Fetal Tachycardia

Fetal tachycardia is a fetal heart rate above 160 beats per minute for more than 10 minutes (above 180 beats per minute is severe). Causes include maternal fever, chorioamnionitis, drugs such as terbutaline, maternal hyperthyroidism, fetal hypoxia, fetal anaemia and fetal arrhythmia. It matters because it may signal intrauterine infection or fetal compromise and needs a cause found quickly.

• Fetal heart rate falls normally with gestation, from about 160 beats per minute at 20 weeks to 140 beats per minute at term.
• Risk factors: prolonged membrane rupture, maternal infection, use of beta agonist drugs, thyroid disease.
• A heart rate above 200 beats per minute is likely an arrhythmia.

The woman may have fever, uterine tenderness, foul-smelling liquor, tachycardia or palpitations, and fetal heart rate above 160 beats per minute on auscultation or cardiotocography.

• Maternal temperature above 38 degrees Celsius.
• Baseline variability may be normal in fever and reduced in hypoxia.
• Differential diagnosis: maternal fever, chorioamnionitis, beta agonist drugs, maternal thyrotoxicosis, fetal hypoxia, fetal anaemia, fetal supraventricular tachycardia, fetal atrial flutter, maternal anxiety.
• Complications: fetal compromise, neonatal sepsis, fetal heart failure and hydrops from sustained arrhythmia, preterm birth, stillbirth.

The approach is to measure the heart rate, find the cause and distinguish sinus tachycardia from arrhythmia.

• Cardiotocography for rate, variability and decelerations; ultrasound M mode to define atrial and ventricular rates (rate above 200 beats per minute with 1 to 1 conduction suggests supraventricular tachycardia).
• Maternal temperature, pulse, blood pressure, uterine tenderness, vaginal examination for liquor, full blood count, C-reactive protein, high vaginal swab, urine culture.
• TSH and free T4 if maternal tachycardia; middle cerebral artery peak systolic velocity for fetal anaemia.
• Fetal echocardiography when rhythm is abnormal.

The principle is to treat the cause, with antibiotics for infection, antiarrhythmic drugs for fetal arrhythmia and delivery if the fetus is compromised.

• Supportive: left lateral position; oral and IV fluids; cooling for fever; stop beta agonist drugs; continuous monitoring; two cannulae.
• Pharmacological: paracetamol 1 g oral 6-hourly (maximum 4 g in 24 hours) for fever; Ringer's lactate 500 mL IV over 30 minutes; ampicillin 2 g IV 6-hourly plus gentamicin 5 mg/kg IV once daily for chorioamnionitis, continued until delivery and then for 24 hours or until afebrile, with metronidazole 500 mg IV 8-hourly added after caesarean section; propranolol 10 to 40 mg oral 8-hourly or propylthiouracil 100 mg oral 8-hourly for maternal thyrotoxicosis; for fetal supraventricular tachycardia without hydrops give maternal digoxin 0.5 mg oral 8-hourly for 3 doses then 0.25 mg 12-hourly, and with hydrops flecainide 100 mg oral 8-hourly (maximum 300 mg daily), both with maternal ECG and drug level monitoring; betamethasone 12 mg IM twice, 24 hours apart, between 24 and 34 weeks and 6 days if preterm delivery is likely.
• Procedures: delivery by induction or caesarean section for chorioamnionitis at any gestation after antibiotics start; intrauterine transfusion for fetal anaemia; cardioversion by fetal medicine for refractory arrhythmia; neonatal resuscitation team present.
• Special groups: maternal thyrotoxicosis needs antithyroid drug adjustment; fetal arrhythmia with hydrops needs transfer to a fetal medicine unit; neonates of mothers with chorioamnionitis receive benzylpenicillin 50 mg/kg IV 12-hourly plus gentamicin 5 mg/kg IV 24-hourly while cultures are awaited.
• Red flags: maternal fever with uterine tenderness, foul liquor, rate above 200 beats per minute, hydrops, decelerations, maternal sepsis.
• Follow-up: continuous monitoring until delivery; maternal temperature 4-hourly; newborn sepsis screen; review at 6 weeks.