Atrial flutter

From IKE

Atrial flutter is a cardiac arrhythmia in which the reentrant circuit is localized to the right atrium, where it circles in a counterclockwise fashion at 300 bpm. While the atria are activated with every beat (1:1), the AV node prevents the ventricles from being activated as frequently. Instead, 2:1 activation most commonly occurs, resulting in a ventricular rate of 150bpm. If through vagal maneuvers or with antiarrhythmic drugs one slows conduction in the AV node such that 3:1 or 4:1 conduction occurs, this results in a ventricular rate of 100 bpm or 75 bpm while the atria remain activated at 300 bpm.