Abstract Body (Do not enter title and authors here):
Background:
Heart failure with preserved ejection fraction (HFpEF) is a heterogeneous syndrome and its pathophysiology is not fully understood. A monoamine, 5-hydroxytryptamine (5-HT), is involved in diverse biological functions and suggested to play a role in cardiovascular diseases. However, the clinical relevance of 5-HT in HFpEF remains unclear.

Aims:
This study aimed to elucidate the clinical significance and prognostic value of 5- HT in patients with HFpEF.

Methods:
We conducted a prospective study involving 240 consecutive hospitalized patients with HFpEF (mean age 72 years, 52% male). We measured circulating blood 5-HT levels using the enzyme-linked immunosorbent assay method. Clinical and outcome data were collected.

Results:
Correlation analysis revealed that 5-HT levels were negatively correlated with blood B-type natriuretic peptide concentration and tricuspid regurgitation pressure gradient. When patients were stratified into two groups based on the median 5-HT levels (90.5 ng/mL), Kaplan-Meier analysis showed that HFpEF patients with low blood 5-HT levels had lower event-free survival rates from the composite event of cardiac death and worsening heart failure over a median follow-up period of 725 days (Figure). In a multivariable Cox proportional hazard model adjusting for confounding variables, low levels of 5-HT were independently associated with increased risks of the composite of cardiac events (hazard ratio, 3.25; P < 0.01).

Conclusion:
Low 5-HT levels are associated with adverse outcomes in patients with HFpEF and 5-HT may serve as a useful biomarker for predicting prognosis in such patients.