Abstract Body (Do not enter title and authors here): Introduction
Heterogeneity in heart failure with preserved ejection fraction (HFpEF) is an important challenge for the development of new medicines for this disease. Clinical trials of SGLT-2 inhibitors and renin-angiotensin inhibition have demonstrated reduced efficacy in patients with higher LVEF. With the advent of myosin modulation, there is renewed focus on those with LVEF >60% as a group that could benefit.

Hypothesis
Patients with HFpEF and LVEF >60% will have impaired cardiac response to exercise stress and reduced functional capacity compared to HFpEF with LVEF <60% with a distinct circulating proteome.

Methods
Patients with HFpEF (n=64, mean age 76.6± 5.5) and age-matched controls (n=16, mean age 71.9 ± 6.2) were prospectively enrolled. Assessment of cardiac volumes, function and stress response was performed using real-time cardiovascular magnetic resonance imaging at rest and sub-maximal exercise (35W for 6 minutes). Plasma samples were acquired immediately after exercise for aptamer-based proteomics to detect 7000 circulating proteins. Pathway enrichment analysis of the proteome was performed to compare both HFpEF groups to controls.

Results
HFpEF patients with high LVEF (> 60%, n=46) had smaller left ventricular volumes (LVEDVi 64.8 ± 14.7 vs 93.6 ± 17.1 ml/m², p<0.001) compared to HFpEF patients with lower LVEF (50-60%, n=18). During exercise, HFpEF patients with high LVEF had attenuated systolic augmentation (LVEF +3.1 ± 4.9 vs +5.5 ± 3.6 %, p=0.03), lower cardiac index (4.5 ± 1.1 vs 5.2 ± 1.3 L/min/m², p=0.06) and lower 6-minute walk distance (348 ± 95 vs 401 ± 76m, p=0.03). When compared to controls, HFpEF patients with high LVEF had a distinct plasma proteomic signature, characterized by altered lipid metabolism pathways whereas those with lower LVEF had perturbed extracellular matrix organisation and inflammatory pathways.

Conclusions
Patients with HFpEF and LVEF >60% have greater functional impairment, smaller LV cavities, blunted systolic augmentation to exercise and a circulating proteome characterised by altered lipid metabolism pathways. These findings support stratified approaches to this group in future clinical trials and identify potential novel biomarkers to assess treatment response.