Atrial flutter is a rapid atrial reentry rhythm, with regular “saw-tooth” pattern flutter (F) waves, reflecting a regular pattern of generalized atrial reentry, best seen in an inferior lead. The atrial rate range is usually between 250 and 350 beats per minute, and classically 300 beats per minute. At times, the saw-tooth appearance is not clear, but a regular atrial rate greater than 200 suggest atrial flutter. Ventricular conduction is often regular but may be variable leading to an irregular QRS rate Although rare, 1:1 conduction, particularly with QRS widening, should lead one to suspect an accessory pathway with pre-excitation, although it may just reflect a rapidly conducting AV node. The following animation illustrates what we have just described.