Updated on December 2, 2023
Normal atrial contraction should produce a proper looking P wave on the ECG.
But in atrial fibrillation, there are multiple foci which generate a potential leading to chaotic rapid incomplete contractions.
These rapid contractions might appear like flickering wave on ECG with no definitive P waves identifiable. 
Among the multiple potentials from the atria, some of them will be allowed to travel through the AV node to the ventricles which will lead to QRS complexes on ECG. This may not follow a particular pattern, leading to irregularly irregular wave form.
The potentials that are allowed by the AV node will pass to the ventricles eventually. As the origin of impulses is above AV node, the resultant QRS complexes will be narrow (QRS will be wide if the impulses originate below AV node).
The net heart rate (based on QRS complexes per minute) can be rapid or controlled (with medication) and will be named accordingly.
AF with rapid ventricular rate.
AF with controlled ventricular rate.
Management of acute AF will be discussed in later lessons.