Spinal anesthesia is the preferred anesthesia method in cesarean section to provide satisfactory analgesia and muscle relaxant with less impact on respiratory system. However, hypotension often occurred due to the block of sympathetic nerve, causing maternal decline of frontal lobe oxygenation, nausea vomit and the decrease of uteroplacental perfusion. Several measures are used to prevent or treat hypotension caused by spinal anesthesia: prehydration, limb compression, left lateral tilt of operation tables or usage of vasopressors. In the past decade, the most recommended vasopressor to prevent or treat hypotension in spinal anesthesia in cesarean section was phenylephrine, an α-adrenergic receptor, maintaining maternal blood pressure and fetal acid-base state. In clinical work, there are two ways to use phenylephrine : intravenous method with less onset time (several seconds and duration (several minutes) and intramuscular method with longer onset time (10-15 minutes) and duration (1 hour). Many trials demonstrated the protective effect of preventive intravenous phenylephrine on maternal hemodynamics and neonatal acid-base status. However, few trials reported the effect of preventive intramuscular phenylephrine on cesarean section under spinal anesthesia.

Hypotension often occurred in parturients undergoing cesarean section in spinal anesthesia. This study aims to determine whether preventive intramuscular phenylephrine can better the fetal acid-base state and maternal hemodynamics.

Inclusion Criteria:

1. Age 18 years to 40 years.
2. Elective cesarean section
3. American Society of Anesthesiologists (ASA) grade from I to Ⅱ, height from 150 cm to 180 cm, BMI<40kg/m2
4. Singleton pregnancy
5. Without pregnancy complications

Exclusion Criteria:

1. Multiple pregnancy
2. Preoperative bradycardia
3. Coagulation dysfunction
4. Parturients with hypertension, diabetes, eclampsia and other pregnancy complications.