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Methods and Results: We reviewed the gross structure of the left atrium by examining the septum, the appendage, and insertions of the pulmonary veins in normal hearts. The limited extent of the true septal component is relevant to procedures asing the transseptal approach. On gross examination, the musculature of the atrial wall is composed of overlapping bundles of aligned fibers that, in the majority of hearts, are arranged in characteristic patterns with only minor individual variations. Muscular sleeves extend into the walls of the pulmonary veins to varying distances. The longest sleeves are in the left upper veins. Bachmann's bundle anteriorly, and other smaller bundles superiorly and posteriorly, bridge the septal raphe to blend with musculature of the right atrium. Tongues of left atrial musculature from the posterior wall also extend into the wall of the coronary sinus.

Conclusion: The left atrium is more complex than usually conceived. Understanding its structure, and the arrangement of its musculature, will help in improving strategies for linear lesions when attempting to compartmentalize the chamber, or when placing focal lesions for ablating ectopic sources.(J Cardiovasc Electrophysiol, Vol. 10, pp. 1525-1533, November 1999) atrial arrhythmia, myoarchitecture, pulmonary vein, radiofrequency ablation