The PR interval on the body surface electrocardiogram is defined as the interval from the onset of the P wave to the onset of the QRS complex. Its components are: atrial depolarization, atrial repolarization, and transmission of the impulse across the atrio-ventricular junction. The latter includes transmission through the AV node and the His-Pukinje system, and/or transmission through an accessory pathway which bypasses the normal AV conduction pathway.