European Journal of Cardiovascular Medicine

Spinal anaesthesia is the preferred technique for caesarean section due to its simplicity, rapid onset, minimal drug transfer to the fetus, and excellent analgesia¹. However, it is frequently associated with maternal hypotension, occurring in up to 80% of parturients². This hypotension, if untreated, can compromise uteroplacental perfusion, resulting in fetal acidosis, neonatal depression, and maternal nausea, vomiting, or even loss of consciousness³.

The primary cause of hypotension in spinal anaesthesia is sympathetic blockade leading to vasodilation and decreased systemic vascular resistance⁴. Multiple strategies have been proposed to counteract this effect, including fluid preloading, positioning, and administration of vasopressors⁵.

Vasopressors remain the cornerstone of hypotension management in obstetric anaesthesia. Phenylephrine and mephentermine are commonly used agents with differing pharmacodynamics. Phenylephrine is a selective α1-adrenergic receptor agonist causing vasoconstriction and rapid restoration of blood pressure⁶. However, it can cause reflex bradycardia and decreased cardiac output⁷. On the other hand, mephentermine is a mixed-acting sympathomimetic with predominantly β-adrenergic activity, leading to increased heart rate and cardiac output with mild vasoconstriction⁸.

Several studies have evaluated the efficacy of these vasopressors. Cooper et al. found that phenylephrine effectively restored blood pressure but with a higher incidence of bradycardia⁹. Jain et al. noted that mephentermine maintains heart rate and cardiac output more effectively¹⁰. Despite their widespread use, the ideal vasopressor for obstetric use remains a subject of debate.

This study aims to compare the effectiveness of IV bolus phenylephrine versus mephentermine for treating spinal anaesthesia-induced hypotension in parturients undergoing caesarean section. Key outcomes assessed include the time to correction of hypotension, number of boluses required, maternal side effects (bradycardia, nausea, vomiting), and neonatal outcomes (Apgar score).

By understanding the differences in response profiles between these agents, clinicians can better tailor vasopressor use based on individual patient needs and optimize both maternal and fetal safety.

This randomized, double-blind, prospective study was conducted over 6 months in the Department of Anaesthesiology at a tertiary care hospital. After ethics committee approval and informed consent, 100 ASA I and II parturients aged 20–35 years scheduled for elective lower segment caesarean section under spinal anaesthesia were enrolled.

Inclusion Criteria:

• ASA Physical Status I or II
• Term singleton pregnancy (≥37 weeks)
• Age 20–35 years
• Undergoing elective caesarean section under spinal anaesthesia

Exclusion Criteria:

• Pregnancy-induced hypertension or preeclampsia
• Multiple gestation
• Cardiovascular, renal, or metabolic disease
• Contraindications to spinal anaesthesia
• Known allergy to phenylephrine or mephentermine

Randomization and Blinding:

Participants were randomly allocated into two groups of 50 using computer-generated random numbers:

• Group P: Phenylephrine 50 µg IV bolus
• Group M: Mephentermine 6 mg IV bolus

The drug was prepared by an anesthetist not involved in the study. The administering anesthetist and the observer collecting data were blinded to group allocation.

Anaesthesia Technique:

All patients received 10 ml/kg Ringer lactate preloading. Spinal anaesthesia was administered in the L3–L4 interspace using 0.5% hyperbaric bupivacaine (2.0 mL). Sensory block to T6 was confirmed.

Definition of Hypotension:

• Systolic blood pressure (SBP) <100 mmHg or a fall >20% from baseline

On occurrence, Group P received phenylephrine 50 µg IV bolus, and Group M received mephentermine 6 mg IV bolus. Repeat doses were given if SBP remained low after 2 minutes.

Parameters Recorded:

• Heart rate (HR), SBP, diastolic BP, and MAP at baseline, every 2 min for 20 min
• Time to restore baseline BP
• Total vasopressor dose required
• Incidence of bradycardia, nausea, vomiting
• Neonatal Apgar scores at 1 and 5 min

Statistical Analysis:

Data were analyzed using SPSS version 22. Student's t-test and Chi-square test were applied. p < 0.05 was considered statistically significant

Table 1: Baseline Demographic and Clinical Characteristics

Table 2: Hemodynamic Response Post-Bolus

Table 3: Time to Restore SBP

In table 3, Group P (phenylephrine) restored SBP in 2.8 ± 1.2 minutes, significantly faster than Group M (4.1 ± 1.3 minutes).

Table 4: Total Number of Boluses Required

In table 4, Group P required fewer bolus doses (1.4 ± 0.6) compared to Group M (2.3 ± 0.8), which was statistically significant.

Table 5: Incidence of Side Effects

In table 5, Bradycardia occurred more often in Group P (24%) vs. Group M (6%), highlighting a major side effect of phenylephrine.

Table 6: Neonatal Apgar Scores

In table 6, No significant difference in 1-minute and 5-minute Apgar scores between the groups.

This study compared the effectiveness of phenylephrine and mephentermine administered as IV boluses to treat spinal anaesthesia-induced hypotension during caesarean section. Our results show that phenylephrine more effectively restores systolic blood pressure and does so faster, whereas mephentermine preserves heart rate but is slower in onset and requires more bolus doses.

Phenylephrine, a pure α1-agonist, causes peripheral vasoconstriction and elevates systemic vascular resistance¹¹. This rapid vasopressor response explains the faster restoration of SBP observed in our study (2.8 vs. 4.1 minutes). However, reflex bradycardia, as seen in 24% of Group P, is a common limitation of phenylephrine, corroborating findings by Cooper et al. ¹² and Ngan Kee et al.¹³.

Mephentermine, with mixed α and β adrenergic activity, enhances cardiac output through increased HR and contractility¹⁴. This explains the higher HR in Group M and fewer incidents of bradycardia (only 6%). Our findings align with Sharma et al., who found mephentermine useful for maintaining cardiac output but less effective in restoring SBP quickly¹⁵.

Interestingly, despite differences in maternal haemodynamics, neonatal Apgar scores were similar across both groups, suggesting both vasopressors are safe for the fetus. This echoes findings from Lee et al. and Rout et al. who reported no difference in neonatal outcomes between phenylephrine and ephedrine, a β-agonist similar to mephentermine¹⁶.

Total vasopressor requirement was higher in the mephentermine group, indicating a shorter duration of action or less potent pressor effect per dose. This has practical implications for clinicians in busy obstetric ORs¹⁷.

Limitations of our study include a modest sample size and exclusion of high-risk pregnancies (e.g., preeclampsia). Future studies could assess continuous infusions versus boluses and include cardiac output monitoring for more comprehensive data.

In conclusion, phenylephrine provides faster and more effective control of hypotension, though mephentermine may be preferred in patients prone to bradycardia. The choice should be tailored to individual patient physiology and institutional protocols.

Both phenylephrine and mephentermine are effective vasopressors for managing hypotension during spinal anaesthesia for caesarean section. Phenylephrine acts faster and restores SBP more reliably but is associated with bradycardia. Mephentermine maintains heart rate better but requires more frequent dosing. Neonatal outcomes were unaffected by the choice of vasopressor. Clinicians should consider maternal heart rate profile and dosing logistics when choosing a vasopressor.

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