Patients younger than 18 scheduled for congenital heart surgery will be assessed during and post-operatively as well as at the first follow-up after 9-12 month for the novel biomarker sST2. We will assess the marker independently and in evaluation with other blood biomarkers to evaluate sings of heart failure. Compared to established biomarkers, sST2 promises thereby to be less variable to factors like age or acute kidney injury, rendering it potentially more reliable in the field of congenital cardiac surgery.

Single-center prospective biomarker validation study for Soluble suppression of tumorigenicity 2 (sST2). All data will be collected upfront to ensure accuracy. The study's findings aim to improve risk stratification and guide better management for pediatric cardiac surgery patients.

Congenital Heart Disease (CHD) is the most common congenital abnormality, affecting about 1 in 100 live births. While surgical interventions have significantly improved survival rates, a considerable number of patients experience long-term complications like ventricular dysfunction and heart failure, which are major causes of death.

Biomarkers are crucial tools that can aid clinicians in risk stratification, treatment guidance, and predicting outcomes. sST2 has shown utility in adult heart surgery cases and is included in the guidelines of the American Heart Association (AHA), while it's use in pediatric cases is largely unexplored.

This study tests if sST2 as biomarker can be a useful prognostic tool for children with CHD undergoing cardiac surgery. The primary objective is to assess if a specific cut-off level of post-surgery sST2 can predict future heart failure. Secondary objectives include comparing sST2 to other established biomarkers.

Inclusion Criteria:

• Consented for cardiopulmonary bypass surgery for cardiac reason
• Conversational skills in German (by child and/or representative) to be able to fully understand and sign a written consent in German language

Exclusion Criteria:

• Know genetic life limiting conditions
• Syndrome patients who are scheduled or highly likely to be operated on more than one organ
• Body weight at time of surgery less than 2.5 kg
• Being recruited and enrolled for an interventional study protocol

All patients in defined age scheduled for congenital cardiac surgery