1. Heart failure with preserved ejection fraction is an urgent clinical challenge As the leading killer of human life and health, cardiovascular disease is a difficult public health problem worldwide. According to the latest statistics of China Cardiovascular Health and Disease Report 2022, the number of patients in China has exceeded 330 million, and the number of patients with heart failure (HF) is as high as 8.9 million, becoming the second leading cause of death related to heart disease after coronary heart disease. According to the 2023 European Society of Cardiology (ESC) guidelines for the diagnosis and management of acute and chronic heart failure, LVEF) classified heart failure into three categories: heart failure with reduced ejection fraction (HFrEF, LVEF≤40%), and heart failure with reduced ejection fraction (HfreF). heart failure with mildly reduced ejection fraction (HFmrEF, Heart failure with mildly reduced ejection fraction) 40% \< LVEF \< 49%) and heart failure with preserved ejection fraction (HFpEF, EF≥50%). Among them, HFpEF accounts for about 50% of all heart failure, and with the improvement of HFpEF cognition and diagnosis rate, and the aggravation of population aging, its prevalence shows an increasing trend year by year. The annual mortality rate is about 15%, the rehospitalization rate is up to 80% within 5 years after hospitalization, and the survival rate is only 25%-50%. At present, the diagnosis and treatment system of HFrEF is maturing, and the quality of life and overall prognosis of patients are gradually improving. However, there are effective therapeutic drugs for HFrEF. These include angiotensin receptor neprilysin inhibitor (ARNi), angiotensin receptor blocker (angiotensin receptor blocker), and angiotensin receptor inhibitor (ARNI). ARB), Mineralocorticoid Receptor Antagonists (MRAs), PDE-5 inhibitors, etc., have no significant effect on HFpEF. Only one HFpEF positive clinical drug, sodium glucose cotransporter 2 inhibitor (SGLT2i), significantly improved patient survival and rehospitalization rate, and was included in the class IA recommendation of the guidelines. The pathogenesis of HFpEF is complex, and the clinical diagnosis and treatment methods are limited. The incidence and rehospitalization rate of HFpef are increasing year by year, resulting in serious social and economic losses and disease burden. Therefore, exploring the pathogenesis of HFpEF and finding effective prevention and treatment methods are major clinical scientific problems that need to be solved urgently.
2. Clinical characteristics and risk factors of HFpEF HFpEF is not a single heart disease, but a complex, heterogeneous, multi-organ, systemic syndrome. It is often associated with patients with advanced age, obesity, metabolic syndrome, type 2 diabetes, hypertension, sedentary behavior, coronary heart disease, and kidney disease. Among them, hypertension and coronary heart disease are common risk factors for HFrEF and HFpEF, while people with obesity, abnormal metabolism and lack of physical activity are more likely to develop HFpEF. Although studies on HFpEF are increasing, little progress has been made in the treatment of HFpEF. At present, the only drug that has been proven to effectively improve the prognosis of patients is SGLT2i, and the primary indication is for patients with type 2 diabetes, further indicating that metabolic disorders such as diabetes are closely related to HFpEF. Systemic inflammation, metabolic disorders, vascular dysfunction, oxidative stress and other factors induce cardiomyocyte hypertrophy, myocardial fibrosis, and intracellular calcium processing disorders, which lead to myocardial active relaxation dysfunction and cardiomyocyte stiffness, and then the decline of left ventricular diastolic function, abnormal left ventricular filling, decreased myocardial compliance, and cardiac overload leading to HFpEF. The dysfunction and metabolic disorders of HFpEF involve multiple organs outside the heart. Therefore, the study of the mechanism of HFpEF should not be limited to the cardiac level.
3. Gut microbiota is closely related to cardiovascular disease Gut microbiota is a complex and dynamic microbial community inhabiting in the human digestive tract. The total number of microorganisms inhabiting in the human gut is up to 100 trillion, and the total number of genomes is about 150 times that of the human genome, which is called the "second genome" of the human body. According to the existing research data, the gut microbiota is generally divided into six major phyla, namely Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria, Fusobacteria and Verrucomicrobia. Firmicutes and Bacteroidetes are the main types. The main functions of gut microbiota include: fermentation of incompletely digested food, synthesis of certain vitamins, fermentation of sugars to produce Short Chain Fatty Acids (SCFAs) such as acetic acid, propionic acid and butyric acid, and regulation of intestinal epithelial mucosal barrier and immune system. In the healthy state, various microorganisms maintain symbiotic or antagonistic relationships in the intestinal tract, forming the microbial homeostasis of the human gastrointestinal tract, which plays an important role in the regulation of the host body's metabolism, nutrition, and immunity. Unhealthy diet, drugs and lifestyle can lead to the imbalance of intestinal flora and induce a variety of diseases. Therefore, the balance of intestinal flora is closely related to human diseases and health. In recent years, with the development and application of 16S rDNA sequencing technology, metagenomic sequencing technology and metabolomics, the role of intestinal microorganisms in the occurrence and development of cardiovascular diseases has been gradually recognized and widely studied. A large number of research data have confirmed that the richness and diversity of intestinal flora in patients with hypertension are decreased compared with healthy people, and intestinal flora is involved in regulating blood pressure through a variety of mechanisms. In the feces of patients with heart failure represented by HFrEF, the diversity of gut microbiota is significantly decreased, and the composition structure of gut microbiota is changed. Pathogenic bacteria such as Campylobacter and Candida are significantly increased, and are related to the severity of the disease, while anti-inflammatory related bacteria such as Blautia are significantly decreased. Gut microbiota can also affect host cardiometabolic processes by regulating metabolites. The distal gut microbiota can promote the production of SCFAs by fermentation processes such as dietary fiber, regulate blood pressure by recognizing specific host receptors, and play an anti-inflammatory role by activating histone acetyltransferase. Gut microbiota in the colon can metabolize unrecycled primary bile acids to produce secondary bile acids, such as deoxycholate, lithocholate, and ursodeoxycholate. Primary bile acids and secondary bile acids directly or indirectly affect cardiovascular function, and regulate atherosclerosis, obesity, metabolism, inflammation, liver pathophysiology by acting on specific receptors. fecal microbiota transplantation (FMT) is the transfer of minimally processed feces from a healthy donor to the gut of a recipient. The purpose is to treat diseases associated with alterations in the gut microbiota, improve the imbalance of gut microbiota, and reestablish a normal functioning gut microbiota. It was previously used for the treatment of recurrent Clostridium difficile infection. In recent years, some countries have classified FMT as tissue transplantation, because feces is not a simple mixture of microorganisms, but a heterogeneous mixture of microorganisms, human cells, water, mucus, and metabolites, which is affected by the donor's own factors (age, lifestyle, etc.). Studies have found that the therapeutic potential of FMT is not limited to gastrointestinal diseases. The loss of gut microbiota alters the composition of the immune system and impairs cardiac repair after MI. The survival and physiological state of mice after fecal reconstitution are improved. Dysbiosis of gut microbiota in metabolic syndrome is associated with a proinflammatory state and impaired mucosal barrier function. Fecal FMT from lean donors improves glucose clearance in obese patients with metabolic syndrome. Fecal microbiota from healthy rats transplanted into hypertensive rats induced by high-salt diet significantly reduced blood pressure in rats, suggesting that FMT is promising for the treatment of patients with chronic hypertension. At present, more and more scholars believe that HFpEF is a metabolic disease. The common comorbidities of HFpEF include obesity, type 2 diabetes, metabolic syndrome, hypertension, coronary atherosclerotic heart disease, and kidney disease, which have been reported to be closely related to intestinal flora in the past. However, the relationship between gut microbiota and HFpEF remains poorly studied. Only a few small sample clinical studies have shown that fecal SCFAs and Trimethylamine N-oxide (TMAO) are altered in HFpEF patients, and TMAO is associated with poor prognosis in HFrEF patients. Subsequently, elevated circulating TMAO in HFpEF patients was found to be positively associated with adverse cardiovascular events such as cardiac death and rehospitalization for heart failure, and these metabolites have been shown to be closely related to gut microbiota. In addition, HFpEF is associated with intestinal epithelial dysfunction, which can easily lead to intestinal congestion or ischemia, change the morphology and permeability of the intestinal wall, and cause changes in the composition and abundance of intestinal flora, suggesting that intestinal flora may play an important regulatory role in HFpEF, a chronic cardiovascular syndrome. Targeting intestinal flora and metabolites is expected to become a new treatment strategy for HFpEF.

This study aims to reveal the role of gut microbiota and metabolites in the regulation of HFpEF and the related mechanisms. Through the establishment of a clinical cohort of HFpEF patients, the changes of gut microbiota and the expression of key metabolites in blood samples of HFpEF patients were confirmed, and their correlation with clinical indicators was analyzed, so as to provide potential biomarkers and therapeutic targets for the diagnosis, prevention and treatment of HFpEF, a major chronic disease.