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Notes: Although atrial fibrillation is a common arrhythmia, especially in elderly men, little is known about age related changes in atrial electrophysiological properties or gender differences. The aim of this study was to analyze the effects of aging on vulnerability to atrial fibrillation and assessed gender differences in those age related changes. An electrophysiological study was performed on 73 patients with no history of atrial fibrillation, structural heart disease, or conditions with potential effects on cardiac hemodynamic or electrophysiological function, including 25 women (mean age 49 ± 18 years; range 12–84 years). The following atrial excitability parameters were assessed: spontaneous or paced (A1) and extrastimulated (A2) atrial electrogram widths, percent maximum atrial fragmentation (A2/A1 × 100) , effective refractory period, wavelength index (effective refractory period/A2), and inducibility of atrial fibrillation. There were no significant differences in percent maximum atrial fragmentation ( 143 ± 28 vs 142 ± 35% ), effective refractory period ( 241 ± 39 vs 238 ± 50 ms ), wavelength index ( 2.9 ± 0.8 vs 3.1 ± 0.9 ), induction of atrial fibrillation (10 [21%] vs 7 [28%]), or age ( 50 ± 17 vs 49 ± 20 years) between men and women. Age was not statistically different between those patients with and without induction of atrial fibrillation in men ( 48 ± 14 vs 50 ± 18 years ) and women ( 48 ± 18 vs 49 ± 21 years ). Percent maximum atrial fragmentation and effective refractory period were directly correlated with age in men ( r = 0.35, P = 0.01; r = 0.46, P 〈 0.001 , respectively) and women ( r = 0.42, P = 0.04; r = 0.45, P = 0.02 , respectively), though wavelength index did not correlate with age in men (r =−0.04) or women (r =−0.04) with no history of atrial fibrillation. Considering these findings, the authors conclude that the mechanism triggering atrial fibrillation may be different between older and younger patients with atrial fibrillation, because younger patients who have no marked substrate for atrial fibrillation may need many trigger beats to induce atrial fibrillation. (PACE 2003; 26:1238–1244)