Simplified Normal ECG
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Horizontal: One small grid = 0.04 s; one large grid = 0.2 s
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Vertical: One small grid = 0.01 mV; one large grid = 0.1 mV
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Normal heart rate: 60–100 bpm, corresponding to 3–5 large grids (≥3 large grids = tachycardia; ≤5 large grids = bradycardia)
| Time (s) | Amplitude (mV) | Mnemonic for Normal Values | |
|---|---|---|---|
| P wave | <0.12 s | <0.25 mV (lead II) | Width: ≤3 small grids; Height: ≤2.5 small grids (½ large grid) |
| QRS complex | <0.12 s | <1.0 mV (V1), <2.5 mV (V5) | Width: ≤3 small grids; Height: ≤10/25 small grids (2/5 large grids) |
Detailed Version
| Physiological Significance | Normal Duration | Abnormal Duration | Normal Amplitude | Abnormal Amplitude | |
|---|---|---|---|---|---|
| P wave | Atrial depolarization | <0.12 s | Normal amplitude: ≤0.25 mV in limb leads; ≤0.15 mV in precordial leads Most prominent in lead II |
If upright in aVR and inverted in leads II, III, and aVF → retrograde P wave, indicating origin from AV junction | |
| PR interval | Delayed conduction through the AV junction | 0.12–0.2 s | Prolonged → AV block | — | — |
| QRS complex | Ventricular depolarization | <0.1 s | ST segment depression must not exceed 0.05 mV in any lead ST elevation: ≤0.3 mV in V1–V3; ≤0.1 mV in other leads |
ST elevation/depression aids diagnosis of myocardial ischemia/infarction, electrolyte disturbances |
|
| Q wave | — | <0.04 s | — | Depth <¼ of R-wave amplitude in same lead; >¼ suggests myocardial infarction (“two fours”) | |
| ST segment | Ventricular repolarization | — | — | Downward deviation ≤0.1 mV; upward deviation ≤0.3 mV in V1–V3, ≤0.1 mV in other leads | Depression >0.1 mV → myocardial ischemia Elevation: 1. Convex upward → acute myocardial infarction 2. Concave upward → acute pericarditis 3. Also seen in: Prinzmetal’s angina, ventricular aneurysm |
| T wave | — | — | — | Direction normally matches main deflection of QRS complex Amplitude > R/10 (in same lead) |
1. Flattened, biphasic, or inverted → myocardial ischemia/injury, hypokalemia, digitalis effect, ventricular hypertrophy, bundle branch block, etc. 2. Prominently tall → early AMI; hyperkalemia |
| U wave | Inverted U wave suggests left main coronary or LAD occlusion | — | — | — | — |
| QT interval | Total time from onset of Q wave to end of T wave (ventricular depolarization + repolarization) | Male <0.44 s; Female <0.46 s (use corrected QT interval) | Prolonged → risk of ventricular tachycardia | — | — |
| PJ interval | — | — | — | — | — |
[[ECG Lead Localization]]
[[QT Interval Correction]]