The objective of this study is to determine the effect of ondansetron on the total consumption of norepinephrine infusion needed to prevent spinal anaesthesia induced hypotension in cesarean section.

Ondansetron has been reported to attenuate the incidence of spinal anaesthesia-induced hypotension (SAIH) and norepinephrine requirement during caesarean section.

The mechanism of hypotension following spinal anaesthesia involves the reduction in vascular resistance caused by the sympathetic block and the activation of the Bezold-Jarisch reflex, leading to vasodilation and hypotension. Peripheral serotonin receptors, 5-Hydroxytryptamine3 (5HT3), are required for the activation of the Bezold-Jarisch reflex. In a rabbit model, a 5-HT3 receptor antagonist was reported to suppress bradycardia and hypotension by preventing the Bezold-Jarisch reflex.

Inclusion Criteria:

• Age from 19 to 40 years old.
• Singleton pregnancies with a gestational age of at least 37 weeks.
• pregnant women are scheduled for elective caesarean delivery.
• Patients with stable vital signs.
• Patients with normal laboratory investigations.
• patients undergoing spinal anaesthesia for caesarean delivery via Pfannenstiel incision with exteriorizaion of the uterus.

Exclusion Criteria:

• Patient's refusal.
• Age < 19 or > 40 years.
• Height<150 cm, weight < 60 kg, body mass index (BMI) ≥40 kg/m2.
• Contraindications to spinal anaesthesia (Coagulopathy, increased intracranial pressure, or local skin infection).
• Patients with cardiac morbidities.
• hypertensive disorders of pregnancy as pre-eclampsia.
• peripartum bleeding.
• Patients with respiratory morbidities.
• Convulsions.
• Bleeding diathesis.
• Known allergy to any drugs used in this study.