Abstract Body (Do not enter title and authors here): Introduction/Background
Left atrial (LA) pericardial and epicardial fat are associated with atrial fibrillation (AF). However, the association of LA intramyocardial fat, i.e. lipomatous metaplasia (LALM), with AF persistence and ablation outcomes is understudied.

Research Question/Hypothesis
Do the total and regional distributions of LALM differ between paroxysmal and persistent AF, and does higher LALM burden predict early AF recurrence after catheter ablation?

Methods/Approach
We retrospectively analyzed 100 patients who underwent contrast-enhanced cardiac computed tomography less than one year before AF ablation. The LA was segmented using ADAS software into standardized regions (Panel A) and LALM was quantified using intensity thresholds [−180 to 0 Hounsfield unit (HU)]. Total and regional LALM burdens were expressed as percentages of total LA myocardial and respective regional volumes. For transmural analysis, the myocardium was equally divided into 20%, 40%, 60%, and 80% wall depth-layers from sub-endocardium to sub-epicardium. Patients were stratified by AF persistence and by freedom from or recurrence of AF (≥30 s) beyond a three-month blanking period.

Results/Data
Total LALM comprised a median of 16.6% (interquartile range: 11.0–21.1%) of the total LA myocardial volume. Regional burden was highest at the right (28.1%) and left (19.8%) pulmonary vein (PV) carinae, followed by the inferior wall (19.6%), interatrial septum (15.0%), posterior wall (14.1%), lateral wall (12.7%), and superior wall (10.7%). Patients with persistent AF (n = 43) had a significantly higher LALM percentage than those with paroxysmal AF (n = 57; 17.2% vs 16.1%, p = 0.03) (Panel B). No significant difference was found between patients with and without AF recurrence following ablation (16.4% vs 16.8%, p = 0.82) (Panel C). A transmural gradient was observed, with LALM increasing from the sub-endocardium (3.5%) to the sub-epicardium (24.9%, p < 0.001) (Panel D).

Conclusions
LALM percentage is significantly greater in persistent than paroxysmal AF. Regionally, the highest percentages are found at the right and left PV carinae, followed by the inferior wall, and interatrial septum. Further investigation into LALM pathogenesis and its association with treatment outcomes is warranted.