Learning Objective
After completing this section, learners will be able to identify the components of a normal ECG, correlate ECG waves and intervals with cardiac action potential phases, and explain how changes in heart rate affect ECG intervals.
Normal ECG Components
| ECG Component | Description | Duration / Notes |
|---|---|---|
| P wave | Atrial depolarization | ~80–100 msec |
| QRS complex | Ventricular depolarization | 40–100 msec |
| R wave | First upward deflection after the P wave | – |
| S wave | First downward deflection after the R wave | – |
| T wave | Ventricular repolarization | – |
| PR interval | Start of P wave to start of QRS complex; reflects AV nodal delay | 120–200 msec |
| QT interval | Start of QRS complex to end of T wave; represents total action potential duration | – |
| ST segment | Ventricles fully depolarized; corresponds roughly to the plateau phase (Phase 2) | – |
| J point | End of S wave; represents the isoelectric point | – |
The height of ECG waves depends on:
- Mass of tissue depolarizing/repolarizing
- Rate of change of membrane potential
- Orientation of the lead relative to the current flow
Correlation with Cardiac Action Potential
- Phase 0 (rapid depolarization): Produces QRS complex
- Phase 2 (plateau): Corresponds to the ST segment
- Phase 3 (repolarization): Produces the T wave
- QT interval: Duration of the ventricular action potential; inversely related to heart rate
- Increased sympathetic stimulation → ↑ heart rate → shorter action potential → decreased QT interval
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