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normal ECG tracing waves intervals
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abnormal ECG ST elevation myocardial infarction bundle branch block
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ECG P wave QRS T wave normal labeled diagram
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Here is a comprehensive overview of Normal vs Abnormal ECG, drawn from Medical Physiology (Boron & Boulpaep) and Harrison's Principles of Internal Medicine.
The ECG at a Glance
ECG Paper Basics
- Small box = 0.04 s (horizontal) / 0.1 mV (vertical)
- Large box = 0.2 s / 0.5 mV
- 5 large boxes = 1 second
Waves, Segments & Intervals
| Component | What It Represents | Normal Value |
|---|---|---|
| P wave | Atrial depolarization | Duration < 0.12 s; positive in I, II, aVF |
| PR interval | AV node conduction time | 0.12–0.20 s (3–5 small boxes) |
| QRS complex | Ventricular depolarization | Duration < 0.12 s |
| ST segment | Ventricular plateau (isoelectric) | At baseline; no elevation/depression |
| T wave | Ventricular repolarization | Upright in I, II, V3–V6; inverted in aVR |
| QT interval | Total ventricular activity | QTc ≤ 0.44 s (men), ≤ 0.46 s (women) |
| U wave | Purkinje/papillary repolarization | Small, same direction as T wave |
Normal Sinus Rhythm — Criteria
- Rate: 60–100 bpm
- P wave before every QRS (upright in II, inverted in aVR)
- Regular PR interval (0.12–0.20 s)
- Narrow QRS (< 0.12 s)
- Isoelectric ST segment
- Concordant T waves (same direction as QRS)
- Normal axis: −30° to +90°
Quick rate estimation: Count large boxes between R–R → Rate = 300 ÷ (number of large boxes)
Common Abnormal ECG Findings
🔴 ST Changes
| Finding | Meaning |
|---|---|
| ST elevation ≥ 1 mm (≥ 2 mm V1–V4) | STEMI, pericarditis, Brugada, early repolarization |
| ST depression ≥ 0.5 mm | NSTEMI, ischemia, digoxin effect |
| Diffuse saddle-shaped ST elevation | Pericarditis |
🟡 Interval Abnormalities
| Finding | Meaning |
|---|---|
| PR > 0.20 s | 1st-degree AV block |
| PR progressively lengthens → dropped QRS | 2nd-degree AV block (Mobitz I / Wenckebach) |
| Sudden dropped QRS, fixed PR | Mobitz II AV block |
| No relationship between P & QRS | Complete (3rd-degree) AV block |
| Short PR + delta wave | WPW syndrome (pre-excitation) |
| QTc > 0.44–0.46 s | Long QT syndrome → risk of Torsades de Pointes |
🟠 QRS Abnormalities
| Finding | Meaning |
|---|---|
| Wide QRS + RSR' in V1, slurred S in V6 | Right Bundle Branch Block (RBBB) |
| Wide QRS + broad notched R in V5/V6, absence of septal Q | Left Bundle Branch Block (LBBB) |
| Pathological Q waves (> 0.04 s, depth > 1/4 R) | Old myocardial infarction |
| Right axis deviation | RVH, RBBB, lateral MI |
| Left axis deviation | LBBB, left anterior fascicular block, inferior MI |
🟢 Rhythm Abnormalities
| Finding | Meaning |
|---|---|
| No P waves, irregular RR | Atrial fibrillation |
| Sawtooth baseline ~300 bpm, regular | Atrial flutter |
| Wide complex regular tachycardia | Ventricular tachycardia (VT) |
| No organized activity | Ventricular fibrillation (VF) |
🔵 Other Key Findings
| Finding | Meaning |
|---|---|
| Peaked T waves + widened QRS | Hyperkalemia |
| Prominent U waves | Hypokalemia |
| Prolonged QT + flat T | Hypocalcemia |
| Low voltage in all leads | Pericardial effusion, obesity, hypothyroidism |
| Electrical alternans | Cardiac tamponade |
Systematic Approach to Reading an ECG
Follow this order every time:
- Rate (bpm)
- Rhythm (regular? P before QRS?)
- Axis (normal −30° to +90°)
- Intervals (PR, QRS, QTc)
- Waveform morphology (P, QRS, T, U)
- ST segments (elevation, depression)
- Clinical correlation
Sources: Medical Physiology (Boron & Boulpaep), BOX 21-2; Harrison's Principles of Internal Medicine, 21st ed.
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