Common Cardiac Diseases and Their ECG Changes

1. Acute Myocardial Infarction (AMI) / Ischemic Heart Disease

Pathophysiology of ECG Changes

ST-Elevation MI (STEMI)

1. Hyperacute T waves - tall, broad, positive T waves (earliest sign, minutes)
2. ST elevation - upward convex or straight, in the territory of ischemia
3. Q wave formation - pathological Q waves (>1mm wide, >1/4 R height) indicating necrosis
4. T wave inversion - develops over hours to days
5. ST normalization, Q waves may persist

Special STEMI Localization Points

• Inferior STEMI (RCA): ST elevation greater in III than II, plus ST depression in I and aVL (reciprocal); if ST elevation also in V1, suspect right ventricular involvement - confirm with right-sided leads (V3R-V4R)
• Posterior STEMI: Tall R wave + ST depression in V1-V3 (mirror image); use posterior leads V7-V9
• Wellens' Syndrome: Deeply inverted or biphasic T waves in V2-V3 (sometimes V1-V4) when pain-free - indicates critical LAD stenosis, high risk for anterior STEMI. Present in ~75% as deep inversion, ~25% as biphasic pattern (Tintinalli's Emergency Medicine)

NSTEMI / Unstable Angina

• ST depression (horizontal or downsloping) - more common in subendocardial ischemia
• T-wave inversion without Q waves
• Normal ECG - does not exclude NSTEMI (biomarkers required)
• When subendocardial ischemia is diffuse: ST depression in multiple leads + ST elevation in aVR

2. Acute Pericarditis

• ST elevation is diffuse (not territory-specific)
• ST elevation is concave upward ("saddle-shaped"), not convex
• PR depression is highly characteristic (absent in STEMI)
• No reciprocal ST changes (unlike STEMI, except aVR/V1)
• No Q waves
• Spodick's sign: downward-sloping TP segment (sensitive but not specific)

3. Heart Failure / Cardiomyopathy

Dilated Cardiomyopathy (DCM)

• Sinus tachycardia (compensatory)
• Left bundle branch block (LBBB) - common; broad QRS >120ms, notched R in I/V5-V6, QS in V1
• Left ventricular hypertrophy (LVH) - Sokolow-Lyon criteria: S in V1 + R in V5/V6 >35mm; or Cornell criteria
• Atrial fibrillation (from atrial dilation)
• Non-specific ST-T changes
• Poor R-wave progression in precordial leads
• Interventricular conduction delay

Hypertrophic Cardiomyopathy (HCM)

• LVH pattern (often striking voltage)
• Deep, narrow Q waves (septal hypertrophy) - especially in inferolateral leads (II, III, aVF, V5-V6) - "dagger-like" Q waves
• T-wave inversions (especially lateral leads)
• ST depression in lateral leads
• Left axis deviation
• Apical HCM variant: giant negative T waves in precordial leads V4-V6 (Yamaguchi pattern)
• ECG Q waves or inferolateral changes help differentiate from athlete's heart (Goldman-Cecil Medicine)

Restrictive Cardiomyopathy / Cardiac Amyloidosis

• Low voltage (small QRS complexes despite thick walls on echo - a striking mismatch)
• Pseudo-infarction Q waves (without actual infarction)
• Conduction abnormalities (AV block, bundle branch blocks)
• Atrial fibrillation

4. Conduction System Diseases / Arrhythmias

Bundle Branch Blocks

• Left anterior fascicular block (LAFB): Left axis deviation < -45°, no other cause
• Left posterior fascicular block (LPFB): Right axis deviation >+110°, rare as isolated finding (Harrison's Principles of Internal Medicine)

Wolff-Parkinson-White (WPW) Syndrome

• Short PR interval (<120ms)
• Delta wave (slurred upstroke of QRS)
• Wide QRS (>120ms, due to preexcitation)
• Can mimic LBBB, RBBB, or MI (pseudo-infarct Q waves)
• Associated with SVT (AVRT) or AF with rapid ventricular response

Atrial Fibrillation

• Irregularly irregular RR intervals
• Absence of distinct P waves (replaced by fibrillatory f waves, best seen in V1)
• Ventricular rate varies

Atrial Flutter

• Sawtooth flutter waves (F waves) at ~300 bpm, best in II, III, aVF
• Ventricular rate typically 150 bpm (2:1 block)

Complete AV Block (3rd Degree)

• P waves and QRS complexes completely dissociated
• P rate faster than QRS rate
• Narrow escape (junctional) or wide escape (ventricular) QRS

5. Infective Endocarditis (IE)

• Usually nonspecific findings
• New AV block (1st, 2nd, or 3rd degree) or bundle branch block - signals perivalvular extension, especially from aortic valve IE extending to the AV node/His bundle (10-20% incidence with perivalvular extension)
• New conduction abnormality is a multivariate predictor of death in IE (Braunwald's Heart Disease)

6. Pulmonary Embolism (PE)

• Sinus tachycardia (most common finding)
• S1Q3T3 pattern: S wave in lead I, Q wave and T-wave inversion in lead III (classic but insensitive)
• Right heart strain: Right axis deviation, right bundle branch block (complete or incomplete), ST depression + T-wave inversions in V1-V4
• T-wave inversions in V1-V4 (right ventricular strain)
• New atrial fibrillation
• Sinus tachycardia is the most sensitive but least specific finding

7. Electrolyte Abnormalities (Cardiac-relevant)

Hyperkalemia

1. Peaked, symmetric ("tent-shaped") T waves (earliest)
2. Prolonged PR interval and widened QRS
3. Loss of P waves
4. Sine wave pattern (severe, precedes cardiac arrest)
5. Ventricular fibrillation / asystole

Hypokalemia

• Flattened or inverted T waves
• Prominent U waves (best seen V2-V3)
• QU prolongation (can trigger TdP)
• ST depression

Hypercalcemia

• Short QT interval (shortened ST segment)

Hypocalcemia

• Prolonged QT interval (prolonged ST segment specifically)

8. Long QT Syndrome (LQTS) & Brugada Syndrome

Long QT Syndrome

• Prolonged QTc (>440ms men, >460ms women)
• Risk of polymorphic VT (Torsades de Pointes - TdP)
• TdP on ECG: twisting of QRS axis around baseline, "spindle-fuseau" pattern

Brugada Syndrome

• Coved ST elevation (≥2mm) in V1-V2, followed by negative T wave (Type 1 - diagnostic)
• Right bundle branch block morphology
• Associated with sudden cardiac death, especially at rest/sleep

9. Aortic Stenosis / Valvular Disease

• LVH (pressure overload - concentric hypertrophy)
• Left axis deviation
• ST-T strain pattern ("LV strain"): ST depression and T-wave inversion in lateral leads (I, aVL, V5-V6) - the "strain pattern" suggests significant hypertrophy

10. Mitral Valve Prolapse (MVP)

• May be normal or show non-specific changes
• T-wave inversions in inferior (II, III, aVF) and lateral leads
• ST-segment changes that can mimic ischemia
• Associated with ventricular ectopy (PVCs) (Grainger & Allison's Diagnostic Radiology)

ECG Localization Quick Reference