Effect of a 20% Reduction in Hyperbaric Bupivacaine Dose on High Spinal Block Incidence in Cesarean Delivery

Study OverviewThis prospective, randomized, double-blind, active-controlled trial investigates the clinical efficacy of a 20% dose reduction of intrathecal hyperbaric bupivacaine (10 mg vs. 12.5 mg) in reducing the incidence of high spinal block (HSB) among parturients undergoing elective or emergency cesarean delivery under spinal anesthesia. Participants are stratified preoperatively using an adapted risk-prediction model to evaluate the predictive validity of dose-blind versus dose-aware scoring frameworks.Study Arms and InterventionsArm A: Reduced-Dose Group (Intervention)Intrathecal Regimen: 10 mg hyperbaric bupivacaine 0.5% (2.0 mL) combined with 25 µg preservative-free fentanyl (0.5 mL of 50 µg/mL solution).Total Injectate Volume: 2.5 mL.Arm B: Standard-Dose Group (Active Comparator)Intrathecal Regimen: 12.5 mg hyperbaric bupivacaine 0.5% (2.5 mL) combined with 25 µg preservative-free fentanyl (0.5 mL of 50 µg/mL solution).Total Injectate Volume: 3.0 mL.Procedural Workflow and MethodologyStep 1: Risk-Prediction Scoring FrameworkPrior to randomization, each participant's risk profile is quantified using a 7-factor model adapted from Benjhawaleemas et al. (2024):Risk FactorCriterionBaseline WeightGestational age< 35 weeks8 pointsMaternal height< 150 cm8 pointsPost-pregnancy BMI> 27.5 kg/m²10 pointsOperator experienceFirst-year resident4 pointsSurgical historyPrevious cesarean delivery during labor6 pointsSurgical indicationElective cesarean delivery9 pointsBupivacaine dose> 11 mgVariable (see below)Timepoint 1 (Pre-randomization / Dose-Blind): The six non-dose variables are scored based on clinical assessment. The dose parameter is provisionally assigned a neutral weight of 5 points (reflecting the expected mean contribution under 1:1 allocation) to allow unbiased baseline stratification into low (score ≤ 14), intermediate (15-21), or high (> 21) risk tiers.Timepoint 2 (Post-randomization / Dose-Aware): Following unmasking of the syringe preparation, the score is recalculated with the actual dose parameter weighted as 0 points (≤ 11 mg, Group A) or 9 points (> 11 mg, Group B). Comparative discrimination metrics (ROC/AUC) assess the diagnostic utility of the pre-randomization model.Step 2: Anesthetic TechniqueAdministration: Performed by supervised 2nd- or 3rd-year anesthesia residents with the patient in the sitting position with maximal lumbar flexion.Access \& Equipment: L3-L4 or L4-L5 interspace identified via Tuffier's line; 25-gauge Whitacre (pencil-point) needle via midline approach with free-flowing clear CSF confirmation.Injection Rate: Intrathecal injection over 15-20 seconds using identical, prefilled syringes prepared by an unblinded pharmacist/anesthetist not involved in outcome assessment.Post-Injection Positioning: Immediate supine positioning with ≥ 15° left uterine displacement (wedge or table tilt).Block Assessment: Bilateral sensory level to cold sensation (ice/alcohol) and motor blockade via the Modified Bromage Scale. Surgical incision proceeds once bilateral sensory block achieves the T6 dermatome.Step 3: Hemodynamic and Safety ProtocolCo-loading: Intravenous balanced crystalloid (Ringer's lactate) at 10-15 mL/kg administered over 15-20 minutes starting at spinal injection.Prophylaxis: Routine prophylactic vasopressor infusions are withheld to prevent pharmacodynamic confounding of the primary endpoint.Monitoring Regimen: Continuous ECG, pulse oximetry ($SpO_2$), and heart rate. Non-invasive blood pressure (NIBP) measured every 2-3 minutes for the first 15 minutes post-injection, and every 5 minutes thereafter until surgery completion.Study Design and Administration DetailsTarget Enrollment: 666 participants (333 per arm; powered at 80%, $\alpha = 0.05$ to detect a reduction in HSB incidence from 5% to 1%, accounting for continuity corrections and attrition).Randomization \& Allocation Concealment: 1:1 computer-generated block randomization managed through sequentially numbered, opaque, sealed envelopes opened solely by the drug preparation team.Blinding Model: Double-blind (participants, clinical care providers, and outcome assessors remain masked).Study Setting: Single-center tertiary academic hospital (Operating Theaters, Aswan University Hospital, Egypt).