When there are clinical signs of hypovolemia, such as hypotension, decreased urine output and central venous pressure less than 5 mmHg, the study will be started. At index fingers of both hand, pulse oximeter sensors are applied by gradually increasing the contact force (from 0 to 1.4N, being increased by 0.2-0.3N). Then, pulse oximeter plethysmography (POP) waveforms are obtained for 90 seconds. Then, intravenous crystalloid fluid 10 mL/kg is infused for 15 min.