To investigate biomarker reflects systemic or specific organ perfusion well, we are going to the observational comparison study using several hemodynamic monitoring methods in the premature infants. It includes near-infrared spectroscopy (NIRS), pulse oximetry with perfusion index (PI) and pleth variability index (PVI) and functional echocardiography.

Hemodynamic observational comparisons will be performed in 6 episodes below on each patient.

1. first 24 hours after birth
2. during red blood cell (RBC) transfusion: from 4 hours before to 4 hours after the transfusion
3. suspicion of necrotizing enterocolitis: from the suspicion point for 48 hours
4. suspicion of sepsis: from the suspicion point for 48 hours
5. oligouria (< 1 mL/kg/hour of urine): for 48 hours
6. hypotension (mean blood pressure < 30 mmHg): for 48 hours

Inclusion Criteria:

• premature infants with 32 weeks or less of gestational age or 1,500 g or less of birth weight

Exclusion Criteria:

• infants without the consent of their guardians

premature infants with 32 weeks or less of gestational age or 1,500 g or less of birth weight