Pathogenesis of atrial flutter.

It is now known that most cases of atrial flutter are due to reentrant excitation in the right atrium. In the usual reentrant circuit, the reentrant excitation wavefront travels up the interatrial septum and down the right atrial free wall. The boundaries of this reentrant circuit include on one side the tricuspid valve ring and on the other side an area of block, which is probably functional, in the region between the venae cavae. The latter area of block forms during the transitional atrial fibrillation rhythm of variable duration that almost always precedes the initiation of atrial flutter. An isthmus of conduction is also present in the reentrant circuit, and is bounded by the tricuspid ring and the inferior vena cava, the Eustachian ridge, and the coronary sinus. It is probable that an abnormal atrial tissue substrate is usually required. Reentrant circuits around a surgical incision in the atria or around the pulmonary veins (in whole or in part) may be also responsible for atrial flutter.