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ECG normal vs abnormal findings interpretation 2025
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Here is a comprehensive guide to ECG Normal vs. Abnormal Findings:
The Normal ECG — Waves, Intervals & Segments
Fig. 4.17 — Normal ECG waveform (Lead II). — Costanzo Physiology, 7th Ed.
The ECG records potential differences on the body surface that reflect the sequence of cardiac depolarization and repolarization.
Normal Parameters
Heart Rate
| Age Group | Normal Range |
|---|---|
| Adult | 60–100 bpm |
| Neonate | Up to 220 bpm (sinus tachycardia) |
Intervals & Segments
| Component | Normal Value | Represents |
|---|---|---|
| P wave | <120 ms, upright in II, negative in aVR | Atrial depolarization |
| PR interval | 120–200 ms (adult) | Atrial → AV node → His bundle conduction |
| QRS complex | <120 ms | Ventricular depolarization |
| ST segment | Isoelectric (flat) | Plateau phase of ventricular action potential |
| T wave | Upright in most leads | Ventricular repolarization |
| QT interval | <440–460 ms (corrected, QTc) | Ventricular depolarization to end of repolarization |
Key ECG Components Explained
P Wave
- Normally positive in lead II, negative in aVR (sinus rhythm)
- May be biphasic in V1 (small positive right atrial component, small negative left atrial component — both normal)
- Abnormal: retrograde P waves (negative in II, positive in aVR) → ectopic atrial or junctional pacemaker
PR Interval
- Normal: ~160 ms (range 120–200 ms)
- Prolonged → AV nodal conduction delay (e.g., 1st-degree heart block)
- Shortened → pre-excitation (e.g., WPW syndrome) or enhanced AV conduction
QRS Complex
- Phase 1: Septal depolarization (left → right) → small r in V1, small q in V6
- Phase 2: LV dominates → S wave in V1, tall R in V6
- Widened (≥120 ms) → bundle branch block, ventricular pacing, hyperkalemia
ST Segment & T Wave
- Normally isoelectric; corresponds to the action potential plateau
- ST elevation → acute STEMI, pericarditis, Brugada pattern, early repolarization, LV aneurysm
- ST depression → subendocardial ischemia, digoxin effect, strain pattern
- T-wave inversion → ischemia, ventricular hypertrophy, bundle branch blocks (secondary), PE
Major Abnormal Findings
1. Bundle Branch Blocks
Fig. 247-10 — Normal, RBBB, and LBBB patterns in V1 and V6. Note secondary T-wave inversions. — Harrison's Principles of Internal Medicine, 22nd Ed.
| Pattern | QRS | V1 | V6 | Clinical Significance |
|---|---|---|---|---|
| RBBB | ≥120 ms | rSR' ("rabbit ears") | qRS | Can be normal; also ASD, PE, ischemia |
| LBBB | ≥120 ms | Wide QS | Tall broad R | Often marker of serious heart disease (CAD, HTN, cardiomyopathy) |
2. ST-Elevation — Differential Diagnosis
| Cause | Pattern |
|---|---|
| Acute MI (STEMI) | Regional, with reciprocal depressions |
| Acute pericarditis | Diffuse, saddle-shaped, PR depression |
| Brugada syndrome | Coved/saddle in V1–V2 with RBBB-like morphology |
| Early repolarization | Concave upward, mostly inferior/lateral leads |
| LV aneurysm | Persistent elevation after prior MI |
| Hyperkalemia | Peaked T waves → wide QRS → sine wave |
| Hypothermia | J (Osborn) waves |
| Pulmonary embolism | S1Q3T3 pattern, RBBB |
3. QT Prolongation
- QTc >440 ms (men) / >460 ms (women) = abnormal
- Causes: drugs (antiarrhythmics, antipsychotics, antibiotics), hypokalemia, hypomagnesemia, congenital long QT syndrome
- Risk: torsades de pointes → ventricular fibrillation
4. Axis Deviation
| Axis | Degrees | Significance |
|---|---|---|
| Normal | −30° to +90° | — |
| Left axis deviation | −30° to −90° | LAFB, inferior MI, LVH, WPW |
| Right axis deviation | +90° to +180° | RVH, LPFB, lateral MI, PE, normal in children |
5. Hypertrophy Patterns
- LVH: Tall R in V5/V6 (>25 mm), deep S in V1; Sokolow-Lyon criterion: S(V1) + R(V5 or V6) ≥35 mm
- RVH: Tall R in V1, right axis deviation, strain pattern in V1–V3
6. Ischemia & Infarction Sequence
| Stage | ECG Change |
|---|---|
| Hyperacute | Tall, peaked ("hyperacute") T waves |
| Acute STEMI | ST elevation, loss of R wave |
| Evolving | Deep Q waves develop, ST normalizes |
| Old infarct | Persistent Q waves, T-wave inversion |
Anterior STEMI: ST elevation in V1–V4, I, aVL; reciprocal depression in II, III, aVF
Inferior STEMI: ST elevation in II, III, aVF; reciprocal depression in I, aVL
Inferior STEMI: ST elevation in II, III, aVF; reciprocal depression in I, aVL
Quick Reference: Normal Adult ECG Values
| Parameter | Normal Range |
|---|---|
| Heart rate | 60–100 bpm |
| PR interval | 120–200 ms |
| QRS duration | <120 ms |
| QTc | <440 ms (men), <460 ms (women) |
| QRS axis | −30° to +90° |
| P wave axis | 0° to +75° (upright in I and II) |
Sources: Harrison's Principles of Internal Medicine, 22nd Ed.; Costanzo Physiology, 7th Ed.; Tintinalli's Emergency Medicine
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