Oxytocin-Induced Hyperstimulation

Oxytocin induced hyperstimulation is excessive uterine activity, with more than 5 contractions in 10 minutes or contractions lasting more than 2 minutes, caused by an oxytocin infusion, usually with fetal heart rate changes. It results from too high a dose, too rapid an increase, a sensitive uterus or a calculation error. It matters because it can cause fetal hypoxia, abruption and uterine rupture, and because oxytocin has a short half life, so stopping it usually relieves the problem within 10 to 15 minutes.

• Risk factors: high infusion rate, rapid dose increase, previous scar, grand multiparity, small baby, concurrent prostaglandin use.
• Oxytocin half life is 3 to 5 minutes.
• Infusion pumps and a written regimen prevent most errors.

Women have strong, frequent painful contractions with little rest, and the fetal heart rate shows late or prolonged decelerations, tachycardia or reduced variability. A hypertonic uterus may not relax.

• Contractions every 1 to 2 minutes.
• Fetal heart rate changes during or after contractions.
• Differential diagnosis: normal strong labour, abruption, uterine rupture, hypertonic dysfunction, cord compression, maternal hypotension.
• Complications: fetal hypoxia and acidosis, fetal death, abruption, uterine rupture, precipitate labour, tears, postpartum haemorrhage from atony, water intoxication with high dose and hypotonic fluid.

The diagnosis is made by tocography and the infusion record.

• Cardiotocography with tocography: more than 5 contractions in 10 minutes over 30 minutes, or a contraction lasting more than 2 minutes, with fetal heart rate changes.
• Check infusion pump rate and the oxytocin dilution (10 IU in 1 litre equals 10 mU/mL).
• Maternal pulse, blood pressure, uterine tone, tenderness, vaginal bleeding; scar assessment.
• Fetal scalp lactate where available (above 4.8 mmol/L is abnormal).

The principle is to stop the oxytocin at once, resuscitate the fetus, and restart at a lower dose only when the tracing is normal.

• Supportive: stop the infusion; left lateral position; oxygen 10 L/minute; empty the bladder; two cannulae; senior review; continuous cardiotocography; reassess after 10 to 15 minutes.
• Pharmacological: Ringer's lactate 500 to 1000 mL IV over 15 to 30 minutes (isotonic fluid only); terbutaline 0.25 mg subcutaneous once if the tracing remains abnormal after 15 minutes; ephedrine 5 to 10 mg IV for hypotension after epidural; restart oxytocin at half the earlier rate after 30 minutes if contractions are 5 or fewer in 10 minutes and the tracing is normal, increasing every 30 minutes to a maximum of 192 mL per hour (32 mU/minute); oxytocin 10 IU IM after birth.
• Procedures: caesarean section if the tracing stays pathological after 30 minutes of treatment, and at once for bradycardia, abruption or scar pain; 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 immediate assessment for rupture and a lower oxytocin ceiling of 120 mL per hour (20 mU/minute); grand multiparas use lower starting rates and slower increases.
• Red flags: fetal bradycardia, absent variability, scar pain, bleeding, shock, headache or vomiting (water intoxication).
• Follow-up: umbilical cord blood gas at birth; newborn Apgar score and examination; observe for 24 hours; check for postpartum atony and bleeding.