Abstract
Fetal arrhythmias are defined as any abnormality in heart rhythm, including heart rates less than 100 beats/min (bradyarrhythmia) or greater than 180 beats/min (tachyarrhythmia) or any irregularity (mainly ectopic beats). Ectopic beats are the most frequent fetal arrhythmia, and usually the prognosis is excellent. Supraventricular trachycardia may associate hydrops and can be treated with maternal oral administration of digoxin or flecainide. Complete atrioventricular block commonly associates cardiac structural anomalies, and postnatal pacing is needed in most cases.
Keywords
bradyarrhythmia, tachyarrhythmia, ectopic beats, supraventricular trachycardia, complete atrioventricular block
Introduction
Arrhythmia is defined as any abnormality in normal heart rhythm. Fetal arrhythmias include regular heart rates less than 100 beats/min or greater than 180 beats/min, or any irregularity in fetal heart rate in the absence of uterine contractions. Fetal arrhythmias occur in 2% of pregnancies, and most are benign, with no requirement for in utero treatment. At least 90% of arrhythmias are caused by irregular rhythms (ectopic beats); 8% by tachyarrhythmias (heart rate >180 beats/min); and 2% by bradyarrhythmias (heart rate <100 beats/min). Tachyarrhythmia most commonly relates to supraventricular reentrant tachycardia, atrial flutter, and sinus tachycardia. Enduring bradycardia may result from sinus node dysfunction, complete heart block or nonconducted atrial bigeminy. In this chapter, the most common arrhythmias during intrauterine life are reviewed: (1) atrial ectopic beats, (2) supraventricular tachycardia, and (3) complete atrioventricular block.
Atrial Ectopic Beats
Definition
Atrial ectopic beats refer to an irregular rhythm also known as extrasystoles.
Prevalence and Epidemiology
Atrial ectopic beats (extrasystoles) typically appear in the third trimester and account for 85% to 90% of fetal arrhythmias, affecting 1% to 2% of all pregnancies.
Etiology and Pathophysiology
Although their etiology is unknown, it is hypothesized that atrial ectopic beats appear after a spontaneous atrial depolarization owing to immaturity or instability of the fetal conduction system. Most cases are isolated; however, 1% to 2% of cases can be caused by an underlying structural heart disease or cardiac tumor. A complete echocardiographic assessment is warranted.
The prognosis is benign, and most cases are self-limited without treatment. Usually, atrial ectopic beats are well tolerated by the fetus, and the fetus remains stable hemodynamically. However, 2% to 5% can progress to tachycardia or bradycardia. For this reason, follow-up of patients with heart rate auscultation every 1 to 2 weeks is recommended to exclude any runs of tachycardia (>200 beats/min).
Manifestations of Disease
Clinical Presentation
Atrial ectopic beats are found by auscultation of a fetal irregular rhythm or observation during an ultrasound (US) scan performed for other reasons.
Imaging Technique and Findings
Ultrasound.
The diagnosis is usually easily made with US because of the typical irregularity of the rhythm. Premature atrial wall movements by M-mode or atrial beats by pulsed Doppler are observed. The space between the ectopic beat and the previous one is shorter than the space between two previous beats. The ectopic beat may be followed by a ventricular beat, and typically the next atrial beat is delayed because of a compensatory pause ( Fig. 95.1 ). Ectopic beats can be coupled to sinus beats leading to bigeminy (1 sinus beat for 1 ectopic beat), trigeminy (2 sinus beats for 1 ectopic beat), and so on ( Fig. 95.2 ).
Irregular rhythm with premature atrial beat followed by a compensating pause
Synopsis of Treatment Options
Prenatal
No treatment is required because most cases are well tolerated and self-limited. However, reduced caffeine intake and smoking withdrawal is empirically recommended by some groups. Perinatal management does not need to be changed, and a vaginal delivery in a nontertiary center can be offered.
Postnatal
Usually, no treatment is required.
Most cases are benign and self-limited; however, a complete echocardiographic assessment and follow-up are required. Follow-up may be through auscultation at routine prenatal visits for any evidence of tachycardia.
Supraventricular Tachycardia
Definition
Supraventricular tachycardia is atrial tachycardia (usually 220 to 260 beats/min) with 1 : 1 atrioventricular conduction.
Prevalence and Epidemiology
Supraventricular tachycardia accounts for 1% to 5% of fetal arrhythmias. Despite being the most common fetal tachyarrhythmia, it is infrequent, affecting 1 in 10,000 pregnancies.
Etiology and Pathophysiology
The mechanism is an atrioventricular reentry, usually based on retrograde atrial activation across a fast-conducting accessory pathway, although antegrade conduction can also occur. Structural heart disease or tumors underlie 5% to 10% of cases. Some cases are secondary to frequent atrial ectopic beats explaining an intermittent pattern of tachycardia, with tachycardia beginning and ending suddenly ( Fig. 95.3 ).
Hydrops can occur in 30% to 50% of cases, typically associated with heart rates greater than 210 to 220 beats/min. A maintained tachycardia can lead to dilated myocardiopathy with valvular insufficiency, decrease of ventricular filling time and cardiac output, congestive heart failure, and finally hydrops. The presence of hydrops is a marker of worse prognosis owing to increased mortality (>30%) or later neurologic impairment.
Manifestations of Disease
Clinical Presentation
Clinical presentation consists of auscultation of maintained fetal heart rate at 220 to 260 beats/min, with or without fetal hydrops.
Imaging Technique and Findings
Ultrasound.
Supraventricular tachycardia is characterized by a fetal heart rate greater than 180 beats/min (usually 220 to 260 beats/min) with 1 : 1 atrioventricular conduction. Atrial rhythm and ventricular rhythm should be almost equal ( 
and 
). Atrial and ventricular rates can be evaluated simultaneously by pulsed Doppler or M-mode ( Figs. 95.4 and 95.5 ).
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