Overview
Square Sizes
The standard ECG is recorded at a speed of 25mm/sec and gain of 10mm/mV.- 40ms = 1mm = 1 small square
- 200ms = 5mm = 1 large square
- 0.1mV = 1mm = 1 small square
The ECG Trace
A normal ECG trace includes a P wave, a QRS complex and a T wave.
Look For
PQRST Complex
- P wave - indicative of atrial depolarisation
- QRS complex - indicative of ventricular depolarisation
- T wave - indicative of ventricular repolarisation
Intervals
- PR interval - the time between atrial depolarisation and ventricular depolarisation, suggestive of AV conduction time
- QT interval - the time between ventricular depolarisation and repolarisation
ECG Leads
A standard 12-lead ECG includes bipolar limb leads, unipolar limb leads and chest leads.
Bipolar Limb Leads
- Electrical potential difference is measured between two limb electrodes.
| Electrodes | Direction | Complex | |
|---|---|---|---|
| I | RA → LA | 0 | Lateral |
| II | RA → LL | +60 | Inferior |
| III | LA → LL | +120 | Inferior |
Unipolar (Augmented) Limb Leads
- Electrical potential difference is measured between one limb electrode and a combination of the other limb electrodes.
| Electrodes | Direction | Complex | |
| aVL | LA | -30 | Lateral |
| aVR | RA | -150 | Right atrial |
| aVF | LL | +90 | Inferior |
Unipolar Chest Leads
- Electrical potential difference is measured between the chest electrode and the average of the limb electrodes.
| Complex | |
|---|---|
| V1 | Right atrial |
| V2 | Septal |
| V3 | Anterior |
| V4 | Anterior |
| V5 | Lateral |
| V6 | Lateral |
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