The purpose of this study is to compare norepinephrine and phenylephrine on the effects of maintaining the cardiac output in Cesarean section under spinal anesthesia.

CO was measured using the NICOM® monitor. Bioreactance, a development from impedance technology, is also emerging as an accessible mode of continuous CO monitoring in the operating room and can provide valuable insight into the hemodynamic effects of our interventions. This method of monitoring provides continuous CO, stroke volume (SV) and heart rate (HR) monitoring, and intermittent BP and SVR assessments at 1-minute intervals.

Either phenylephrine (100 mcg/ml) or norepinephrine (0.5 mcg/ml) in 50-ml syringe that were labeled "study drug". She was not involved in patient's care or study assessment.

The hypotension was defined as systolic blood pressure <80% of baseline or < 90 mmHg, and managed with study drug (1 ml) intravenously. The bradycardia was defined as HR < 60 beats/min and was recorded and managed with atropine (0.5 mg) intravenously as HR < 45 beats/min. The presence of nausea and vomiting was measured on a 3 point scale (1 = no nausea and vomiting, 2 = nausea only, and 3 = both nausea and vomiting) and managed with ondansetron (4mg) intravenously.

Inclusion Criteria:

• ASA class I and II, term pregnancy (gestational age>37weeks), planned spinal anesthesia for elective cesarean section

Exclusion Criteria:

• preexisting or gestational hypertension, cerebrovascular or cardiovascular disease, any contraindication to the spinal anesthesia, fetal anormaly, weight<50kg or >100kg, and height<140cm or >180cm.