Spinal anesthesia is the preferred technique for elective cesarean section as per ASA guidelines. Hypotension is the main complication of this technique and is secondary to both sympatholysis and its associated decrease in systemic vascular resistance and to the Bezold-Jarisch reflex, which causes hypotension and bradycardia in response to noxious stimuli detected in the cardiac ventricles. In pregnant patients, spinal anesthesia induced hypotension is worsened by compression of the aorta and inferior vena cava by the gravid uterus. In this setting, hypotension could lead to uteroplacental hypoperfusion and fetal distress. In its 2020 guidelines for enhanced recovery after cesarean section, SOAP states that preventing spinal-induced hypotension is an important strategy to enhance maternal and neonatal outcomes in cesarean delivery Recent studies showed that 5-hydroxytryptamine-3 receptor antagonists, mostly used as nausea and vomiting prophylaxis agents, also contributed to inhibit the Bezold-Jarisch reflex and its associated hemodynamic consequences. Ondansetron is the most studied molecule in this field. Many recent studies and meta-analyses show renewed interest in the use of norepinephrine as a first line agent for preventing and treating spinal anesthesia-induced hypotension in obstetric anesthesia practice instead of phenylephrine. Norepinephrine has the advantage of a better cardiac output and cardiac frequency as compared to phenylephrine without any fetal side effect. The combination of ondansetron and phenylephrine for the prevention of spinal anesthesia-induced hypotension has been studied, but not the combination of ondansetron and norepinephrine. The main objective of this study is to evaluate the sparing effect of a standard dose of ondansetron on norepinephrine consumption during elective cesarean section under spinal anesthesia by determining the effective dose in 50% of subjects (ED50) of a prophylactic norepinephrine infusion after receiving a single dose of 4 mg of ondansetron or a saline control.
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ED50 Norepinephrine
Timeframe: At the fetal delivery