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.