Since 2011, phenylephrine was recommended as the preferred drug to maintain blood pressure in
obstetric anesthesia.Phenylephrine, an α adrenoceptor agonist, can induce peripheral
vasoconstriction to maintain blood pressure, while reflexly decrease heart rate and result in
cardiac output (CO) reduction.Norepinephrine acts not only as an α adrenoceptor receptor
agonist, but also as a weaker β adrenergic receptor agonist. It can elevate blood pressure
the same asphenylephrine, meanwhile produce positive inotropic effect including increasing
heart rate.Thus, the administration of norepinephrine can maintain blood pressue and avoid
the decline of CO. The purpose of this study is to evaluate the effect of norepinephrine and
phenylephrine on maternal CO in cesarean section by transesophageal echocardiography.