The weight-based dosing of ondansetron to reduce hypotension has never been investigated. The aim of this study is to determine the optimal dose of ondansetron required based on the patient's weight to reduce hypotension following spinal anesthesia for cesarean section.

In this perspective, triple-blinded, parallel group, randomized controlled trial, a total of 228 pregnant women were randomized to receive either normal saline (group NS) or ondansetron 0.05 mg/kg (group O1) or ondansetron 0.1 mg/kg (group O2) intravenously 5 min before induction of spinal anesthesia.

Hypotension

The incidence of hypotension, mean arterial pressure, heart rate, vasopressor requirements, and blood loss between the three groups were compared. Maternal and neonatal complications were also assessed. Changes in blood pressure and heart rate were compared using the generalized estimating equations method.

Thirteen patients were excluded from the analysis because of no intervention (n = 12) and protocol violation (n = 1). Two hundred and fifteen patients remained for the intention-to-treat analysis.

The incidence of hypotension in groups NS (n = 72), O1 (n = 71), and O2 (n = 72) were 81.9%, 84.5%, and 73.6%, respectively (P = 0.23). The episodes of hypotension before delivery (first 14 min after spinal anesthesia) were significantly higher in group O1 compared to NS (5 vs 2, P = 0.02).

Ephedrine

The overall heart rates throughout the operations were not different among the three groups. The ephedrine requirements and amount of blood loss were also similar among the three groups.

Neonatal Complications

The metoclopramide requirement was significantly lower in group O2 compared to group NS (2.8% vs 16.7%, P = 0.01). There were no serious adverse events in terms of maternal or neonatal complications.

Ondansetron 0.05 mg/kg or 0.1 mg/kg administered before spinal anesthesia did not reduce the incidence of hypotension in this study.