Tachycardia

During normal fetal activity, the fetal heart rate can increase to about 180bpm but if it goes over 200bpm, this is abnormal and is termed a tachycardia. Very rarely, a sinus tachycardia of around 200bpm can be due to maternal hyperthyroidism. A fast rate can be anywhere between 200 and 300bpm, although a rate of around 240 is the most common. This may be atrial or ventricular in origin, with the latter rare. Atrial flutter is less common than a re-entrant mechanism. Tachycardias are rarely associated with congenital heart disease but may occur with Ebstein’s malformation or cardiac tumours. A tachycardia can cause intrauterine cardiac failure. Treatment should usually be attempted prenatally. The drug of choice will depend on the type of arrhythmia and the presence or absence of cardiac failure. Many different drugs have been tried in the treatment of prenatal tachycardias. The majority of cases will be successfully converted prenatally with a good outcome. Most children will not require long-term anti-arrhythmic therapy and there is no underlying conduction abnormality.