This study evaluate the ability of phenylephrine and of noradrenaline to maintain normal systemic arterial blood pressure and heart rate in healthy pregnant women submitted to cesarean section with spinal anesthesia. Half of participants will receive bolus of phenylephrine while the other half will receive bolus of noradrenaline.

This is a prospective, randomized, double-blind, comparative two-group clinical study to observe two vasopressor drugs, phenylephrine and noradrenaline, in the ability to maintain systemic arterial pressure in pregnant women after spinal anesthesia for cesarean section; based on the hypothesis that, since noradrenaline has a predominant alpha-adrenergic action and a weak 1- beta-adrenergic beta action, it could be as capable of maintaining blood pressure effectively as phenylephrine, but with a lower frequency of bradycardia; patients who meet the inclusion criteria will be randomly assigned to one of the groups to be studied: group F - phenylephrine at a dose of 100mcg bolus and N - noradrenaline at the dose of 5mcg. One of these drugs will be used whenever systolic blood pressure drops below 10% of the baseline, additional doses will be allowed whenever necessary, aiming at maintaining the systolic blood pressure at baseline values; the blood pressure will be measured non-invasively and the heart rate by pulse oximeter and electrocardiogram.

Inclusion Criteria:

• Pregnant women, with single gestation, with cesarean programming to be performed under spinal anesthesia;
• Age above 18 years;
• Physical State American Society of Anesthesiologists (ASA) II and III;
• Weight between 50kg and 120kg;
• Height between 140cm and 180cm.

Exclusion Criteria:

• Refusal to participate in the study;
• Pregnant women with fetus with known abnormalities;
• Pregnant women with cardiovascular disease;
• Pregnant women with pregnancy-specific hypertensive disease;
• Allergy to any medication to be used in the study;
• Users of monoamine oxidase inhibitors;
• Users of tricyclic antidepressants;
• Emergency caesarean section due to acute fetal distress;
• Situations in which the sensory level after single intrathecal injection of the local anesthetic does not reach at least T6 within 5 minutes after spinal anesthesia.