(A) Schematic of AR generation. (B) Graded AR severities (VTI_ratio) quantified in the aortic arch using Doppler ultrasound. (C) Post-AR mitral inflow changes consistent with restrictive left ventricular (LV) filling. (D) Twenty-four-hour LV systolic and diastolic pressures in Sham (red) and AR (black) mice; 12:12 light/dark cycles indicated. (E) LV end-diastolic pressure (LVEDP) increased immediately after AR, remaining elevated compared with Shams (2-way repeated measures ANOVA with Šidák’s multiple comparison test). (F) Gross morphology and LV M-mode 4 weeks after AR. Scale bars: 1 mm. (G) Changes in LV structural, functional, and hemodynamic indices with VTI_ratio. (H and I) Atrial electrograms with programmed stimulations showing inducible AF in AR (10/16), but not Sham (0/15) mice. (J) AF durations increased with AR, correlating with VTI_ratio (gamma generalized linear model, 95% CI shown). (K) Atrial weight–to–tibia length ratio increased in AR versus Sham mice, increasing with VTI_ratio. (L) Collagen deposition increased in left atrial appendages (LAAs) of AR versus Sham mice (left), with fibrosis increasing with VTI_ratio (right). Scale bars: 200 μm (×3 magnification) and 100 μm (insets) (×20 magnification). (M) Confocal LAA micrographs showing increased F4/80⁺ macrophage infiltration (arrows) in AR mice (left), with counts increasing with VTI_ratio (right). Scale bars: 20 μm (×40 magnification). (N and O) In vivo atrial effective refractory periods (AERPs) and ex vivo action potential durations (80% repolarization) (APD80) were shortened in AR mice. (P) Isochronal activation maps in paced (90 ms) atria revealed conduction slowing with AR. (Q and R) Ex vivo AF inducibility and optically mapped AF episodes were increased in isolated AR atria (5/9) but absent in Sham (0/8) mice. Data presented as mean ± SEM; n values indicated. *P < 0.05 by 2-tailed Student’s t test.