10 Common Heart Diseases with ECG Changes

1. 🫀 ST-Elevation Myocardial Infarction (STEMI)

• Hyperacute T waves — earliest change; tall, broad, peaked T waves
• ST-segment elevation ≥ 1 mm in ≥ 2 contiguous limb leads or ≥ 2 mm in precordial leads (convex "tombstone" shape)
• Reciprocal ST depression in opposite leads
• Pathological Q waves (>40 ms wide, >25% QRS height) — evolve over hours/days
• T-wave inversion follows in the recovery phase

ECG — Anterolateral STEMI:

ECG — Inferior STEMI:

2. 🫀 Atrial Fibrillation (AF)

• Absent P waves — replaced by irregular fibrillatory (f) waves, best seen in V1
• Irregularly irregular R-R intervals (hallmark finding)
• Narrow QRS (unless aberrant conduction/BBB)
• Ventricular rate varies (bradycardic if controlled, tachycardic if uncontrolled)
• Baseline shows fine, chaotic oscillations (350–600 impulses/min from atria)

ECG — Atrial Fibrillation with rapid ventricular response:

3. 🫀 Complete (Third-Degree) AV Block

• Complete AV dissociation — P waves and QRS complexes march independently
• Regular P-P intervals (atrial rate, typically 60–100 bpm)
• Regular R-R intervals (slow escape rhythm, typically 20–45 bpm)
• No constant PR interval — P waves can fall before, within, or after QRS
• Wide QRS if escape is ventricular (>120 ms); narrow QRS if junctional escape
• Requires urgent pacemaker implantation

ECG — Complete Third-Degree AV Block:

4. 🫀 Ventricular Tachycardia (VT)

• Wide complex tachycardia (QRS ≥120 ms), rate 100–250 bpm
• Regular or slightly irregular rhythm
• AV dissociation (P waves independent of QRS) — pathognomonic when seen
• Fusion beats and capture beats (Dressler beats) — diagnostic
• Concordance — all precordial leads positive (positive concordance) or negative (negative concordance)
• Extreme axis deviation ("northwest" axis, −90° to ±180°)
• Brugada criteria and Vereckei criteria used to differentiate from SVT with aberrancy

ECG — Ventricular Tachycardia (monomorphic wide complex):

5. 🫀 Left Bundle Branch Block (LBBB)

• QRS duration ≥120 ms
• Broad, notched (M-shaped) R waves in lateral leads (I, aVL, V5, V6)
• Deep, broad S waves or QS pattern in right precordial leads (V1, V2)
• No septal Q waves in I, V5, V6
• Discordant ST/T changes (ST and T in opposite direction to main QRS deflection — secondary repolarization change)
• Left axis deviation common

ECG — Left Bundle Branch Block:

6. 🫀 Wolff-Parkinson-White (WPW) Syndrome

• Short PR interval (<120 ms) — bypass of AV nodal delay
• Delta wave — slurred upstroke at start of QRS (pre-excitation of ventricle)
• Wide QRS (>120 ms) — fusion of pre-excited and normally conducted activation
• Secondary ST/T changes discordant to delta wave
• Accessory pathway localization possible by delta wave polarity (negative delta in inferior leads → posteroseptal pathway; positive V1 → left-sided pathway)

ECG — Wolff-Parkinson-White Syndrome:

7. 🫀 Pulmonary Embolism (PE)

• Sinus tachycardia — most common finding
• S1Q3T3 pattern (McGinn-White sign) — S wave in lead I + Q wave in lead III + inverted T wave in lead III
• Right axis deviation
• Incomplete or complete RBBB — due to RV strain
• T-wave inversions in V1–V4 — reflects right ventricular strain pattern
• P pulmonale (tall P in II >2.5 mm) — right atrial enlargement
• Atrial fibrillation may occur
• Normal ECG does NOT exclude PE

ECG — Pulmonary Embolism (S1Q3T3 pattern, annotated):

8. 🫀 Acute Pericarditis

• PR segment depression in most leads (II, V2–V6) — highly specific; aVR shows reciprocal PR elevation
• Diffuse ST elevation — concave upward, not confined to one coronary territory
• No reciprocal ST depression (except aVR and V1)
• Spodick's sign — downsloping TP segment
• No pathological Q waves (differentiates from MI)

ECG — Acute Pericarditis:

9. 🫀 Hyperkalemia

ECG — Severe Hyperkalemia:

10. 🫀 Hypertrophic Cardiomyopathy (HCM)

• LVH voltage criteria — Sokolow-Lyon: SV1 + RV5/V6 ≥35 mm; Cornell criteria >28 mm men / >20 mm women
• Deep, "giant" T-wave inversions in V2–V6 — especially in apical HCM (Yamaguchi syndrome); T waves can be >10 mm deep
• Deep, narrow Q waves in inferior (II, III, aVF) and lateral leads (V4–V6, I, aVL) — from septal hypertrophy
• Abnormal septal Q waves absent in I, V5, V6 (opposite of normal)
• ST-segment depression with lateral strain pattern
• Prolonged QTc in some variants
• Left axis deviation
• Increased risk of VT/VF → sudden cardiac death in young athletes

ECG — Hypertrophic Cardiomyopathy (HCM):

HCM ECG variants compared (concentric, septal, apical):

Quick Reference Summary

Clinical Pearl: Always interpret ECGs in the clinical context. A single ECG can be normal even in serious disease (e.g., early STEMI, PE), and pathological findings (e.g., LBBB, LVH) require clinical correlation to guide management.