• The primary outcome will be the minimum dose of phenylephrine which effectively prevents both hypotension and nausea/vomiting between intrathecal injection of the local anesthetic and delivery of the fetus.
  

• Maternal Demographics: Maternal age-weight-height
  
• Nausea and Vomiting
  
• Time of intrathecal injection, skin incision, uterine incision, and delivery
  
• Total dose of phenylephrine
  
• Hypertension (> 120 % of baseline)
  
• Bradycardia (< 50 bpm)
  
• Upper sensory level of anesthesia by pinprick upon delivery
  
• Umbilical artery and vein blood gases
  

The purpose of this study is to determine the minimum effective bolus dose of phenylephrine to prevent post-spinal hypotension in CS in 95 % of our patients. The study will be conducted in a randomized double-blinded fashion. This dose-response study will be conducted as per the up-down sequential allocation method, modified by the Narayana rule, designed to cluster the results around the ED95.

Blood pressure will be monitored every minute from intrathecal injection to the delivery of the child. Phenylephrine will be administered every time the blood pressure is equal to or lower than the control value.Hypotension will be defined as a SBP less than 80 % of the baseline value, in which case the treatment will be a failure. Hypertension will be defined as a SBP more than 120 % of baseline value. If a patient presents hypertension for two consecutive measurements, the case will be considered a drop out.An adequate response will be defined as the absence of hypotension or nausea/vomiting during the period from induction of spinal anesthesia to uterine incision prior to fetal delivery.

The results of this study will define the minimum effective dose of phenylephrine to prevent post-spinal hypotension and nausea/vomiting secondary to hypotension in Cesarean section, thus optimizing the safety profile with respect to maternal and fetal side effects of this vasopressor.