Atrial flutter and fibrillation may occur in the same patient. Indeed, it is quite common for atrial flutter to convert spontaneously to atrial fibrillation. The ECGs shown here are from the same patient. The upper panel shows atrial flutter with 4:1 AV block and a regular ventricular rhythm with a rate of 75. The flutter rate is 300 and the flutter waves are upright in lead II indicating clockwise rotation of the flutter waves around the right atrial re-entry loop. The lower panel reveals atrial fibrillation with a well controlled ventricular rate. The rhythm is now irregular and the well defined flutter waves have been replaced by less well defined and more rapidly occurring fibrillatory waves.