Comparison with Previous Studies
LA WTs have been evaluated in post-mortem studies, revealing a significant variation in the WTs among regions, patients, and studies.1,2 Most of these studies reported average thickness of LA myocardium between 1.0 and 5.0 mm with a range from 0.5 to 12 mm.1,2 The results in this study were almost within this range but the average WT values were slightly thicker than those in previous post-mortem studies. We measured the distance between endocardial and epicardial boundary, which might be greater than the actual muscular thickness in histological examinations. Fresh tissue specimens also shrink after formalin fixation, causing a reduction of WT. These factors likely contribute to the difference in the WT measurement between previous histological studies and our ICE study. Cardiac CT imaging has been used to evaluate LA WT in AF patients,2,6-8 and therefore we compared the ICE-based WTs measurements to cardiac CT-based WT measurements in the same patients to validate the accuracy and reliability of our method; the ICE-based WT measurements were almost similar to, or slightly thicker than, the cardiac CT-based WT measurements in our study, suggesting the similar accuracy of these measurements. The CT-derived WT averages approximately 1.0-3.0 mm in previous reports. Of note, when viewed alongside the results of the pathological studies in the similar location, the CT measurements are reportedly lower than the histological measurement.2
The left-PV anterior segment, the LLR, had the thickest wall among all segments in this study. Cabrera et al. have reported that this region has a folding structure with the thick muscle ranging from 2.2 mm to 12.3 mm.9 The morphology can be flat, round, or pointed, as seen in this study (Figure 4).9 A previous CT study has also shown that the LLR had the thickest walls in the area surrounding the PV, which is consistent with our results.6
ICE provides higher spatial resolution (0.2-0.3 mm) than cardiac CT (0.4 mm-) and MRI (1.4 mm-)2,10 and therefore seems to be a good modality for the WT evaluation in the LA. ICE was used to assess right atrial WTs in an experimental study.11 However, it has not been used to assess WT in the LA of AF patients, likely because of the invasive nature of this examination. The ICE has been limited to use in procedures where its value has been clearly established. The ICE catheter is usually placed in the RA for the trans-septal puncture during AF ablation. In this study, the ICE catheter was advanced into the LA, allowing clearer visualization of the anatomy of the area surrounding the PV and reducing the angulation errors of the view perpendicular to the LA/PV surface for the WT measurement. Nevertheless, this assessment lacks a gold standard and merits further evaluation.