Effect of Three Different Prophylactic Bolus Doses of Phenylephrine on Hypotension Following Caesarean Section Under Combined Spinal-Epidural Anaesthesia

Abstract

Background: Hypotension is a frequent and potentially serious complication of spinal anaesthesia during caesarean section. Prophylactic administration of phenylephrine is commonly employed to counteract this effect; however, the optimal bolus dose remains uncertain.

Objectives: To compare the efficacy of three different prophylactic bolus doses of phenylephrine—50 µg, 75 µg, and 100 µg—in preventing maternal hypotension following combined spinal–epidural anaesthesia for elective caesarean section.

Methods: A randomized controlled study was conducted on 180 parturients (ASA II–III), equally divided into three groups (n = 60 each) to receive 50 µg, 75 µg, or 100 µg of phenylephrine immediately after spinal block. Demographic and baseline data were comparable across groups. Primary outcome was incidence of hypotension (≥20% fall in systolic blood pressure or SBP < 90 mmHg). Secondary outcomes included rescue vasopressor requirement, maternal side effects, and neonatal parameters. Data were analyzed using ANOVA, Chi-square, and multivariate logistic regression.

Results: The incidence of hypotension showed a clear dose-dependent decline: 40% in Group P50, 20% in Group P75, and 10% in Group P100 (p < 0.001). Higher phenylephrine doses were associated with delayed onset of hypotension and reduced rescue vasopressor requirements (p < 0.001). Mild bradycardia occurred more frequently with higher doses but without clinical significance (p = 0.08). Maternal nausea and vomiting were significantly less common in higher-dose groups (p = 0.02), and neonatal Apgar scores and umbilical pH remained comparable across groups (p > 0.05). Logistic regression confirmed phenylephrine dose as an independent predictor of hypotension, with adjusted odds ratios of 0.35 for 75 µg and 0.15 for 100 µg versus 50 µg.

Conclusion: Prophylactic phenylephrine bolus doses of 75 µg and 100 µg effectively reduced the incidence and severity of spinal-induced hypotension during caesarean section without adverse maternal or neonatal effects. A 100 µg dose provided the most stable haemodynamics, suggesting it as the optimal prophylactic dose in routine obstetric anaesthesia practice.