The purpose of this study is to evaluate the variations of respiratory changes in pulse oximeter plethysmographic amplitude (delta POP) according to different contact forces between finger and sensor of pulse oximeter. During general anesthesia without surgical stimuli, pulse oximeter sensor is applied with serial contact force, from 0 to maximal 1.5N (increased by 0.2N) and POP waveform is obtained for 60 seconds. Delta POP will be calculated and compared.

Inclusion Criteria:

• children who are aged between 2 and 5 years
• no cardiopulmonary and vascular diseases
• obtained written informed consent from their guardians
• no hemodynamic instability from massive bleeding

Exclusion Criteria:

• significant cardiopulmonary or vascular disease
• significant hemodynamic instability due to massive bleeding etc.
• cyanotic patients