The atria are complex structures that are still not completely understood. Characterization of atrial bundles has been done in diseased hearts, magnetic resonance tractography, and computational models. To date, no technique has been able to map and characterize endo-epicardial fibers using a standard mapping catheter. The primary purpose of this study was to assess the informational content of far-field electrograms recorded in the left atrium during sinus rhythm.

We included patients who underwent left atrial mapping for atrial fibrillation ablation. A conventional HD-Grid™ catheter was used to perform electro-anatomical mapping of the left atrium, strictly during sinus rhythm and in non-diseased atria. Software programming was used to display additional electrograms selected from electrode pairs at increased inter-electrode distance along the mapping catheter. Four configurations of high density electrograms of varying inter-electrode distances were generated and analyzed.

We included 22 patients in the study. The mean age was 50 years, and the mean atrial volume was 30.4 ml/m2. We found two main patterns of left atrial activation. 70 % of patients had a type 1 pattern, where activation initiated in the Bachman's bundle- corresponding region, and type two pattern, where activation started in the septo-atrial bundle-corresponding region. When we increased the interspaced in the electrodes of the mapping catheter, areas of far-field activation were detected and when we compared them to the prior descriptions of tracts and bundles, they seemed to correspond to epicardial activation.

We introduce a new technique of confocal mapping that allows for the assessment of endo-epicardial activation and aids in the understanding of the complex atrial structure.