If each small box is equal to 0.04 seconds or 1 mm, then the standard ECG speed is 1 mm per 0.04 seconds, or 25 mm per second. The standard approach to reading an ECG includes, in this order: ...

The main part of an ECG contains a P wave, QRS complex and T wave. Each will be explained individually in this tutorial, as will each segment and interval. The P wave indicates atrial depolarization.