oxytocin
uterotonic· Brands: Syntocinon, PitocinIMIV
Mechanism of action
Oxytocin binds to oxytocin receptors on uterine myometrial cells, increasing intracellular calcium and triggering rhythmic contractions. It also acts on myoepithelial cells in the breast to stimulate milk ejection.
Pharmacokinetics
Following IV administration, effects on uterine contraction are immediate. The drug is metabolised by peptidases in plasma and tissues, with only small amounts excreted unchanged in urine. Its plasma half-life is short, around 3 to 5 minutes, requiring continuous infusion for sustained uterine contraction.
Indications & dosing
Induction of labour
Titrate to uterine contractions; usual range 1-4 milliunits/min, adjust as needed.
- adult · IV: 1–4 milliunits/min continuous infusion · Titrate to uterine contractions; usual range 1-4 milliunits/min, adjust as needed.
Augmentation of labour
Titrate to uterine contractions; monitor fetal heart rate and uterine activity.
- adult · IV: 1–4 milliunits/min continuous infusion · Titrate to uterine contractions; monitor fetal heart rate and uterine activity.
Prevention of postpartum haemorrhage
Administer after delivery of anterior shoulder or placenta.
- adult · IM: 5 units once · Administer after delivery of anterior shoulder or placenta.
Treatment of postpartum haemorrhage
Slow IV bolus; may be repeated as needed. Follow with infusion if required.
- adult · IV: 5 units single dose · Slow IV bolus; may be repeated as needed. Follow with infusion if required.
Incomplete or missed abortion
Dilute in IV fluid; titrate to response. Monitor for uterine cramping and bleeding.
- adult · IV: 20–40 milliunits/min infusion · Dilute in IV fluid; titrate to response. Monitor for uterine cramping and bleeding.
Contraindications & precautions
- Hypersensitivity to oxytocin · Rare but contraindicates use.
- Fetal distress or non-reassuring fetal heart rate pattern · Oxytocin may worsen fetal hypoxia.
- Placenta praevia or vasa praevia · Vaginal delivery is contraindicated.
- Uterine hyperstimulation · Oxytocin should be stopped if hyperstimulation occurs.
- Cephalopelvic disproportion · Vaginal delivery likely impossible.
- Previous classical caesarean section · Risk of uterine rupture; protocol-dependent.
- Administer via an infusion pump to control the rate accurately.
- Continuously monitor fetal heart rate and uterine contraction frequency and strength.
- Avoid prolonged use or high doses to reduce the risk of water intoxication.
- Use only the intravenous route for induction or augmentation of labour.
- Ensure appropriate pain relief and anaesthesia are available if needed.
- Adjust the dose according to uterine response and fetal condition.
Adverse effects
- Uterine hypertonicity or hyperstimulationserious
- Fetal bradycardia or fetal distressserious
- Nausea and vomiting
- Hypotension (especially with rapid intravenous bolus)serious
- Water intoxication and hyponatraemiaserious
- Tachycardia or arrhythmiasserious
Monitoring, pregnancy & toxicity
- Uterine contractions · Continuous during infusion
- Fetal heart rate · Continuous during induction/augmentation
- Maternal vital signs · Regularly, more frequent during infusion
- Fluid balance · Monitor for signs of water intoxication
- Blood loss · Assess after delivery
- Uterine tone · Assess after delivery
- Pregnancy: Oxytocin is used in pregnancy for induction or augmentation of labour and the management of postpartum haemorrhage. It should be administered under close medical supervision with appropriate monitoring of uterine activity and fetal wellbeing. The lowest effective dose should be used.
- Toxicity: Uterine hyperstimulation · Stop the infusion immediately. Administer a tocolytic if necessary. Monitor for uterine rupture and fetal distress.
- Toxicity: Water intoxication (hyponatraemia) · Discontinue oxytocin, restrict fluids, and correct sodium deficit with hypertonic saline if severe.
- Toxicity: Hypotension and arrhythmias · Stop infusion, support blood pressure, and treat arrhythmias with appropriate antiarrhythmic drugs.
Clinical notes
Oxytocin is used to induce or augment labour, control postpartum haemorrhage after delivery, and facilitate milk let-down. Dosing must be individualised and closely monitored to avoid uterine hyperstimulation and fetal distress. It should be administered with caution in cases of malpresentation, previous caesarean section, or known hypersensitivity.
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BNF; local formulary