Comparison of fractionated versus bolus administration of different doses of hyperbaric bupivacaine combined with opioids in spinal anesthesia for caesarean section – a prospective, randomized, participant-blind clinical trial

Spinal anesthesia with hyperbaric bupivacaine and neuraxial opioids is the preferred technique for cesarean section, but maternal hypotension remains a common complication. Fractionated administration of local anesthetic may provide hemodynamic stability while maintaining effective anesthesia. This study aimed to compare the efficacy and safety of fractionated versus bolus administration of different doses of hyperbaric bupivacaine combined with opioids in spinal anesthesia for cesarean section. This prospective, randomized, participant-blind clinical trial included 99 pregnant women (ASA II-III) scheduled for cesarean section. Patients were randomized into three groups (n = 33 each): Group B received a bolus injection of bupivacaine (0.07 mg/cm height); Group F7 received the same total dose but fractionated (i.e. divided into two injections separated by an interval); and Group F6 received a lower fractionated dose (0.06 mg/cm height). All patients also received 10 µg fentanyl and 100 µg morphine intrathecally. The primary outcome was the incidence of clinically significant hypotension – defined as a decrease in systolic blood pressure (SBP) to below 100 mmHg or below 80% of the individual baseline value – occurring despite a standardized prophylactic phenylephrine infusion protocol. Secondary outcomes included: systolic blood pressure, diastolic blood pressure, and especially mean arterial pressure changes over time; vasopressor requirements; block characteristics; duration of analgesia; perioperative complications. The three groups were comparable in demographic characteristics (age, height, BMI). The incidence of hypotension did not differ significantly: 24.2% in Group B, 27.3% in Group F7, and 27.3% in Group F6 (p > 0.05). Mean phenylephrine requirements were numerically lower in the fractionated groups (Group F6: 1325.2 ± 909.83 µg, Group F7: 1462.1 ± 1070.56 µg) than in the bolus group (Group B: 1616.7 ± 1182.2 µg), but the differences were not statistically significant (p > 0.05). The duration of analgesia was comparable between groups (Group B: 216 ± 49 min, Group F7: 212 ± 56 min, Group F6: 207 ± 57 min, p = 0.79). The duration of motor block showed a trend towards being shorter in the reduced-dose fractionated group (Group B: 178 ± 43 min, Group F7: 169 ± 53 min, Group F6: 155 ± 33 min, p = 0.12), though this difference did not reach statistical significance. Fractionated administration of hyperbaric bupivacaine combined with opioids in our study did not significantly reduce hypotension incidence compared to bolus administration. However, the lower dose (0.06 mg/cm) fractionated technique showed a trend toward reduced vasopressor requirements and shorter duration of motor block. Moreover, both fractionated techniques provided effective postoperative analgesia, suggesting that fractionated spinal anesthesia may be a reasonable alternative for cesarean section. Trial Registration Number on ClinicalTrials.gov: NCT07249814. The trial was registered retrospectively at ClinicalTrials.gov on November 25, 2025. Study timeline: Ethics Committee approval (23 June 2024) → patient enrolment (July 2024-June 2025) → trial registration (25 November 2025). At the time when the trial was started, research team was unfortunately not aware of the policy of the ICMJE, which requires prospective registration of all interventional clinical trials. No changes to the study design, primary outcome, or statistical analysis plan were made after data collection commenced.