Maternal hypotension is a frequent complication after spinal anesthesia for cesarean delivery. Many vasopressors have been studied and used, but the perfect vasopressor is yet to be found. Phenylephrine is the most common used in obstetric anesthesia but its cardiac depressant activity, being an only alpha-adrenergic agonistic, is linked to frequent side effects such as bradycardia and decreased cardiac output.~Norepinephrine is a vasopressor characterized by both alpha and minor beta-adrenergic agonistic activity, it has then a minimal cardiac depressant activity. Hence it would provide a better stability of hemodynamic and cardiac output, and appears as a better alternative to phenylephrine.~In this study, the investigators will compare prophylactic continuous variable infusion of both vasopressors. Phenylephrine started at the dose of 0,5mcg/kg/min and Norepinephrine tartrate started at the dose of 0,1mcg/kg/min. The doses will be adjusted according to maternal systolic blood pressure in order to prevent hypotension (defined by a systolic blood pressure under 80% of baseline).
Comparison between prophylactic continuous variable infusion of phenylephrine (starting dose 0,5mcg/kg/min) and norepinephrine tartrate (starting dose 0,1mcg/kg/min) to prevent hypotension and maintain cardiac output under spinal anesthesia during cesarean delivery.