The investigator will evaluate the influence of colloid administration on postoperative acute kidney injury in pediatric patients undergoing cardiac surgery under cardiopulmonary bypass.

The investigator will randomly divide pediatric patients aged less than 7 years old into crystalloid and colloid groups. Crystalloid group receive only crystalloid, and colloid group receive hydroxyethyl starch (HES, 130/0.4). The maximum volume of HES is limited less than 30 ml/kg. The same transfusion protocol will be applied in both groups. From the day of operation to postoperative 7 days, glomerular filtration rate (GFR), serum creatinin level (Cr) will be checked. Acute kidney injury (AKI) is defined when there is more than 50% increase in Cr when compared to baseline level. The investigator will compare the clinical outcomes including AKI between two groups.

Inclusion Criteria:

• Children aged less than 7 years old
• American Society of Anesthesiology (ASA) physical status 1-3

Exclusion Criteria:

• Preoperative creatinine > 1.5mg/dl
• History of dialysis
• Liver function abnormality
• Diabetes Mellitus
• History of allergic reaction
• Coagulation abnormality