Common heart disorder with ecg and explanation

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atrial fibrillation ECG 12 lead

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Common Heart Disorders: ECG Findings & Explanations

Understanding the ECG is central to diagnosing cardiac conditions. Below are the most clinically important heart disorders, each with their characteristic ECG pattern, explanation, and a real ECG example.

1. 🫀 Atrial Fibrillation (AF)

The most common sustained cardiac arrhythmia — affects over 33 million people worldwide.

Mechanism

Chaotic, disorganized electrical firing from multiple foci within the atria (often near the pulmonary vein ostia) replaces the coordinated P-wave generated by the sinoatrial node. The AV node is bombarded with impulses at 350–600/min and conducts them irregularly to the ventricles.

ECG Features

Feature	Finding
P waves	Absent — replaced by fine fibrillatory (f) waves
Rhythm	Irregularly irregular R-R intervals
QRS	Narrow (<120 ms) unless aberrant conduction
Rate	Variable; rapid if uncontrolled (>100 bpm = AF with RVR)

Clinical Significance

Reduces cardiac output by ~20% (loss of atrial "kick")
High stroke risk (Virchow's triad: stasis in left atrial appendage)
Rate control: beta-blockers, calcium channel blockers, digoxin
Anticoagulation: warfarin or DOACs (dabigatran, rivaroxaban, apixaban)

Atrial Fibrillation ECG — classic irregularly irregular rhythm with absent P waves

Classic AF: absent P waves, fine fibrillatory baseline, irregularly irregular narrow QRS complexes with rapid ventricular response.

2. ❤️‍🔥 ST-Elevation Myocardial Infarction (STEMI)

A cardiac emergency requiring immediate reperfusion (door-to-balloon <90 min).

Mechanism

Complete occlusion of a coronary artery (most commonly the Left Anterior Descending = LAD) causes transmural ischemia. Injured myocytes alter their ion channel behavior, producing the characteristic ST changes.

ECG Features — Evolving Sequence

Stage	ECG Finding
Hyperacute (minutes)	Tall, peaked (hyperacute) T waves
Acute (hours)	ST elevation ≥1 mm in ≥2 contiguous leads; convex ("tombstone") morphology
Established	Q waves develop (>40 ms, >25% of R height)
Reperfusion/old	T-wave inversion, persistent Q waves

Localization by Lead

Territory	Leads with ST Elevation	Artery
Anterior	V1–V4	LAD
Inferior	II, III, aVF	RCA
Lateral	I, aVL, V5–V6	LCx
Posterior	Tall R in V1–V2 + ST depression	RCA/LCx

Acute anterior STEMI: convex ST elevation in V1–V6 (greatest in V2–V4), consistent with LAD occlusion. Note reciprocal changes in inferior leads.

3. 🚨 Complete (Third-Degree) AV Block

Life-threatening conduction failure — the atria and ventricles beat independently.

Mechanism

No impulses pass from the atria through the AV node to the ventricles. The ventricles maintain themselves via a slow escape rhythm (junctional at 40–60 bpm, or ventricular at 20–40 bpm).

ECG Features

Feature	Finding
P waves	Present, regular, at normal atrial rate (~60–100/min)
QRS	Present, regular, but SLOWER and independent of P waves
PR interval	No fixed PR interval — complete AV dissociation
QRS width	Narrow if junctional escape; wide (>120 ms) if ventricular escape

Classic Sign

"P waves marching through QRS complexes" — P waves appear before, within, and after QRS complexes with no consistent relationship.

Complete heart block: regular P waves and regular (but slower, wide) QRS complexes occurring independently — classic AV dissociation pattern.

Management

Urgent pacing (transcutaneous → permanent pacemaker).

4. ⚡ Ventricular Tachycardia (VT)

A life-threatening ventricular arrhythmia, especially in structural heart disease.

Mechanism

A reentrant circuit or automatic focus within the ventricular myocardium fires rapidly (>100 bpm), bypassing the normal His-Purkinje system → wide, aberrant QRS complexes.

ECG Features

Feature	Finding
Rate	100–250 bpm
Rhythm	Regular
QRS	Wide (>120 ms), bizarre morphology
P waves	May be dissociated (AV dissociation) or absent
Fusion/capture beats	Pathognomonic when present

Brugada Criteria (VT vs SVT with aberrancy)

AV dissociation → VT
QRS >160 ms → VT
Concordance (all QRS same direction V1–V6) → VT

Wide-Complex Ventricular Tachycardia ECG

Monomorphic VT: rapid rate, wide bizarre QRS complexes (~200 ms), no clear P waves — this is a medical emergency.

5. 🔀 Bundle Branch Blocks (BBB)

Conduction delay in either the left (LBBB) or right (RBBB) bundle branch causes delayed ventricular activation.

Right Bundle Branch Block (RBBB)

QRS ≥120 ms
rSR' ("rabbit ears") in V1
Broad slurred S wave in leads I, V5, V6
Secondary T-wave inversion in V1–V3
Causes: pulmonary embolism, ASD, right heart strain

Left Bundle Branch Block (LBBB)

QRS ≥120 ms
Broad notched R wave in V5–V6, I, aVL
Deep QS or rS in V1
No septal Q waves in lateral leads
New LBBB = treat as STEMI equivalent

RBBB vs LBBB QRS-T patterns in V1 and V6

Harrison's diagram comparing normal QRS-T morphology (top) vs RBBB (middle: rSR' in V1, broad S in V6) vs LBBB (bottom: broad R in V6, deep S/QS in V1).

6. 📈 Comparative Summary Table

Disorder	Rate	Rhythm	P Waves	QRS	Key Feature
Atrial Fibrillation	Variable	Irregularly irregular	Absent (f waves)	Narrow	No P waves
STEMI	Normal	Regular	Normal	Normal (early)	ST elevation ≥1 mm
3° AV Block	20–60	Regular (escape)	Present, unrelated	Wide or narrow	AV dissociation
Ventricular Tachycardia	100–250	Regular	Dissociated	Wide >120 ms	Wide-complex tachycardia
RBBB	Normal	Regular	Normal	Wide (rSR' V1)	Rabbit ears in V1
LBBB	Normal	Regular	Normal	Wide (notched R)	Broad R in V5/V6

Quick ECG Reading Framework

Rate — count R-R intervals (300 ÷ large squares)
Rhythm — regular or irregular?
P waves — present? morphology? relationship to QRS?
PR interval — 120–200 ms (3–5 small squares)
QRS duration — <120 ms = narrow; ≥120 ms = wide
ST segment — elevation or depression?
T waves — peaked, inverted, or biphasic?
QT interval — corrected QTc <440 ms (male), <460 ms (female)

Sources: Braunwald's Heart Disease (10th ed.); Harrison's Principles of Internal Medicine (21st ed.)

With images

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Common Heart Disorders — ECG with Images & Explanation

1. 🫀 Atrial Fibrillation (AF)

What you see on the ECG

No P waves — replaced by a chaotic, irregular fibrillatory baseline (best seen in V1 and lead II)
Irregularly irregular R-R intervals (no two beats are equally spaced)
Narrow QRS complexes (unless aberrant conduction exists)
Rapid ventricular rate (here ~120 bpm = AF with rapid ventricular response)

Why it happens

Multiple chaotic electrical wavelets circulate through the atria at 350–600 impulses/min. The AV node acts as a gatekeeper, letting through random impulses — producing the hallmark irregularly irregular rhythm. The atria quiver rather than contract, eliminating the "atrial kick" that contributes ~20% of cardiac output.

Key Clinical Points


Stroke risk	High — blood pools in the left atrial appendage → thrombus → embolism
Rate control	Beta-blockers, diltiazem, verapamil, digoxin
Rhythm control	Cardioversion (electrical or chemical), ablation
Anticoagulation	DOACs or warfarin (CHA₂DS₂-VASc score guides use)

2. ❤️‍🔥 ST-Elevation Myocardial Infarction (STEMI)

What you see on the ECG

Massive ST elevation across V1–V6 (precordial leads) with a convex "tombstone" morphology
Hyperacute T waves (tall, peaked) in V2–V4
Reciprocal ST depression in inferior leads (II, III, aVF)
Sinus rhythm maintained

Why it happens

A complete coronary artery occlusion (here the Left Anterior Descending artery) cuts off blood supply to the anterior wall. Ischemic myocytes leak K⁺ and accumulate Ca²⁺, altering their resting membrane potential — this shifts the ST segment above the isoelectric line. The longer the occlusion, the more cells die → Q waves develop (irreversible necrosis).

Localization Table

Territory	Leads with ST↑	Culprit Artery
Anterior	V1–V4	LAD
Inferior	II, III, aVF	RCA
Lateral	I, aVL, V5–V6	LCx
Posterior	Tall R + ST↓ in V1–V2	RCA/LCx

⚡ Emergency: Call cath lab immediately. Door-to-balloon time <90 min saves lives.

3. 🚨 Complete (Third-Degree) AV Block

What you see on the ECG

P waves are present — regular, at normal atrial rate (~80–100/min)
QRS complexes are present — regular, but slower (~30–40/min here)
No fixed PR interval — P waves "march through" QRS complexes completely independently
Wide QRS complexes — indicating a ventricular escape rhythm (infra-Hisian origin)

Why it happens

The AV node or His-Purkinje system is completely diseased — zero impulses cross from atria to ventricles. The ventricles rely on a slow escape pacemaker (junctional at 40–60 bpm, or ventricular at 20–40 bpm) to keep beating. This is called AV dissociation.

Causes

Inferior MI (RCA occlusion), Lyme disease, surgical/procedural trauma, degenerative conduction disease (Lenègre disease), hyperkalemia, drugs (digoxin toxicity, beta-blockers in excess).

⚡ Emergency: Transcutaneous pacing → transvenous pacing → permanent pacemaker.

4. ⚡ Ventricular Tachycardia (VT)

What you see on the ECG

Rapid rate (~116 bpm here)
Very wide, bizarre QRS complexes (~200 ms) — markedly abnormal morphology
Tall, peaked R waves in V1–V2; deep S waves in V4–V6 — discordant T waves
No identifiable P waves — AV dissociation

Why it happens

A reentrant circuit or automatic focus fires within the ventricular myocardium, bypassing the normal His-Purkinje system. Activation spreads slowly cell-to-cell → wide QRS. The fast ventricular rate reduces filling time, drops cardiac output, and can degenerate into ventricular fibrillation (VF) → sudden cardiac death.

VT vs SVT with Aberrancy — Key Differentiators

Feature	Points to VT
AV dissociation	✅ Highly specific
QRS >160 ms	✅
Concordance (all V1–V6 same direction)	✅
Fusion beats	✅ Pathognomonic
History of structural heart disease	✅

⚡ Emergency: If unstable → synchronized cardioversion. If stable → amiodarone, lidocaine.

5. 🔀 Bundle Branch Blocks — RBBB & LBBB

Harrison's Principles of Internal Medicine — Figure 240-10: QRS-T patterns in RBBB and LBBB compared to normal, in leads V₁ and V₆.

What you see

	RBBB	LBBB
V1	rSR' ("rabbit ears")	Deep broad QS or rS
V6	Broad slurred S wave	Broad notched R wave
QRS duration	≥120 ms	≥120 ms
T-wave	Inverted in V1–V3	Inverted in V5–V6, I, aVL
Septal Q waves	Present in I, V5–V6	Absent

Why it happens

RBBB: Impulse can't travel down the right bundle → right ventricle activated late via slow cell-to-cell spread → terminal R' in V1
LBBB: Impulse can't travel down the left bundle → left ventricle activated late → the septum activates right→left (opposite of normal), erasing normal septal Q waves, creating broad R in lateral leads

Clinical Significance

	RBBB	LBBB
May be normal	✅ (isolated RBBB)	❌ (usually pathological)
New = Emergency	Only if with MI	✅ New LBBB = STEMI equivalent
Causes	PE, ASD, aging	IHD, HTN, cardiomyopathy, LBBB

6. 📊 Multi-Arrhythmia Comparison

Comparative rhythm strips: (a) Normal sinus rhythm; (b) Isolated PVCs; (c) Multifocal PVCs; (d) Atrial tachycardia; (e) Sinus arrhythmia; (f) Frequent PVCs; (g) Left Bundle Branch Block (LBBB) — widened QRS with secondary ST-T changes.

Quick ECG Reading Checklist

1. RATE          → 300 ÷ (large squares between R waves)
2. RHYTHM        → Regular? Irregularly irregular?
3. P WAVES       → Present? Morphology? Each followed by QRS?
4. PR INTERVAL   → 120–200 ms (3–5 small squares) normal
5. QRS WIDTH     → <120 ms = narrow; ≥120 ms = wide (BBB or VT)
6. ST SEGMENT    → Elevation ≥1mm? Depression?
7. T WAVES       → Peaked (hyperacute)? Inverted?
8. QTc           → <440 ms (male), <460 ms (female)

Summary Table

Disorder	Rate	Rhythm	P Waves	QRS	Signature Finding
Atrial Fibrillation	Variable	Irreg. irreg.	Absent (f waves)	Narrow	No P waves
STEMI	Normal	Regular	Normal	Normal	ST elevation
3° AV Block	20–60	Regular (escape)	Present, unrelated	Wide	AV dissociation
Ventricular Tachycardia	100–250	Regular	Dissociated	Wide >120 ms	Wide-complex tachycardia
RBBB	Normal	Regular	Normal	Wide	rSR' in V1
LBBB	Normal	Regular	Normal	Wide	Broad R in V5/V6

Sources: Braunwald's Heart Disease (11th ed.); Harrison's Principles of Internal Medicine (21st ed.)

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