Common Pathological ECG Findings

1. Ischemia & Infarction

ST-Segment Elevation (STEMI pattern)

• Transmural (full-thickness) ischemia shifts the ST vector toward the epicardium → ST elevation in leads overlying the ischemic zone
• Earliest sign: tall, broad hyperacute T waves (before ST elevation develops)
• "Tombstoning" / "shark fin" morphology = massive ST elevation merging with T wave → high-risk, often proximal LAD or multi-vessel occlusion
• aVR elevation with diffuse ST depression = left main or proximal LAD occlusion

ST-Segment Depression / Subendocardial Ischemia

• Subendocardial ischemia shifts the ST vector toward the endocardium → ST depression in overlying leads (e.g., anterior precordial leads)
• Typically horizontal or downsloping ≥1 mm

Pathological Q Waves

• Represent myocardial necrosis (transmural scar)
• Criteria: duration ≥40 ms (1 small square) OR depth >25% of the R wave in that lead
• Persist after healed infarction as a permanent marker
• Location indicates infarct territory:
  • Q in II, III, aVF = inferior (RCA)
  • Q in V1–V4 = anterior (LAD)
  • Q in I, aVL, V5–V6 = lateral (LCx)

T-Wave Inversion

• Deep, symmetrical T-wave inversion in anterior leads (V2–V5) = Wellens' syndrome (critical proximal LAD stenosis, reperfused STEMI pattern)
• Diffuse T-wave inversion = may indicate pericarditis (after initial ST elevation), myocarditis, cardiomyopathy, or PE (right precordial leads)

2. Bundle Branch Blocks

Right Bundle Branch Block (RBBB)

• QRS ≥120 ms
• rSR' ("rabbit ears") in V1 — M-shaped wide complex
• Wide, slurred S wave in I and V6
• Secondary T-wave inversions in V1–V3 (discordant with last QRS deflection)
• Causes: PE (acute), RV pressure overload, CAD, congenital heart disease, normal variant

Left Bundle Branch Block (LBBB)

• QRS ≥120 ms
• Broad, notched R wave in V5–V6, I, aVL (no preceding Q)
• Deep, wide S wave in V1
• Secondary T-wave inversions in lateral leads
• Always pathological — associated with CAD, hypertensive heart disease, aortic valve disease, cardiomyopathy
• New LBBB + chest pain = treat as STEMI equivalent

Fascicular Blocks

• Left Anterior Fascicular Block (LAFB): QRS axis more negative than −45° (marked left axis deviation); most common cause of left axis deviation in adults
• Left Posterior Fascicular Block (LPFB): axis >+110°; rare, must exclude other causes
• Bifascicular block (RBBB + LAFB): broad S wave in I/V6 + left axis deviation

3. Arrhythmias

Atrial Fibrillation (AF)

• Irregularly irregular R-R intervals
• Absent P waves replaced by chaotic fibrillatory baseline (best seen in V1)
• Narrow QRS (unless aberrant conduction)
• Associated with LVH, structural heart disease, thyrotoxicosis, alcohol

AV Blocks

Pre-excitation (Wolff-Parkinson-White)

• Short PR (<120 ms) + delta wave (slurred upstroke of QRS) + wide QRS
• Due to accessory pathway bypassing AV node

4. Ventricular Hypertrophy

Left Ventricular Hypertrophy (LVH)

• Sokolow-Lyon criteria: S in V1 + R in V5 or V6 ≥35 mm
• Cornell criteria: R in aVL + S in V3 ≥28 mm (men) / ≥20 mm (women)
• Associated strain pattern: ST depression + T-wave inversion in lateral leads (I, aVL, V5–V6)
• Causes: hypertension (most common), aortic stenosis, HCM

Right Ventricular Hypertrophy (RVH)

• Right axis deviation (>+90°)
• Dominant R wave in V1 (R > S in V1)
• Deep S waves in V5–V6
• ST depression + T-wave inversion in right precordial leads (V1–V3)
• Causes: pulmonary hypertension, mitral stenosis, tetralogy of Fallot

5. Electrolyte & Drug Effects

6. QT Prolongation

• QTc >440 ms (men) / >460 ms (women) = pathological
• Predisposes to Torsades de Pointes (polymorphic VT) → VF → sudden death
• Causes: congenital long QT syndrome, drugs (antiarrhythmics, antipsychotics, antibiotics), hypokalemia, hypomagnesemia, hypothyroidism

7. Pericarditis

• Saddle-shaped ST elevation in multiple leads (diffuse, not territory-specific)
• PR segment depression (pathognomonic early finding)
• T-wave inversions develop after ST elevation resolves
• No pathological Q waves

8. Right Heart Strain (Acute PE — "S1Q3T3")

• S wave in lead I + Q wave in III + T-wave inversion in III
• Sinus tachycardia (most common ECG finding in PE)
• New right bundle branch block
• T-wave inversions V1–V4

9. Brugada Pattern

• RBBB morphology + ST elevation in V1–V3 (coved or saddle-back pattern)
• Predisposes to sudden cardiac death from polymorphic VT/VF
• Exacerbated by fever, sodium-channel blocking drugs

Summary Table

Source: Harrison's Principles of Internal Medicine 22E (2025, McGraw Hill Medical), Chapter 247 — ECG section