## 2 Materials and Methods  
The PATHFNDER-CHD Registry is a prospective, multicenter, non-interventional cohort study launched in 2022. It aims to collect structured real-world data on adults (\(\geq\)18 years) with CHD and either manifest, previous, or preclinical HF. Participating centers are tertiary care institutions across Germany, all certified for their expertise in the management of ACHD [6]. The registry was established with a strong focus on maintaining high-quality standards [11].  
The ongoing registry includes data collected between June 2023 and February 2025, with February 2025 serving as the cut-off date for this analysis.  
Patients were eligible for inclusion if they were aged 18 years or older, had a confirmed diagnosis of any form of CHD and HF, provided informed consent (either personally or via a legal guardian), and were suitable for long-term follow-up within the registry. The primary exclusion criterion was participation in an interventional clinical trial, as this could compromise the validity of real-world data collection. No additional exclusion criteria were applied in order to reflect the full spectrum of clinical presentations seen in specialized ACHD care. Data collection is performed via a secure, web-based electronic data capture (EDC) system in compliance with the European General Data Protection Regulation (GDPR). All data are pseudonymized and stored in encrypted form. Ethical approval was granted by the leading ethics committee from the Friedrich-Alexander-University Erlangen-Nurnberg (Ref: 23-460-Bn) and the Technical University of Munich (Ref: 2022-582-S-KH), with additional further local approvals obtained as required.  
The registry captures both mandatory core data and optional extended variables at baseline and follow-up visits. Documented variables include demographics, type and complexity of CHD, prior surgical or interventional procedures, ventricular morphology, functional classification (Perloff and modified ACC/AHA) [2; 12], comorbidities, medication use, as well as weight and height to calculate BMI.  
Overweight and obesity are categorized according to the World Health Organization classification: normal weight (BMI 18.5-24.9 kg/m\({}^{2}\)), overweight (BMI 25.0-29.9 kg/m\({}^{2}\)), and obesity (BMI \(\geq\) 30.0 kg/m\({}^{2}\)) [13]. Heart failure was classified using the Perloff's functional class and the ACC/AHA Heart Failure Classification (2022) modified for CHD. Comorbidities such as arterial hypertension, diabetes mellitus, and sleep apnea were documented based on physician diagnosis and standard clinical criteria as routinely applied in participating certified ACHD centers. Surgical status, ventricular morphology, and types of congenital heart disease were recorded in accordance with standard cardiological practice.  
The study complies with the principles of Good Pharmacoepidemiology Practice (GPP) [14] and Good Practice in Secondary Data Analysis (GPS) [15]. All statistical evaluations were conducted on pseudonymized, non-personal data. Descriptive and inferential statistical analyses were performed to identify patterns and associations. Methods included frequency analyses, cross-tabulations, and one-sided chi-squared tests for categorical variables, with statistical significance defined as \(p<0.05\). No imputation or statistical adjustments were applied; missing values were reported as observed. All analyses were performed using SPSS version 29.0 (IBM Corp., Armonk, NY, USA). Longitudinal analyses are planned for future registry follow-up data.