This study compares the effectiveness of morphine and fentanyl. Its goal is to investigate myocardial protection in patients. It focuses on acyanotic children undergoing congenital open-heart surgery. The main conditions being monitored are congenital heart defects and myocardial reperfusion injury. Within the study, Fentanyl (intravenously) and morphine are administered. The study was sponsored by Baskent University and Baskent University Ankara Hospital. This study is already completed.