P atrial depolarisation, QRS ventricular depolarisation, T ventricular repolarisation
Mnemonic
Normal values worth carrying, in small squares at 25 mm per second, where 1 small square = 40 ms and 1 large square = 200 ms:
- P wave - under 2.5 small squares tall and under 3 wide (120 ms)
- PR interval - 3 to 5 small squares, 120 to 200 ms
- QRS - under 3 small squares, 120 ms
- QT - under half the R-R interval at normal rates
- Rate - 300 divided by the number of large squares between R waves
“Little squares are 40, big squares are 200” is the only conversion needed.
P wave abnormalities: tall (over 2.5 mm) is P pulmonale, right atrial enlargement; broad or bifid (over 120 ms) is P mitrale, left atrial enlargement.
“The rate landmarks are 300, 150, 100, 75, 60, 50” counting large squares, and for an irregular rhythm count the complexes on the rhythm strip and multiply by 6.
Expansion
| Component | Event | Normal duration |
|---|---|---|
| P wave | Atrial depolarisation | under 0.12 s |
| PR interval | Atrial depolarisation plus atrioventricular nodal delay | 0.12 to 0.20 s (3 to 5 small squares) |
| QRS complex | Ventricular depolarisation | under 0.12 s (3 small squares) |
| ST segment | Ventricular plateau, phase 2 | isoelectric |
| T wave | Ventricular repolarisation | |
| QT interval | Total ventricular depolarisation and repolarisation | under 0.44 s, rate corrected |
Atrial repolarisation is hidden inside the QRS complex, which is far larger.
At the standard paper speed of 25 mm per second, one small square is 0.04 seconds and one large square is 0.20 seconds. Rate can be read as 300 divided by the number of large squares between successive R waves.