best seen in V2-V3
Significance: prominent U waves seen in hypokalemia

Ventricular Depolarization Vectors

Ventricular depolarization phases diagram

Two phases of ventricular depolarization. Phase 1: septal depolarization left→right (small r in V1, small q in V6). Phase 2: LV dominance directed left and posterior (S wave in V1, tall R in V6). - Harrison's Principles of Internal Medicine, 22e

Normal ECG - Systematic Approach (7 Steps)

Step	What to Assess
1. Rate	Bradycardia (<60), Normal (60-100), Tachycardia (>100)
2. Rhythm	Regular vs irregular; P before every QRS?
3. Axis	Normal: -30° to +90°
4. P wave	Morphology, PR interval
5. QRS	Duration, morphology, R-wave progression
6. ST/T wave	Elevation, depression, T-wave changes
7. QT interval	Calculate QTc

Axis Determination

Axis	Lead I	aVF
Normal (-30° to +90°)	Positive	Positive
Left axis deviation (< -30°)	Positive	Negative
Right axis deviation (> +90°)	Negative	Positive
Extreme axis ("northwest")	Negative	Negative

Causes of left axis deviation: Left anterior fascicular block, inferior MI, LVH Causes of right axis deviation: RVH, left posterior fascicular block, PE, lateral MI

Key Pathological Patterns

Bundle Branch Blocks

RBBB: RSR' ("M" pattern) in V1, wide S in I and V6; QRS > 0.12 sec
LBBB: broad notched R in I, V5, V6; QS in V1; no septal q waves; QRS > 0.12 sec
Mnemonic: WiLLiaM MaRRoW (LBBB = W in V1, M in V6; RBBB = M in V1, W in V6)

Myocardial Infarction (STEMI)

Territory	ST Elevation Leads	Culprit Artery
Anterior	V1-V4	LAD
Inferior	II, III, aVF	RCA (or LCx)
Lateral	I, aVL, V5-V6	LCx
Posterior	ST depression V1-V3 (reciprocal)	RCA/LCx

Evolution of MI on ECG:

Hyperacute T waves (minutes)
ST elevation (hours)
Q wave development (hours-days)
T-wave inversion (days-weeks)
Q waves may persist permanently

Chamber Enlargement/Hypertrophy

Finding	ECG Pattern
Right atrial overload (P pulmonale)	Peaked P waves > 2.5 mm, best in II
Left atrial abnormality (P mitrale)	Broad notched P waves; biphasic P in V1 (negative component > 1 mm²)
LVH (Sokolow-Lyon)	SV1 + RV5 or RV6 > 35 mm
RVH	Dominant R in V1 (R > S), right axis deviation

Arrhythmias (Quick Reference)

Arrhythmia	Key Features
Sinus tachycardia	Rate >100, P before every QRS, normal morphology
Sinus bradycardia	Rate <60, otherwise normal
Atrial fibrillation	Absent P waves, irregularly irregular R-R, fibrillatory baseline
Atrial flutter	Sawtooth flutter waves ~300 bpm, usually 2:1 or 3:1 block
SVT (AVNRT)	Narrow complex tachycardia, P waves hidden in or just after QRS
1° AV block	PR > 0.20 sec, every P followed by QRS
2° AV block Mobitz I (Wenckebach)	Progressive PR lengthening until dropped beat
2° AV block Mobitz II	Constant PR, then sudden dropped QRS
3° AV block (complete)	No relation between P and QRS; escape rhythm present
VT	Wide complex tachycardia (QRS > 0.12), AV dissociation
VF	Chaotic, no organized complexes - emergency

Electrolyte Effects

Hyperkalemia: peaked T waves → wide QRS → sine wave → VF
Hypokalemia: flat T waves, prominent U waves, QT prolongation
Hypercalcemia: short QT
Hypocalcemia: long QT

Common Clinical ECG Patterns

Condition	ECG Finding
Pulmonary embolism	S1Q3T3, sinus tachycardia, new RBBB
Pericarditis	Diffuse saddle-shaped ST elevation, PR depression
Digitalis toxicity	ST scooping ("digitalis effect"), arrhythmias
Wolff-Parkinson-White	Short PR, delta wave, wide QRS
Brugada syndrome	RBBB morphology + coved ST elevation V1-V2
Long QT syndrome	QTc > 440-460 ms, risk of Torsades
Hypothermia	Osborn (J) waves, bradycardia

Sources: Harrison's Principles of Internal Medicine, 22e (2025) - Chapter 247; Ganong's Review of Medical Physiology, 26e; Braunwald's Heart Disease, 2 Vol Set

This is a broad overview. Let me know if you want to go deeper on any specific area - arrhythmia interpretation, MI localization, bundle branch blocks, pediatric ECG norms, or anything else.

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