## 2 Methods  
### Participants and Study Design  
An observational study was conducted involving 43 patients (30 males and 13 females) and 86 volunteers who comprised the control group (44 males and 42 females), with all participants aged between 39 and 88. Due to the limited sample size of the case group, a higher control-to-case ratio was implemented to improve statistical power. This resulted in an overall control-to-case ratio of approximately 2:1, with a ratio of about 3:1 for patients with myocardial infarction (MI) and around 5:1 for patients with congestive heart failure (CHF). To match the age of the CHF patients, who were 58 years and older, a subgroup of 46 participants from the control group was formed to discard the age effect on amine levels in CHF patients. The control-to-case ratio was approximately 3:1.  
Patients were recruited consecutively, from February 2016 to December 2019, without bias toward sex, from the Hospital Universitario Central de Asturias in Spain. This group included individuals diagnosed with AMI upon admission to the Coronary Care Unit and those suffering from CHF who were attending cardiology follow-ups, with at least 5 years of evolution. The control group was selected from healthy blood donors at the Centro Comunitario de Sangre y Tejidos de Asturias, specifically those aged up to 65 years. Participants were included if they had no known diseases and were not undergoing any pharmacological treatments. Additionally, older adults were recruited from medical consultations, provided they had no cardiovascular, renal, respiratory, or metabolic diseases.  
This study was explained to the participants, who signed an informed consent form to collect 4.5 mL blood samples in tubes containing EDTA to prevent clotting. Blood sampleswere collected on the first day of admission to the Coronary Care Unit for patients with MI. For patients with CHF, samples were taken on the day of their follow-up appointment. The data used for the clinical analysis were collected on the same date as the blood sample collection in both groups.  
This study received approval from the Comite de Etica de la Investigacion del Principado de Asturias (Spain) (CEImPA 28/10; 90/17; 277/18; 28/19) and adhered to the guidelines of the Declaration of Helsinki.  
The variables considered in this study included age, sex, and biogenic amines in peripheral blood samples, which were determined using HPLC. The presence of cardiovascular disease, AMI, or CHF was also examined. Additionally, clinical and analytical parameters, including heart rate, systolic and diastolic blood pressure, left ventricular ejection fraction ( LVEF), left interventricular septum measurements, and left atrial diameter as assessed by 2D echocardiography, were analysed for possible correlations with the determined polyamines. Biochemical data (Table 1) for each patient were collected on the same date as the blood samples for polyamine determination.  
Heart failure (HF) was classified in accordance with 2021 European Society of Cardiology Guidelines [32], based on the value of the left ventricular ejection fraction (LVEF) in HF with preserved EF (HFpEF), LVEF > 50%, HF with mildly reduced EF (HFmref), LVEF 40-49%), and HF with reduced EF (HFrEF), LVEF < 40%.