First-lineEntonox / Nitronox
Entonox
Inhaled 50:50 N₂O / O₂ analgesic
Ubiquitous UK labour analgesia — patient-controlled, rapid on/off.
Inhaled (demand valve)
First-line for
- Labour analgesia (1st and 2nd stage)
- Procedural analgesia (manual removal, perineal repair)
Mechanism
NMDA antagonism + endogenous opioid release + non-specific membrane effects → analgesia and mild dissociation.
"Entonox = Every contraction — inhale early, exhale late"
- Onset 30–60 s — start deep breaths at contraction onset
- No neonatal effect (rapid off)
- Ensure scavenging + Vit B12 in staff
Dosing
- Use
- Self-administered via mouthpiece/mask during contractions
Contraindications
- AbsolutePneumothorax or bowel obstruction
- AbsoluteRecent (<6 wk) middle-ear or intraocular surgery
- AbsoluteB12 deficiency (repeat use)
Side effects
- Nausea
- Lightheadedness / paraesthesia
- Rare bone marrow suppression with chronic exposure
Fetal & maternal notes
- • No fetal accumulation — safe every stage
Key interactions
IV opioids
CautionAdditive sedation — supervise closely
Clinical pearls
- 💡 Coach breathing pattern — many benefits are technique-dependent
- 💡 Room ventilation matters for occupational exposure
Content last reviewed: REPLACE_ME (e.g. 2026-07)
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