What electrical phenomenon initiates atrial fibrillation?

Atrial fibrillation is primarily initiated by abnormal electrical impulses within the atria of the heart. These impulses disrupt the normal rhythm, leading to a chaotic electrical activity that causes the atria to fibrillate or quiver instead of contracting effectively. This is often due to factors such as structural remodeling of the atria, increased atrial stretch, or ectopic foci, particularly in the pulmonary veins, which produce rapid electrical signals.

In contrast, normal sinus rhythm represents a healthy, organized electrical impulse originating from the sinoatrial node, which is not involved in the initiation of atrial fibrillation. External electrical stimuli, while they can influence heart activity, are not the underlying cause of atrial fibrillation itself. Pacemaker activity refers to the normal pacing of the heart's electrical system, but in atrial fibrillation, this regular pacing is overridden by the abnormal impulses leading to the disorder.