There is little research about the administration of norepinephrine in obstetric anesthesia.Ngan Kee et al.compared phenylephrine (0.57μg/kg/min) with norepinephrine (0.035μg/kg/min) in the treatment of hypotension in obstetric anesthesia. CO monitored by a suprasternal Doppler ultrasound every 5 minutes. CO and heart rate was significantly higher in norepinephrine group than that in phenylephrine group, while peripheral vascular resistance was significantly lower in norepinephrine group than that in phenylephrine group. And the oxygen content in umbilical venous was significantly higher in norepinephrine group, which may be related to the norepinephrine induced lower peripheral vascular resistance and high cardiac output.Monitoring of CO can give a comprehensive understanding of hemodynamics in the pregnant. It enables obstetric anesthesiologist to use liquid expansion rationally and administrate vasoactive agents properly.Although the accuracy of suprasternal Doppler ultrasound was high in that study, the interval of 5min to monitor CO may miss the rapid changes in hemodynamics of pregnant women.In the present study, we will monitor CO continuously by a small diameter (6 mm) transesophageal echocardiography, which can be retained in the patient's esophagus.