Based on the patient's heart rate variability, the analgesia nociception index (ANI) measures the activity of the autonomic nervous system and the sympathetic/parasympathetic nervous system and expresses it as a numerical value through a specific algorithm. The investigators will analyze it and see if it has the ability to predict severe hypotension following spinal anesthesia.
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The change of averaged value
Timeframe: 15 min, 10 min and 5 min before entering the operating room; every 5 min after entering the operating room
The change of instantaneous value
Timeframe: 15 min, 10 min and 5 min before entering the operating room; every 5 min after entering the operating room
Total dose of injected phenylephrine
Timeframe: During the anesthesia (from initiation of anesthesia until the end of anesthesia)
Total dose of injected glycopyrrolate
Timeframe: During the anesthesia (from initiation of anesthesia until the end of anesthesia)
The change of systolic blood pressure
Timeframe: Every 1 min after entering the operating room until finishing the operation