Introduction: The "flutter isthmus," the part of the lower right atrium between the eustachian valve and the tricuspid annulus inferior to the coronary sinus os, is considered the crucial zone for conduction delay necessary for the genesis of atrial flutter. However, the underlying mechanism remains unclear.
Methods and results: We studied the "flutter isthmus" in 50 hearts obtained at autopsy from patients without atrial tachyarrhythmias. The muscular trabecular arrangement was dissected carefully by peeling off the endocardium. Documentation of the trabecular arrangement focused, in particular, on the question of whether there was a uniform pattern of well-aligned muscle trabeculae or a nonuniform architecture. It appeared that a nonuniform trabecular pattern prevailed (37/50 [74%]). In these hearts, the muscular arrangement showed abundant cross-overs and interlacing trabeculae, particularly in the zone immediately inferior to the coronary sinus os. Connections also occurred along the inferior rim of the os.
Conclusion: The normal anatomy of the lower right atrium favors nonuniform muscular trabeculation, with interlacing bundles and a multitude of cross-overs. The potential for conduction delay is present in the vast majority of normal hearts. This raises the question as to what has changed in the hearts of patients with atrial flutter such that the potential for conduction delay and reentry has become effective.