During atrial fibrillation the atrial impulses discharge at a rate of 350-600 per minute, resulting in small (or “fine”), irregular f (fibrillation) waves. The amplitude of these waves varies and may be especially prominent (or “coarse”) in lead V1. As only occasional impulses penetrate the atrioventricular node, a totally irregular ventricular rhythm results, which is the characteristic of this arrhythmia.

Rapid atrial fibrillation with a rapid ventricular response may easily be mistaken for other supraventricular arrhythmias—for example, atrial flutter or supraventricular tachycardias. Variation in the RR interval is the important clue. At very high heart rates, with a short RR interval, beat to beat variation may be subtle but may become more obvious if the carotid sinus is massaged or the speed of the electrocardiogram trace increased. In a young patient with fast atrial fibrillation it is important to consider an underlying pre-excitation syndrome, such as the Wolff-Parkinson-White syndrome, as traditional drugs such as digoxin or verapamil will accelerate the ventricular response by blocking atrioventricular node impulses and increasing conduction through the accessory pathway.

Finally, atrial fibrillation with a bundle branch block pattern on the QRS complex may be difficult to distinguish from a ventricular tachycardia: again, the important difference is the irregularity of the RR interval present in atrial fibrillation.