(A) Bottom: A normal ECG recording showing sinus rhythm. Top: Schematics of major events in one cardiac activation cycle: rhythm is initiated by the SA node pacemaker, resulting in atrial activation, followed by atrioventricular conduction via the AV node and His-Purkinje conducting system, leading to ventricular activation. (B) ECG showing onset of AF after one regular sinus beat. Atrial activation is now rapid and irregular, producing an undulating baseline that is visible when not obscured by larger QRS and T waves (continuous atrial activity during this phase is represented by dotted lines). During AF, rapid and uncoordinated atrial activity leads to ineffective atrial contraction. Ventricular activations (QRS complexes) now driven by the fibrillating atria occur rapidly and irregularly, weakening cardiac contraction efficiency and causing clinical symptoms.