Atrial fibrilliation is a disorganized rhythm where random electrical activity causes the atria to quiver or "fibrillate." Some of the random electrical impulses are able to travel across the AV node and cause the ventricles to contract. However, this is not a regular event. Therefore, the ventricular rate associated with atrial fibrillation is irregular with no specific pattern. The term used to describe this is "irregularly irregular" and it is the hallmark of atrial fibrillation. In lone atrial fibrillation, the chaotic heart rate may occur paroxysmally.
| Rate | atrial rate usually between 400-650/bpm. |
| P wave | not present; wavy baseline is seen instead. |
| QRS | normal |
| Conduction | variable AV conduction; if untreated the ventricular response is usually rapid. |
| Rhythm | irregularly irregular. (This is the hallmark of this dysrhythmia). |