Colloid Co-hydration and Vasoconstrictor Infusion for Prevention of Hypotension During Cesarean Section

in parturients allocated to the phenylephrine group, a phenylephrine infusion will be started as soon as spinal anesthesia is initiated

in parturients allocated to the norepinephrine group, a norepinephrine infusion will be started as soon as spinal anesthesia is initiated

in parturients allocated to the placebo group, a normal saline infusion will be started as soon as spinal anesthesia is initiated