Fetal tachycardia is an abnormal increase in the fetal heart rate. It is variably defined as a heart rate above 160-180 beats per minute (bpm) and typically ranges between 170-220 bpm (higher rates can occur with tachyarrhythmias).

Epidemiology

The estimated prevalence is ~0.4-1% of pregnancies ^3,7.

Pathology

In the majority of cases, the abnormal electrical impulses originate from the atria. A fetal tachycardia can range from simple sinus tachycardia to various fetal tachyarrhythmias. In sinus tachycardia, there is a 1:1 conduction from the atria through to the ventricles.

Associations

A fetal tachycardia can be associated with many maternal, as well as fetal conditions, which include:

• maternal

  • maternal hyperthyroidism

  • maternal medications

  • maternal tachycardia (e.g. systemic infection)

• fetal

  • in utero infection

  • in utero hypoxia

  • fetal anemia

  • chromosomal anomalies

    • trisomy 13

    • Turner syndrome

Radiographic features

Ultrasound

Fetal echocardiography

An M-mode Doppler study is best for assessment of heart rate. It is recommended that the sampling line intercepts both the atrial and ventricular walls, thereby allowing simultaneous assessment of both ventricular and atrial contractility.

Ancillary features

Ultrasound may also show evidence of associated complications, such as signs of hydrops fetalis.

Treatment and prognosis

The long-term prognosis for most fetuses diagnosed with sinus tachycardia is generally good, with the abnormal rhythm resolving spontaneously during the first year of life in the majority of cases ⁵. Treatment options (if required) include transplacental administration of antiarrhythmic drugs.

History and etymology

It was first recognized by A S Hyman in 1930 ².

Differential diagnosis

Considerations include:

• fetal premature atrial contraction(s): transient and not sustained

• fetal premature ventricular contraction(s): transient and not sustained