The basic mechanism of atrial fibrillation is intra-atrial reentry, typically provoked by PAC’s. The ventricular response is irregularly irregular, unless there is complete AV block with an escape or electronic pacemaker rhythm. There are no discernable P waves. Atrial fibrillatory wave rates range from 350-700 per minute. While the fibrillatory waves in this tracing are fine, they may also be coarse or even finer. In younger individuals with normal AV node function, the untreated ventricular rate is typically 100-180 beats per minute, and as low as 100 in older patients. However, depending on sympathetic tone the rate may be well under 100 in any patient with a slowly conducting AV node. A QRS rate greater than 200, particularly with wide bizarre appearing QRS, should lead one to suspect an accessory pathway and ventricular pre-excitation (WPW pattern). The following animation illustrates what we have just described.