Some vasopressive drugs including ephedrine and phenylephrine have been widely used to prevent maternal hypotension. Although ephedrine has mixed a-adrenoceptor activity , it maintains arterial pressure mainly by increases in cardiac output and heart rate as a result of its predominant activity on β1-adrenoceptors. It has been demonstrated that ondansetron preloading with crystalloid infusion reduces maternal hypotension in parturient women undergoing cesarean delivery.~Ondansetron has been widely used in the clinic to prevent nausea and vomiting caused by cancer chemotherapy, radiation therapy, and surgery. Ondansetron has been proven as a well-tolerated drug, but the most common side effects of ondansetron include headache, constipation, diarrhea, asthenia, and somnolence.
More than 30% of the patients receiving spinal anesthesia develop hypotension. Hypotension developed during cesarean section (C/S) under spinal anesthesia may jeopardize uteroplacental circulation leading to fetal compromise and even fetal death.~The effect of prophylactic ondansetron on blood pressure after spinal anesthesia has not been compared in a clinical trial with that of a vasoconstrictor. The investigators will compare ephedrine and ondansetron for the prevention of maternal hypotension after spinal anesthesia for elective cesarean delivery.