Abstract Body (Do not enter title and authors here): Introduction: Heart failure (HF) and atrial fibrillation/flutter (AF) often co-occur in older adults. Understanding the temporal relationship of HF and its subtypes (HFpEF and HFrEF) with AF and their prognosis are understudied and was the objective herein.

Methods: This prospective cohort study included 31,171 post-menopausal women from the Women’s Health Initiative (WHI) study with adjudicated HF outcomes, merged with Medicare database for adjudicated AF outcomes during a median follow-up of 17 years. Participants were categorized into 4 groups: No incident AF or HF (n=22578); incident AF (n=6466); incident HF (n=1004); and both incident AF-HF (n=1123). Cox proportional hazards regression analysis was performed to study temporal association between AF and HF, all-cause and cardiovascular death (CVD).

Results: Participants had a mean age of 63.2 (6.9) years at baseline. The risk of all-cause and CVD related death was elevated in incident AF [all-cause death HR 95% CI: 1.15 (1.08, 1.21); CVD death HR 95% CI: 1.32 (1.17, 1.50)], incident HF [All-cause death HR 95% CI: 1.84 (1.69, 2.01); CVD death HR 95% CI: 3.43 (2.94, 4.00)], and incident AF-HF [All-cause death HR 95% CI: 1.72 (1.59, 1.87); CVD death HR 95% CI: 3.57 (3.12, 4.10)] compared to no AF or HF. Risk of developing incident HFpEF [HR, 95% CI: 2.03 (1.77, 2.32)] and incident HFrEF [HR, 95% CI 1.33 (1.12, 1.59)] was higher with prior AF development compared to no prior AF.

Conclusion: All-cause and CVD mortality is elevated with development of new AF, however mortality is much higher with incident HF or HF co-occurrence with AF. Risk of both HFpEF and HFrEF is elevated with presence of AF, with risk much higher for HFpEF in post-menopausal women. These findings underscore the importance of early detection and treatment of AF in post-menopausal women which can help reduce HF incidence and thereby overall mortality.