Explain me how to analyse ecg from basic to advanced with all conditions along with proper diagrams

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ECG interpretation guide basic to advanced with diagrams all conditions

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I now have comprehensive content from multiple authoritative textbooks and real ECG images. Let me compile the complete guide.

Complete ECG Analysis Guide: Basic to Advanced

Sources: Medical Physiology (Boron & Boulpaep), Goldman-Cecil Medicine, Braunwald's Heart Disease, Rosen's Emergency Medicine, Frameworks for Internal Medicine, Harrison's Principles of Internal Medicine 22E, Ganong's Review of Medical Physiology

PART 1 - THE BASICS: Understanding the ECG

What is an ECG?

An ECG (electrocardiogram) provides a direct measurement of the rate, rhythm, and time-dependent electrical vector of the heart. It records the time versus amplitude of electrical projections and represents the heart's conduction system. Because different parts of the heart activate sequentially, we can attribute time-dependent changes in the electrical vector to different regions.

P wave - atrial depolarization
QRS complex - ventricular depolarization
T wave - ventricular repolarization
U wave - delayed repolarization of Purkinje fibers (may be seen in hypokalemia)

(Medical Physiology, Boron & Boulpaep)

The ECG Paper

ECG paper uses a standard grid:

Vertical axis: 0.1 mV per mm
Horizontal axis: 0.04 seconds per small box (1 mm), 0.2 seconds per large box (5 mm)
5 large boxes = 1 second

At the standard paper speed of 25 mm/s:

Box Size	Width	Time
Small (1 mm)	-	0.04 sec
Large (5 mm)	-	0.20 sec

The 12 Leads: What Each Sees

A 12-lead ECG uses 10 electrodes to generate 12 views of the heart from different angles:

Limb Leads (Frontal Plane)

Lead	View
I	Lateral (0°)
II	Inferior (60°)
III	Inferior (120°)
aVR	Cavity/right (-150°)
aVL	Lateral (-30°)
aVF	Inferior (90°)

Precordial Leads (Horizontal Plane)

Lead	Position	View
V1	Right sternal border, 4th ICS	Septal/RV
V2	Left sternal border, 4th ICS	Septal
V3	Between V2 and V4	Anterior
V4	Midclavicular line, 5th ICS	Anterior
V5	Anterior axillary line	Lateral
V6	Midaxillary line	Lateral

Coronary territory localization:

Leads with Changes	Territory	Artery
V1-V4	Anterior	LAD
I, aVL, V5-V6	Lateral	LCx
II, III, aVF	Inferior	RCA
V1-V2 (tall R, ST depression)	Posterior	RCA/LCx
V3R-V4R	Right ventricle	RCA

PART 2 - THE SYSTEMATIC APPROACH (7-Step Method)

Always read every ECG using the same sequence. Never skip steps.

Step 1 - Rate

Direct method: Count seconds between two R waves (R-R interval). Heart rate = 60 / R-R interval in seconds.

Quick method (for regular rhythms): Count large boxes between R waves and use the "300 rule":

Large Boxes (R-R)	Heart Rate
1	300 bpm
2	150 bpm
3	100 bpm
4	75 bpm
5	60 bpm
6	50 bpm

For irregular rhythms: Count QRS complexes in a 10-second rhythm strip and multiply by 6.

Normal: 60-100 bpm
Bradycardia: < 60 bpm
Tachycardia: > 100 bpm

Step 2 - Rhythm

Ask four questions:

Is there a P wave before every QRS?
Is there a QRS after every P wave?
Are P-P intervals regular?
Are R-R intervals regular?

Normal sinus rhythm requires:

P wave before each QRS
P wave upright in leads II, III, aVF
PR interval 0.12-0.20 sec
Regular rate 60-100 bpm

Step 3 - Axis

The normal QRS axis lies between -30° and +90°.

Quick method - look at leads I and aVF:

Lead I	aVF	Axis
Positive (up)	Positive (up)	Normal (0° to +90°)
Positive (up)	Negative (down)	Left axis deviation (LAD)
Negative (down)	Positive (up)	Right axis deviation (RAD)
Negative (down)	Negative (down)	Extreme (Northwest) axis

Causes of LAD: LBBB, left anterior fascicular block, inferior MI, LVH, WPW Causes of RAD: RBBB, left posterior fascicular block, RVH, lateral MI, PE, WPW

Step 4 - Intervals

Interval	Normal	Meaning
PR interval	0.12-0.20 sec (3-5 small boxes)	AV conduction time
QRS duration	< 0.12 sec (< 3 small boxes)	Ventricular depolarization
QT interval	Varies with rate; corrected QTc < 450 ms (men), < 470 ms (women)	Ventricular action potential duration
QTc formula	QT / √(R-R interval in seconds)	Bazett's formula

Step 5 - P Waves and Atrial Enlargement

Normal P wave: < 0.12 sec wide, < 2.5 mm tall, upright in I and II, biphasic in V1

Finding	Meaning
Tall peaked P > 2.5 mm in II	Right atrial enlargement ("P pulmonale")
Wide notched P > 0.12 sec in II	Left atrial enlargement ("P mitrale")
Biphasic P in V1 with large terminal negative component	Left atrial enlargement

Step 6 - QRS Complex

Normal Q waves: Small septal q waves in I, aVL, V5-V6 (< 1 small box wide, < 25% height of R wave)

Pathological Q waves: > 1 mm wide OR > 1/3 height of QRS = old infarction (not in III and aVR where Q waves can be normal)

R wave progression: R waves should grow from V1 to V5/V6 (poor R-wave progression = anterior MI or LVH)

Step 7 - ST Segment and T Waves

Finding	Meaning
ST elevation > 1 mm in 2+ contiguous leads	STEMI, pericarditis, Prinzmetal's angina, LV aneurysm
ST depression	Ischemia (NSTEMI/UA), reciprocal changes, digitalis effect
T wave inversion	Ischemia, strain pattern, PE, Wellens' syndrome
Tall peaked T waves	Hyperkalemia, hyperacute STEMI
Flat/inverted T waves	Hypokalemia, ischemia

PART 3 - ARRHYTHMIAS

3.1 Sinus Arrhythmias

Arrhythmia	Rate	Rhythm	P waves	PR	QRS
Normal sinus rhythm	60-100	Regular	Upright in II, before QRS	0.12-0.20	Normal
Sinus tachycardia	>100	Regular	Normal	Normal	Normal
Sinus bradycardia	<60	Regular	Normal	Normal	Normal
Sinus arrhythmia	60-100	Irregular (varies with breathing)	Normal	Normal	Normal
Sinus pause/arrest	Variable	Pause	Missing P	-	Missing QRS

3.2 Atrial Arrhythmias

Below is a classic textbook ECG showing atrial extrasystole, atrial tachycardia, atrial flutter, and atrial fibrillation side by side:

Atrial arrhythmias ECG: extrasystole, tachycardia, flutter, and fibrillation

FIGURE - Atrial arrhythmias. Top to bottom: atrial extrasystole (arrow = abnormal P buried in preceding T wave); atrial tachycardia; atrial flutter with 4:1 AV block (sawtooth pattern); atrial fibrillation with totally irregular ventricular rate. (Ganong's Review of Medical Physiology)

Atrial Extrasystoles (PACs)

Abnormal P wave morphology (ectopic atrial focus)
QRST usually normal (same ventricular activation)
Followed by a compensatory or non-compensatory pause
Clinically benign in most individuals

Atrial Tachycardia

Rate: 150-220 bpm from a single atrial focus
P waves present but different morphology from sinus
May have AV block (especially in digitalis toxicity)

Atrial Flutter

Atrial rate: 200-350 bpm, typically ~300 bpm
Classic sawtooth "flutter waves" at 300/min in leads II, III, aVF (negative) and V1 (positive)
Usually 2:1 AV block → ventricular rate ~150 bpm (always suspect flutter when rate is exactly 150)
Mechanism: large counterclockwise reentrant circuit in right atrium around cavo-tricuspid isthmus

The stress test below shows the natural progression from sinus tachycardia → rapid AF → 2:1 atrial flutter → AF → sinus rhythm:

Exercise stress test showing progression from sinus tachycardia to AF to atrial flutter to sinus rhythm

FIGURE - Exercise stress test showing sinus tachycardia, rapid AF, 2:1 atrial flutter, AF, and spontaneous return to sinus rhythm. (Braunwald's Heart Disease)

Atrial Fibrillation (AF)

Atrial rate: 300-500/min, completely irregular and disorganized
No distinct P waves - replaced by fibrillatory baseline
Irregularly irregular R-R intervals (pathognomonic)
Ventricular rate: 80-160 bpm
Mechanism: multiple concurrent reentrant excitation waves in both atria; some paroxysmal AF from ectopic foci in pulmonary veins (Ganong's Review of Medical Physiology)

3.3 AV Nodal Arrhythmias

AVNRT (AV Nodal Reentrant Tachycardia)

Most common SVT
Rate: 150-250 bpm, regular
P waves buried in or just after QRS (retrograde P, pseudo-R' in V1, pseudo-S in inferior leads)
Narrow QRS (unless aberrant)
Terminates with vagal maneuvers or adenosine

AVRT (AV Reentrant Tachycardia) - WPW

Accessory pathway connects atria and ventricles
Delta wave on baseline ECG: slurred upstroke of QRS, short PR (<0.12 sec), wide QRS
Orthodromic AVRT: narrow complex (down accessory, up normal)
Antidromic AVRT: wide complex (down normal, up accessory) - looks like VT!

3.4 Heart Blocks (AV Blocks)

The three general types are first-degree, second-degree, and third-degree AV block.

First-Degree AV Block

ECG finding: PR interval > 200 ms (> 1 large box), every P conducts to QRS

FIGURE - First-degree AV block. The PR interval is >200 ms in duration. (Frameworks for Internal Medicine)

Generally benign
Causes: inferior MI, increased vagal tone, digoxin, electrolyte abnormalities
No treatment needed for isolated first-degree block

Second-Degree AV Block - Mobitz Type I (Wenckebach)

ECG findings: Progressively lengthening PR intervals until a QRS is dropped, then the cycle resets.

Mobitz I (Wenckebach) second-degree AV block ECG

FIGURE - Mobitz I second-degree AV block. The P-wave rate is constant, but PR interval progressively lengthens until a QRS is completely blocked (after the 4th P wave). (Frameworks for Internal Medicine)

Level of block: AV node
Associated with: right coronary artery territory MI (inferior leads II, III, aVF)
Usually benign, may be transient
Rarely requires pacing

Second-Degree AV Block - Mobitz Type II

ECG findings: Constant PR interval with sudden unexpected drop of a QRS (no progressive PR lengthening).

Level of block: His-Purkinje system (below AV node)
High risk of progression to complete heart block
Permanent pacemaker usually required (Frameworks for Internal Medicine)

Second-Degree AV Block - 2:1 Block

2:1 AV block ECG with alternating conducted and non-conducted P waves

FIGURE - Second-degree AV block with 2:1 AV conduction. There are twice as many P waves as QRS complexes - every other atrial impulse is blocked. (Frameworks for Internal Medicine)

Every other P wave is non-conducted
Cannot distinguish Mobitz I from II electrocardiographically in pure 2:1 block
PR interval of conducted beats helps: prolonged = more likely Mobitz I; normal = more likely Mobitz II

Third-Degree (Complete) AV Block

ECG findings: Complete AV dissociation - P waves and QRS complexes are completely independent.

Atrial rate (P waves) is faster than the ventricular rate (QRS)
Escape rhythm: junctional (narrow QRS, 40-60 bpm) or ventricular (wide QRS, 20-40 bpm)
Causes: inferior MI (transient), anterior MI (permanent), Lyme disease, digoxin toxicity, fibrosis
Treatment: atropine acutely; permanent pacemaker for persistent cases (Goldman-Cecil Medicine)

3.5 Bundle Branch Blocks

Feature	RBBB	LBBB
QRS duration	≥ 0.12 sec	≥ 0.12 sec
V1 morphology	rSR' ("rabbit ears"/ M-shaped)	Broad rS or QS
V6 morphology	qRS with wide S	Broad monophasic R, no Q
Mnemonic	WiLLiaM MaRRoW (W in V1, M in V6 = LBBB; M in V1, W in V6 = RBBB)	Same mnemonic
ST/T changes	Discordant (opposite to QRS direction)	Discordant
Axis	Normal or RAD	Normal or LAD
Significance	Often benign; new RBBB can indicate PE, RV strain	Significant - rules out STEMI interpretation; new LBBB = possible ACS

Memory trick - "WiLLiaM MaRRoW":

LBBB: W in V1, M in V6 (= "WilLiam")
RBBB: M in V1, W in V6 (= "MaRRow")

3.6 Ventricular Arrhythmias

Premature Ventricular Contractions (PVCs)

Wide, bizarre QRS (> 0.12 sec), not preceded by a P wave
Followed by full compensatory pause (SA node not reset)
T wave opposite in polarity to QRS
If PVCs appear every other beat = bigeminy; every 3rd beat = trigeminy; consecutive pairs = couplets

Ventricular Tachycardia (VT)

Rate: > 100 bpm (usually 140-250), wide QRS (> 0.12 sec)
AV dissociation (P waves march through independently) = diagnostic of VT
Fusion beats (sinus + ventricular = hybrid QRS) = diagnostic of VT
Capture beats (sinus impulse successfully captures ventricle = narrow QRS amid wide ones) = diagnostic of VT

The algorithm below - from Goldman-Cecil Medicine - is the definitive clinical tool for distinguishing wide-QRS tachycardia VT from SVT with aberrancy:

Wide QRS complex tachycardia ECG algorithm: distinguishing VT from SVT with aberrancy

FIGURE - ECG algorithm for diagnosis of wide-complex tachycardias. Key decision points: regularity, AV relationship, QRS morphology in precordial leads. (Goldman-Cecil Medicine)

Ventricular Fibrillation (VF)

Completely chaotic, disorganized electrical activity
No identifiable QRS complexes, P waves, or T waves
"Coarse" VF = larger amplitude; "fine" VF = nearly flat
Immediately life-threatening; requires immediate defibrillation

PART 4 - ISCHEMIA AND INFARCTION

4.1 The ECG Progression of STEMI

Ischemia → Injury → Infarction follows a predictable ECG sequence:

Stage 1 (Minutes): Hyperacute T waves

Tall, peaked, symmetric T waves (often first sign)

Stage 2 (Hours): ST elevation

Convex ("tombstone") ST elevation ≥ 1 mm in limb leads, ≥ 2 mm in precordial leads in ≥ 2 contiguous leads
Reciprocal ST depression in opposite leads

Stage 3 (Hours-Days): Q waves develop

Pathological Q waves: > 1 mm wide or > 25% of QRS height

Stage 4 (Days-Weeks): T-wave inversion

T waves invert in the leads showing ST elevation

Stage 5 (Weeks-Months): Normalization or persistent Q waves

Q waves may persist indefinitely (scar)

4.2 STEMI by Territory

Below is a real 12-lead ECG of an anterior STEMI (Rosen's Emergency Medicine):

Anterior STEMI 12-lead ECG - ST elevation in V1-V4

FIGURE - Anterior wall STEMI. ST elevation in V1-V4 with straight/oblique morphology. 90% LAD stenosis confirmed on catheterization. (Rosen's Emergency Medicine)

And an anterolateral STEMI involving V2-V6, I, and aVL:

Anterolateral STEMI 12-lead ECG - ST elevation V2-V6, I, aVL

FIGURE - Anterolateral STEMI. STE in V2-V6, I, and aVL. 100% in-stent LAD thrombosis on catheterization. (Rosen's Emergency Medicine)

Infarct Location	Leads with Changes	Artery
Anterior	V1-V4	LAD
Anterolateral	V1-V6, I, aVL	LAD or LCx
Lateral	I, aVL, V5-V6	LCx
Inferior	II, III, aVF	RCA (80%) / LCx
Posterior	Tall R and ST depression V1-V2 (mirror test)	RCA or LCx
Right ventricular	ST elevation in V3R-V4R, inferior ST elevation	RCA proximal
Left main/proximal LAD	ST elevation in aVR + diffuse ST depression	Left main or proximal LAD

4.3 Special STEMI Patterns

de Winter Pattern (proximal LAD occlusion without classic STE):

Upsloping ST depression with tall, peaked T waves in precordial leads V1-V6
ST elevation in aVR
Treat as STEMI equivalent

Wellens' Syndrome (proximal LAD critical stenosis, pain-free):

Type A: Biphasic T waves (upright then inverted) in V2-V3
Type B: Deep symmetric T wave inversions in V2-V3
Do NOT stress test; high risk for massive anterior MI

Sgarbossa Criteria (STEMI in LBBB):

Concordant ST elevation ≥ 1 mm in leads with positive QRS (+5 points)
Concordant ST depression ≥ 1 mm in V1-V3 (+3 points)
Discordant ST elevation ≥ 5 mm (+2 points)
Score ≥ 3 = high specificity for acute MI

PART 5 - HYPERTROPHY

Left Ventricular Hypertrophy (LVH)

Voltage criteria (any one sufficient with appropriate clinical context):

Sokolow-Lyon: S in V1 + R in V5 or V6 > 35 mm
Cornell: R in aVL + S in V3 > 28 mm (men) or > 20 mm (women)
R in aVL > 11 mm
Strain pattern: ST depression and T-wave inversion in V5-V6, I, aVL (lateral "strain")

Right Ventricular Hypertrophy (RVH)

Tall R wave in V1 (R > S in V1)
Right axis deviation (> +90°)
ST depression and T-wave inversion in V1-V4 (right "strain")
Causes: pulmonary hypertension, cor pulmonale, severe pulmonary stenosis, PE (acute)

PART 6 - METABOLIC AND DRUG CHANGES

Hyperkalemia (progressive changes with rising K+)

K+ Level	ECG Changes
5.5-6.5 mEq/L	Tall, peaked, "tented" T waves
6.5-7.5 mEq/L	Widened QRS, prolonged PR, flattened P waves
> 7.5 mEq/L	Absent P waves, "sine wave" pattern
> 9 mEq/L	VF or asystole

Hypokalemia

Flattened T waves
U waves become prominent (U > T in same lead)
ST depression
Prolonged QU interval (often misread as long QT)

Hypercalcemia

Shortened QT interval
Short ST segment

Hypocalcemia

Prolonged QT interval (long ST segment)

Digoxin effect (not toxicity)

"Reverse tick" or "sagging" ST depression (scooped ST depression)
Shortened QT
T-wave flattening or inversion

Digoxin toxicity

Any arrhythmia, especially PAT with block, junctional tachycardia, bidirectional VT
PVCs (bigeminy very common)

PART 7 - OTHER IMPORTANT CONDITIONS

Pericarditis

Stage I (acute): Diffuse concave ("saddle-shaped") ST elevation in nearly all leads except aVR and V1 (where ST is depressed). PR depression is pathognomonic.

Key differentiator from STEMI: In pericarditis, STE is diffuse (not in a single vascular territory), concave (not convex), and PR depression is present.

Stage II-IV: ST normalizes → T-wave inversion → normalization.

Pulmonary Embolism

Classic (but not universal) ECG findings:

S1Q3T3: S wave in lead I, Q wave and T-wave inversion in lead III
Sinus tachycardia (most common finding)
New RBBB or right heart strain pattern
T-wave inversions in V1-V4
Right axis deviation

Brugada Syndrome

Type 1 (diagnostic): Coved ST elevation ≥ 2 mm with T-wave inversion in V1-V2 (unprovoked or with sodium channel blockers)
Type 2: Saddle-back ST elevation in V1-V2 (not diagnostic alone)
Associated with sudden cardiac death from VF in structurally normal hearts
More common in males, South/Southeast Asians

Long QT Syndrome

QTc > 450 ms (men), > 470 ms (women), or > 500 ms (high risk)
Risk of torsades de pointes (polymorphic VT)
Causes: congenital (LQTS1, 2, 3) or acquired (drugs - quinidine, sotalol, erythromycin, antipsychotics; hypokalemia, hypomagnesemia)

Hypothermia

Osborn (J) wave: Positive deflection at the J-point (junction of QRS and ST segment), best seen in inferior and lateral leads
Shivering artifact
Bradycardia, prolonged intervals
Osborn wave size correlates with depth of hypothermia

PART 8 - QUICK REFERENCE SUMMARY TABLE

Condition	Rate	Rhythm	P Waves	PR	QRS	ST/T
Normal sinus	60-100	Regular	Upright, before each QRS	0.12-0.20	Narrow <0.12	Normal
Sinus tachycardia	>100	Regular	Normal	Normal	Normal	Normal
AF	Variable	Irregularly irregular	No P (fibrillatory baseline)	None	Narrow (usually)	Variable
Atrial flutter	~150 (with 2:1)	Regular or variable	Sawtooth 300/min	Variable	Narrow	Variable
SVT/AVNRT	150-250	Regular	Buried/retrograde	None visible	Narrow	Normal
1st-degree AV block	Normal	Regular	Normal	>0.20 sec	Normal	Normal
Mobitz I (Wenckebach)	Normal	Regularly irregular	Normal	Progressive lengthening then drop	Normal	Normal
Mobitz II	Normal	Regularly irregular	Normal	Constant, then QRS drops	Normal/wide	Normal
3rd-degree block	Slow	Regular (but dissociated)	Normal atrial rate	Completely dissociated	Wide (escape) or narrow	Normal
RBBB	Normal	Regular	Normal	Normal	≥0.12, rSR' in V1, wide S in V6	Discordant T
LBBB	Normal	Regular	Normal	Normal	≥0.12, broad R in V6, QS in V1	Discordant T
STEMI	Variable	Any	Normal	Normal	May have Q waves	ST elevation, convex
NSTEMI/UA	Variable	Any	Normal	Normal	Normal	ST depression/T inversion
LVH	Normal	Regular	May be enlarged	Normal	Tall voltage	Lateral strain
Hyperkalemia	Slow if severe	Variable	Tall peaked T first, then P disappears	Prolonged	Widened	Tall T, then sine wave
VT	>100	Regular	Dissociated	None	Wide ≥0.12	Discordant
VF	-	Chaotic	None	None	No QRS	Chaotic

PART 9 - CLINICAL MEMORY AIDS

Rate calculation: 300 ÷ (number of large boxes between R waves) = rate in bpm

Axis quick rule: "If I and aVF are both up → normal axis. I up, aVF down → LAD. I down, aVF up → RAD."

Bundle branch block mnemonic: WiLLiaM MaRRoW - LBBB = W in V1, M in V6; RBBB = M in V1, W in V6

AV blocks mnemonic: "1st degree = PR prolonged, everyone gets through. 2nd degree = some are dropped. 3rd degree = none get through."

STEMI localization: "Inferior = II, III, aVF; Anterior = V1-V4; Lateral = I, aVL, V5-V6"

Hyperkalemia progression: "Peaked T → Wide QRS → No P → Sine wave → Death"

Key reference for further reading: The step-by-step approach above is derived from Boron & Boulpaep's Medical Physiology, Braunwald's Heart Disease, Goldman-Cecil Medicine, Rosen's Emergency Medicine, and Frameworks for Internal Medicine. For video-based learning, this Ninja Nerd lecture is an excellent companion:

Watch "https://www.youtube.com/watch?v=CNN30YHsJw0" on YouTube

https://www.youtube.com/watch?v=CNN30YHsJw0

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