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ECG electrocardiogram waves PQRST normal

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ECG 12 lead interpretation axis heart rate intervals

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ECG ST elevation myocardial infarction STEMI changes

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atrial fibrillation ECG rhythm strip

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📟 ECG (Electrocardiogram) — A Complete Visual Guide

1. What Is an ECG?

An ECG records the electrical activity of the heart at the body surface. It captures differences in electrical potential generated by the heart as impulses travel through the conduction system, producing the characteristic waves we see on the tracing.

2. The Cardiac Conduction System & ECG Waveforms

The electrical journey through the heart creates each component of the ECG:

Wave/Interval	Origin	Represents
P wave	SA node → atria	Atrial depolarization
PR interval	AV node delay	Atrioventricular conduction time
QRS complex	Bundle branches → Purkinje → ventricles	Ventricular depolarization
ST segment	Ventricular plateau	Isoelectric ventricular repolarization phase
T wave	Ventricular myocardium	Ventricular repolarization
QT interval	Start of QRS → end of T	Total ventricular electrical activity

Normal Values

Parameter	Normal Range
Heart rate	60–100 bpm
PR interval	120–200 ms (3–5 small squares)
QRS duration	<120 ms (<3 small squares)
QTc	<440 ms (men), <460 ms (women)

3. The Cardiac Cycle — ECG in Context

This diagram from Miller's Anesthesia shows exactly how each ECG wave relates to mechanical events (valve openings, ventricular pressure, blood flow):

Cardiac cycle showing ECG, ventricular pressure, aortic flow, ventricular volume, heart sounds, and venous pulse correlated over time

Key correlations:

P wave → atrial systole begins
End of R wave → mitral/tricuspid valve close (S1)
QRS → ventricular depolarization → start of isovolumic contraction
T wave → ventricular repolarization → pulmonic/aortic valve close (S2)

Miller's Anesthesia, 10e, p. 1364–1367

4. Reading the ECG Paper

The standard ECG grid:

Small square = 1 mm = 0.04 sec (horizontal) / 0.1 mV (vertical)
Large square = 5 mm = 0.2 sec

Heart rate calculation (regular rhythm):

Rate = 300 ÷ number of large squares between R-R intervals

Common quick memory: 300 → 150 → 100 → 75 → 60 → 50

5. The 12-Lead ECG

A standard 12-lead ECG views the heart from 12 angles:

Group	Leads	View
Inferior	II, III, aVF	Inferior wall (RCA territory)
Lateral	I, aVL, V5, V6	Lateral wall (LCx territory)
Anterior	V1–V4	Anterior wall (LAD territory)
High lateral	I, aVL	High lateral (LAD/LCx)
aVR	—	Cavity; normally negative

This labeled 12-lead shows P waves (green arrows), QRS complexes (blue arrows), and T waves (red arrows):

12-lead ECG with labeled P waves, QRS complexes, and T waves across all leads including rhythm strips

6. Systematic Approach to Reading Any ECG

Use the mnemonic RRAPQRST:

Rate — fast/slow/normal?
Rhythm — regular or irregular?
Axis — normal (−30° to +90°)?
P waves — present? Morphology? 1:1 with QRS?
PR interval — short/normal/long?
QRS — narrow or wide? Any Q waves?
R-wave progression — V1 → V6 increasing?
ST segment & T waves — elevation/depression/inversion?

7. Normal Sinus Rhythm vs. Common Pathologies

A) Atrial Fibrillation

Key features:

No P waves (replaced by chaotic fibrillatory "f" waves)
Irregularly irregular R-R intervals
Narrow QRS (unless aberrant conduction)

Atrial fibrillation 12-lead ECG showing absent P waves, irregular R-R intervals marked with green brackets, fibrillatory baseline (red arrow) in Lead II and V1, with narrow QRS complexes

B) Anterior STEMI (ST-Elevation MI)

Key features:

ST elevation in anterior leads (V1–V6) ± I, aVL → LAD occlusion
Reciprocal ST depression in inferior leads (II, III, aVF)
Hyperacute T waves in early phase
Q waves develop with full-thickness infarction

Anterolateral STEMI 12-lead ECG showing convex ST-segment elevation in V1-V6 and leads I and aVL with reciprocal ST depression in inferior leads II, III, aVF — consistent with proximal LAD occlusion

C) Inferior STEMI

Key features:

ST elevation in II, III, aVF → RCA or LCx occlusion
Reciprocal depression in I and aVL
Check right-sided leads (V3R/V4R) for right ventricular infarction

Inferior STEMI 12-lead ECG demonstrating ST-segment elevation in leads II, III, and aVF with reciprocal ST depression in leads I, aVL, and V4-V6 — consistent with right coronary artery territory infarction

D) Right Bundle Branch Block (RBBB)

Key features:

QRS ≥ 120 ms (wide)
RSR' ("M-shape") in V1–V3 (right precordial leads)
Broad, slurred S wave in I, aVL, V5, V6
Secondary T-wave inversions in V1–V3

Right bundle branch block ECG showing RSR' (M-shaped) pattern highlighted with blue arrows in leads V1-V3 and slurred S waves pointed to by red arrows in leads I, aVL, V5, V6. QRS duration 154ms.

8. ECG Changes by Electrolyte Disturbance

Electrolyte	ECG Change
Hyperkalemia	Peaked T waves → wide QRS → sine wave → VF
Hypokalemia	Flat T waves, prominent U waves, ST depression
Hypercalcemia	Short QT interval
Hypocalcemia	Prolonged QT interval

9. Lead Grouping Quick Reference

INFERIOR:    II, III, aVF       → RCA
LATERAL:     I, aVL, V5, V6    → LCx
ANTERIOR:    V1–V4             → LAD
SEPTAL:      V1–V2             → Septal branch of LAD

Reciprocal changes always appear in leads opposite the infarcted territory — their presence strongly confirms true ST elevation vs. benign variants.

10. Summary: ECG Interpretation Checklist

Step	What to Look For
Rate	>100 = tachycardia; <60 = bradycardia
Rhythm	Regular? P before every QRS?
Axis	Normal (I+, aVF+); LAD (I+, aVF−); RAD (I−, aVF+)
P wave	Upright in I, II; inverted in aVR
PR interval	120–200 ms; prolonged = AV block
QRS	<120 ms; wide = BBB or ventricular origin
ST segment	Elevation (injury/STEMI); depression (ischemia/NSTEMI)
T wave	Normally upright in I, II, V3–V6; inverted = ischemia
QT/QTc	Prolonged = risk of Torsades de Pointes

This covers the foundations of ECG interpretation. Want to go deeper into any specific topic — arrhythmias, axis calculation, hypertrophy criteria, or a specific pathology?

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