The purpose of this study is to investigate the effect of different crystalloid coload volumes on the 90% effective dose of prophylactic norepinephrine infusions for preventing postspinal anesthesia hypotension during cesarean section.

Post-spinal anesthesia hypotension is a frequent complication during spinal anesthesia for cesarean section. The incidence of post-spinal anesthesia hypotension is as high as 62.1-89.7% if prophylactic measures are not taken. Coload has been highly demonstrated for prevention and/or treatment of post-spinal anesthesia hypotension. In addition, there's some evidence that prophylactic infusion of norepinephrine could effectively reduce the incidence of post-spinal anesthesia hypotension in parturients undergoing cesarean section. The purpose of this study is to investigate the effect of different crystalloid coload volumes on the 90% effective dose of norepinephrine infusion prophylaxis for hypotension during spinal anesthesia for cesarean section.

Inclusion Criteria:

• 18-45 years
• Primipara or multipara
• Singleton pregnancy ≥37 weeks
• American Society of Anesthesiologists physical status classification I to II
• Scheduled for cesarean section under spinal anesthesia

Exclusion Criteria:

• Body height < 150 cm
• Body weight > 100 kg or body mass index (BMI) ≥ 40 kg/m2
• Eclampsia or chronic hypertension or baseline blood pressure ≥180 mmHg
• Hemoglobin < 7g/dl
• Fetal distress, or known fetal developmental anomaly