The purpose of this study is to investigate the optimal preload guided by inferior vena cava collapsibility index in parturients with prophylactic norepinephrine infusion undergoing 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. Vasopressors has been highly recommended for routine prevention and/or treatment of post-spinal anesthesia hypotension. As a potential substitute drug for phenylephrine, norepinephrine has gradually been used in parturients undergoing cesarean section. There's some evidence that prophylactic infusion of norepinephrine could effectively reduce the incidence of post-spinal anesthesia hypotension in parturients undergoing cesarean section. However, the investigator's study (NCT03997500) had been shown that inferior vena cava collapsibility index (IVC-CI) markedly decreased in prophylactic norepinephrine infusion versus normal saline after spinal anesthesia and fetal delivery, suggesting that more prudent fluid management is required. The ideal dose of preload and coload is still unknown. Thus, the purpose of this study is to investigate the optimal preload guided by IVC-CI in parturients with prophylactic norepinephrine infusion undergoing cesarean section.

Inclusion Criteria:

• 18-40 years
• Primipara or multipara
• Singleton pregnancy ≥37 weeks
• American Society of Anesthesiologists physical status classification I to II
• Scheduled for elective 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 ≥ 160mmHg
• Hemoglobin < 7g/dl
• Fetal distress, or known fetal developmental anomaly