Postoperative mediastinitis is an important cause of postoperative morbidity in children. The main objective of this study is to describe the distribution of cefazolin, using as surgical antibiotic prophylaxis, in the mediastinal compartment in children after cardiac surgery for congenital heart disease. The investigators aims to build a population pharmacokinetic model of cefazolin using plasma and tissue concentrations in order to optimize and individualize cefazolin dosing regimens. Cefazolin tissue pharmacokinetics will use a microdialysis procedure.

Children with congenital heart disease are very fragile during the postoperative period and at risk of life-threatening mediastinitis due to various factors: post-extracorporeal circulation inflammation, frequent organ failure, possible delayed thorax closure. These patients are at risk of unpredictable pharmacokinetics alteration and inter-individual variability of the drug tissue: significant fluid intake, drug metabolism and elimination functions alteration, extracorporeal circulation. The effective prevention of postoperative mediastinitis in this population is therefore a major issue and challenge. Studying the distribution of cefazolin in the mediastinum and identifying the factors of inter-individual variability would improve the prevention of this pathology by optimizing the dosing.

During cardiac surgery, a microdialysis probe is placed by the surgeon in the mediastinum. The outgoing fluid will be collected every 30 minutes to every 2 hours, for approximately 28 hours. Six plasma samples will be drawn over these 28 hours to measure plasma concentrations. The probe's recovery will then be calculated and the probe will be removed up to 36 hours after insertion.

Inclusion Criteria:

• Child aged over 6 months and under 6 years
• Hospitalized in pediatric cardiac intensive care after cardiac surgery for congenital heart disease.
• Selected congenital heart diseases: ventricular septal defect auricular septal defect, pulmonary stenosis, tetralogy of Fallot
• Surgery with median sternotomy and intraoperative cardiopulmonary bypass
• Intraoperative antibiotic prophylaxis with cefazolin
• Patient affiliated with a social security plan or eligible
• whose two parents or legal guardians have accepted and signed the study consent

Exclusion Criteria:

• Emergency cardiac surgery
• Patient already enrolled in the study
• Patient who has previously undergone cardiac surgery
• Patient with haemostasis disorders or immune deficiency
• Patient with bacterial colonization indicating antibiotic prophylaxis other than cefazolin
• Participation in another interventional research study