Target controlled infusion with remifentanil is widely used during cardiac surgery, wich is performed using the Minto model. It was derived from patients undergoing general surgery. However, pharmacokinetics of remifentanil can be changed during cardiopulmonary bypass. The investigators tested whether Minto model for target controlled infusion produces constant plasma remifentanil concentration during the cardiac surgery.

Inclusion Criteria:

• Adult patients with the ASA class I~III undergoing elective cardiac surgery using cardiopulmonary bypass

Exclusion Criteria:

• Patients not using target controlled infusion during the operation
• Patients with drug/substance abuse
• Patients using analgesics before this study starts
• Pregnant women
• Patients who rejected study participation