Labour analgesia: Remifentanil

Where it fits in labour analgesia

• IV Systemic opioid option for labour analgesia
  • Typically delivered as patient-controlled analgesia (PCA) with 1:1 midwifery monitoring due to risk of maternal respiratory depression
  • Analgesia quality generally inferior to epidural but can be superior to IM opioids; high maternal satisfaction in some units
• Key principle: rapid onset/offset allows titration to contractions but requires robust safety systems

Practical set-up

• Delivery: IV PCA with bolus-only regimen
  • Common starting bolus 20–40 micrograms with lockout 2 minutes; titrate to effect and side effects
  • Timing: patient presses at onset of contraction; peak effect may lag contraction peak, coaching improves efficacy
• Monitoring: continuous pulse oximetry; regular sedation scoring and respiratory rate; consider capnography if high risk patient
  • Supplemental oxygen (nasal cannula) often used but can mask hypoventilation—do not rely on SpO2 alone
• Staffing: 1:1 midwife presence; anaesthetist immediately available; clear escalation pathway
• Equipment: dedicated IV cannula, anti-siphon/one-way valve, PCA pump with drug library, naloxone available, airway/ventilation equipment immediately accessible

Immediate management of adverse events

• Excess sedation/respiratory depression: stop PCA, call for help, stimulate, airway manoeuvres, high-flow O2, support ventilation; consider naloxone titration
  • Naloxone: small IV aliquots (e.g., 40–100 micrograms) titrated to adequate ventilation while avoiding acute pain/sympathetic surge; observe for re-sedation (less likely with remifentanil due to short context-sensitive half-time)
• Fetal concerns: inform obstetric team; remifentanil crosses placenta but is rapidly metabolised—maternal hypoxia/hypotension are bigger threats

Mechanism of action and pharmacology

• Mechanism: potent μ-opioid receptor agonist → analgesia, sedation, respiratory depression, bradycardia
• Metabolism: rapid hydrolysis by non-specific plasma and tissue esterases (not dependent on hepatic/renal function)
  • Predictable offset; context-sensitive half-time ~3–5 minutes (relatively independent of infusion duration)
• Onset/offset: rapid onset (~1 minute); short duration (minutes) → suited to PCA boluses timed to contractions
• Placental transfer: crosses placenta; fetal/neonatal effects usually limited by rapid metabolism, but neonatal respiratory depression can occur (especially if maternal dosing close to delivery or maternal hypoventilation)

Counselling points

• Aim: reduce pain and anxiety; may not remove pain completely; epidural provides superior analgesia
• How it works: you press the button at the start of a contraction; the pump limits dosing to keep you safe
• Common effects: sleepiness, dizziness, nausea/itch; you must stay in bed and be closely monitored
• Important risks: slowed breathing and low oxygen levels; requires continuous monitoring and 1:1 midwife; partner must not press the button
• Baby: usually unaffected, but if you become very sleepy or your breathing slows, baby can be affected; neonatal team may attend at delivery if concerns
• Alternatives: epidural/spinal techniques; inhaled nitrous oxide; IM/IV opioids; non-pharmacological methods

Indications

• Maternal preference
• Neuraxial analgesia contraindicated (e.g., significant coagulopathy/anticoagulation, infection at site, patient refusal)
• Neuraxial unavailable or delayed (e.g., urgent analgesia while awaiting epidural/anaesthetist)
• Failed/ineffective epidural where immediate replacement not feasible (as a bridge)

Contraindications / relative contraindications

• Prematurity <36/40
• Multiple pregnancy
• Pre-eclampsia
• Pethidine within 2 hours, diamorphine / codeine / other opioid within 4 hours
• Intrauterine death (lower volume of distribution – use a morphine PCA instead) 
• Inability to provide required monitoring/staffing (no 1:1 midwife, no continuous SpO2, inadequate escalation capability)
• High risk of respiratory compromise: severe OSA, morbid obesity with hypoventilation, significant respiratory disease, baseline hypoxia/hypercapnia
• Reduced ability to use PCA safely: confusion, language barrier without adequate support, inability to understand instructions, partner likely to activate PCA

Maternal side effects and complications

• Respiratory depression/apnoea; desaturation; airway obstruction (especially supine/OSA/obesity)
• Sedation, dizziness, impaired coordination; aspiration risk if vomiting with reduced airway reflexes
• Nausea/vomiting; pruritus
• Bradycardia and hypotension (less common than with neuraxial but can occur, especially with high doses)
• Chest wall rigidity (rare; more associated with rapid bolus/high dose) → difficult ventilation
• Hyperalgesia/tolerance with prolonged exposure (more relevant to ICU/infusion; less typical in labour PCA but possible)

Fetal/neonatal considerations

• Potential neonatal respiratory depression if significant maternal dosing close to delivery or if maternal hypoxia/hypercapnia occurs
• Interpretation: fetal compromise is more often secondary to maternal physiological disturbance than direct drug effect
• Plan: inform neonatal team if high maternal dose, sedation episodes, or delivery imminent; ensure resuscitation readiness

Comparison with other labour analgesia options

• Versus epidural: less effective analgesia; no sympathectomy; avoids neuraxial complications but has higher risk of maternal sedation/respiratory depression
• Versus IM pethidine/diamorphine: faster onset/offset, better titratability, less prolonged neonatal depression; but requires intensive monitoring and has significant respiratory risk
• Versus Entonox: more potent analgesia but more systemic opioid adverse effects and monitoring burden