During general anesthesia, hypotension might occur during positional change. It is reported that hypotension after positional change from supine to prone is mainly caused by the decrease in preload. The investigators aimed to investigate whether dynamic arterial elastance might predict hypotension associated with positional change from supine to prone.

Inclusion Criteria:

• Patients planned for invasive arterial catheter insertion, undergoing elective spine surgery in prone position under general anesthesia

Exclusion Criteria:

• arrythmia
• pulmonary disease
• increased intracranial pressure
• decreased cardiac function (ejection fraction < 50%)
• obesity (body mass index > 40)

Patients who are scheduled for spine surgery and need positional change from supine to prone positioin.