Atrial flutter represents another example of an anatomically defined re-entry circuit. The circuit usually involves the tricuspid annulus anteriorly and the crista terminalis and Eustachian ridge posteriorly. A region of slow conduction is located between the coronary sinus and tricuspid annulus. The most common form of atrial flutter involves rotation around this circuit in a counterclockwise direction with the impulse traveling superiorly up the anterior portion of the circuit and inferiorly down the posterior portion of the circuit. This results in flutter waves that are inverted, or negative, in leads II, III and aVF. Less commonly, rotation around this circuit occurs in a clockwise direction, with the flutter waves traveling superiorly up the posterior portion of the circuit, and inferiorly down the anterior portion. This results in flutter waves that are upright (positive) in leads II, III and aVF.