A randomised double-blind trial of phenylephrine and metaraminol infusions for prevention of hypotension during spinal and combined spinal–epidural anaesthesia for elective caesarean section. McDonnell NJ, Paech MJ, Muchatuta NA, Hillyard S, Nathan A. Anaesthesia 2017;72:609-617

Presented by: Dr K. Kelly

Background

  • Preventing a substantial decrease in maternal blood pressure after spinal anaesthesia for caesarean section is considered fundamental to avoid maternal nausea, vomiting and syncope, as well as neonatal hypoxaemia and acidosis.
  • Maintenance of maternal systolic arterial pressure at near-baseline values has been recommended
  • Metaraminol has undergone limited investigation in obstetric anaesthesia for this purpose, particularly in comparison with phenylephrine.
  • Prophylactic vasopressor administration is commonly recommended to reduce maternal hypotension during spinal anaesthesia for caesarean section.
  • Ephedrine has largely been replaced by phenylephrine, because of improved maternal outcomes and reduced neonatal acidosis when using phenylephrine.
  • Authors hypothesised that use of a prophylactic metaraminol infusion to reduce maternal hypotension would not be inferior to phenylephrine infusion with respect to neonatal acid-base status.

Design and Setting

  • A randomised, double-blind, active control, parallel group, multicentre non-inferiority trial was to compare prophylactic infusions of phenylephrine and metaraminol in women undergoing elective caesarean section under spinal or combined spinal–epidural (CSE) anaesthesia.
  • The study was conducted at between February 2013 and June 2015 in two Australian Hospitals.

Subjects

  • Women of ASA 1–2
  • BMI 20–35 with a singleton term pregnancy,
  • Scheduled for elective caesarean section under spinal or CSE anaesthesia.

Exclusions

  • Diabetes, pre-eclampsia or other cardiovascular or cerebrovascular disease, a foetal abnormality or intra-uterine growth restriction.
  • Inadequate sensory block to cold sensation up to T4.

Outcomes

Primary outcome

  • The primary outcome was the difference in umbilical arterial pH between groups.

Secondary outcomes

  • Other neonatal acid-base measures,
  • Maternal haemodynamic changes.

Interventions

Neuroaxial Technique

  • 2.2– 2.5 ml hyperbaric bupivacaine 0.5% together with 15mcg of fentanyl.

Vasopressors

Commenced at the time of spinal injection.

  1. 30 ml/hr infusion of the study drug, equivalent to 50 mcg/min of phenylephrine (10mg in 100ml)
  2. 250 mcg/min of metaraminol (10mg in 20ml)

Results

  • The mean (SD) umbilical arterial pH was 7.28 (0.06) in the phenylephrine group vs. 7.31 (0.04) in the metaraminol group (p = 0.0002).
  • Apgar scores did not significantly differ between groups.
  • There was a higher incidence of hypotension, defined as systolic arterial pressure < 90% baseline, in the phenylephrine group.
  • There was a higher incidence of hypertension and severe hypertension (systolic arterial pressure > 110% and > 120% baseline, respectively) in the metaraminol group.
  • There was no significant difference between groups in the incidence of nausea, vomiting or maternal bradycardia.

Conclusions

  • There was a higher incidence of hypertension and severe hypertension (systolic arterial pressure > 110% and > 120% baseline, respectively) in the metaraminol group.
  • There was no significant difference between groups in the incidence of nausea, vomiting or maternal bradycardia.

Strengths

  • Well described outcome measures.
  • Well matched groups.
  • Statistical tests clearly outlined.

Weaknesses

  • Large proportion of CSE.
  • BMI may not represent local population.
  • It was conducted during elective surgery in healthy term pregnant women and infants.
  • Findings may not apply to premature infants or to the non-elective setting.
  • Based in two hospitals only

Implications

  • Metaraminol may be a viable alternative if phenylephrine is not available.
  • Use of infusions preferable to bolus dosing to pre –empt the predicted drop in blood pressure due to neuroaxial block.

Potential for Impact

  • Largest clinical trial to compare phenylephrine and metaraminol for the prevention of hypotension during spinal and CSE anaesthesia at caesarean section.
  • Should we move to infusions rather than bolusing?