The purpose of this study is to evaluate predictability of a respiratory variation in pulse oximeter plethysmography for fluid responsiveness in children undergoing major surgery.

The purpose of this study is to evaluate predictability of a a respiratory variation in pulse oximeter plethysmography (delta POP) for fluid responsiveness in children undergoing major surgery.

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.

To evaluate the change of cardiac output, transesophageal or transthoracic echocardiography is performed before and after fluid administration. In addition, hemodynamic parameters including pulse pressure variation, systolic pressure variation, pleth variability index and central venous pressure are also recorded before and after fluid administration.

Finally, patients will be divided into fluid responder group and non-responder group. If stroke volume index measured using echocardiography increases over 15% after fluid administration, the patient will be fluid responder.

Delta POP (%) obtained from each finger with different contact force is calculated as follows;(POPmax - POPmin)/{(POPmax + POPmin)/2}. Using receiver operating characteristic curve, diagnostic power of delta POPs from different contact forces for fluid responsiveness will be evaluated. In addition, difference between delta POP from low contact force and that from high contact force will be evaluated.

Inclusion Criteria:

- Children who will be scheduled for major surgery requiring invasive hemodynamic monitoring

Exclusion Criteria:

• renal, hepatic and pulmonary disease
• preoperative infection: increased C-reactive protein, whith blood cell count over 10,000, and with fever
• genetic and hematologic disease
• ventricular dysfunction
• increased intracranial pressure