Phenylephrine infusion for prevention of hypotension during spinal anesthesia for elective cesarean section

INTERVENTION: All participants are divided into two groups, phenylephrine group and conventional group. In phenylephrine group, at the same time of spinal anesthesia, starting preventive infusion of phenylephrine 0.5 microgram/kg/min and gradually discontinuing based on the protocol. Treat hypotension in a same way for both groups. CONDITION: elective caesarean section PRIMARY OUTCOME: Occurrence of hypotension SECONDARY OUTCOME: Heart rate variability, total dose of phenylephrine, occurrence of nausea or/and vomiting, occurrence of bradycardia, apgar score, PH BE of umbilical cord blood gas INCLUSION CRITERIA: 1.elective caesarean section under spinal anesthesia 2.age over 20 years old 3.written informed consent