Uterine Hyperstimulation

Uterine hyperstimulation is excessive uterine activity (tachysystole or a contraction lasting more than 2 minutes) that is associated with an abnormal fetal heart rate. It results from oxytocin or prostaglandin excess, and sometimes from abruption or a sensitive uterus. It matters because it can cause fetal hypoxia, acidosis and death, and needs immediate treatment.

• Risk factors: induction with misoprostol or dinoprostone, high oxytocin rate, previous scar, multiparity, small baby.
• Misoprostol can cause delayed effects up to 4 hours after the dose.
• Prompt treatment prevents most harm.

Women have strong and frequent contractions, with severe pain and distress, and the fetal heart rate shows late or prolonged decelerations, tachycardia or reduced variability. A hypertonic uterus may not relax between contractions.

• Contractions more often than every 2 minutes or lasting more than 2 minutes.
• Fetal heart rate abnormality following contractions.
• Differential diagnosis: normal labour, placental abruption, uterine rupture, cord compression, maternal hypotension, hypertonic dysfunction.
• Complications: fetal hypoxia and acidosis, fetal death, uterine rupture, abruption, precipitate labour with tears, caesarean section, neonatal encephalopathy.

The diagnosis is by tocography with an abnormal fetal heart rate.

• Cardiotocography with tocography: contraction frequency and duration with late decelerations, prolonged decelerations or loss of variability.
• Drug history and timing of oxytocin, misoprostol or dinoprostone.
• Maternal pulse, blood pressure, uterine tone, tenderness, vaginal bleeding.
• Fetal scalp lactate where available (above 4.8 mmol/L is abnormal).

The principle is to stop the drug, resuscitate the fetus, relax the uterus and deliver quickly if the tracing does not recover.

• Supportive: stop oxytocin and remove the dinoprostone insert, wiping any misoprostol remnants; left lateral position; oxygen 10 L/minute; empty the bladder; two cannulae; senior review; continuous cardiotocography.
• Pharmacological: Ringer's lactate 500 to 1000 mL IV over 15 to 30 minutes; terbutaline 0.25 mg subcutaneous once (repeat after 15 minutes if needed, maximum 0.5 mg in 30 minutes); glyceryl trinitrate 400 micrograms sublingual as an alternative where terbutaline is unsuitable; ephedrine 5 to 10 mg IV for hypotension after epidural; restart oxytocin at half the earlier rate after the tracing recovers; oxytocin 10 IU IM after birth.
• Procedures: caesarean section for no recovery of the tracing after 30 minutes of treatment, and at once for bradycardia; assisted vaginal delivery if fully dilated; amnioinfusion with sodium chloride 0.9 percent 500 mL over 30 minutes then 3 mL per minute for variable decelerations with oligohydramnios.
• Special groups: a uterine scar needs urgent assessment for rupture; women with heart disease or hypertension avoid terbutaline and use glyceryl trinitrate 400 micrograms sublingual.
• Red flags: bradycardia below 100 beats per minute, absent variability, scar pain, bleeding, shock, thick meconium.
• Follow-up: umbilical cord blood gas at birth; newborn Apgar score and neurological examination; observe for 72 hours if depressed; review at 6 weeks.