Extrasystoles account for 60–90 % of fetal arrhythmia. Ectopic beats may arise in the atria, junctional tissue, or ventricle. Supraventricular ectopy (SVE) is most common. Ventricular ectopy (VE) comprises fewer than 10 % of fetal extrasystole. Frequency of ectopic beats in the normal fetus has not been well established; however, the rate of extrasystoles in healthy premature infants is 20–30 %, with a slightly lower frequency in term infants.

Ectopic beats present as an irregular FHR. As in older patients, very early occurring SVE results in blocked atrioventricular conduction, either complete or partial (in the form of bundle branch block with aberrant depolarization). Hence, SVE can present as bradycardia. The majority of fetuses with premature beats are healthy, and the ectopy resolves over time.

Although the vast majority of fetuses with extrasystoles have a structurally normal heart, SVE and VE can be associated with anatomic congenital heart disease, cardiac tumors, and fetal genetic abnormalities such as Trisomy 18. In addition, SVE precedes supraventricular tachycardia (SVT) in a number of cases. Thus, all fetuses with frequent premature beats should have more frequent FHR monitoring until delivery, or until resolution of the ectopy has been sustained.

Premature atrial and junctional depolarizations occur most often as single beats. Diagnosis is made with a combination of Doppler and M-mode echocardiography. With M-mode, simultaneous atrial and ventricular recording is performed, demonstrating a normal sequence of A–V contractions and an early atrial contraction wave. The atrial beat after the premature contraction demonstrates an incomplete compensatory pause.

Doppler sampling during normal rhythm displays at the junction of the mitral inflow and left ventricular outflow tract an E-wave (early, rapid ventricular filling) followed by an A-wave (atrial contraction with active ventricular filling) and ventricular systole (with semilunar valve outflow). SVE will cause early active ventricular filling (A-wave), obscuring part of the early ventricular filling (E-wave) tracing with subsequent early ventricular systole (Fig. 20.4a). In cases of fully blocked SVE, no ventricular systole will follow the premature atrial contraction. The post-extrasystolic contraction will demonstrate a prolonged filling (diastolic) time interval. Doppler sampling of the SVC/ascending aorta will demonstrate an early flow reversal wave in the IVC during early atrial contraction, and lack of systolic wave in the aorta with a blocked SVE (Fig. 20.4b).

The fetal PR interval can be measured using a gated pulsed Doppler technique. Premature atrial contractions should demonstrate a normal or prolonged PR interval (if slow conduction occurs, Fig. 20.5).

The prognosis is favorable for the majority of fetuses with SVE. In addition to the conditions mentioned earlier, fetal SVE can be associated with maternal drug use or hyperthyroidism. For patients with associated maternal disease, structural congenital heart disease, tumors, or sustained tachyarrhythmias, the prognosis correlates with the associated condition. No treatment is indicated for isolated fetal SVE.

M-mode echocardiography may demonstrate subtle distortion of normal ventricular contraction due to aberrant muscle conduction (Fig. 20.6). Also, a complete atrial compensatory pause is seen after most premature ventricular depolarizations. Doppler ultrasound demonstrates a characteristic AV valve inflow pattern with decreased diastolic antegrade flow. Marked retrograde flow in the IVC is seen during atrial contraction.

In addition to conditions mentioned previously, VE has been associated with fetal myocarditis, cardiomyopathy, long QT syndrome, and complete AV block with a slow escape rate. Premature ventricular contractions also occur in healthy fetuses as well as those with structural or functional abnormalities. Prognosis is dependent on the associated abnormality if present. No treatment of isolated premature ventricular contractions is indicated.

Most cases of elevated FHR are due to sinus tachycardia. SVT and atrial flutter are less common and atrial fibrillation and ventricular tachycardia (VT) are rare. In one large single-center retrospective report, SVT and atrial flutter together accounted for 12 % of fetal arrhythmia diagnoses.

Fetal sinus rates rarely exceed 210 bpm, whereas fetal SVT rarely falls below 200 bpm. In sinus tachycardia, fetal M-mode echocardiography shows synchronous atrioventricular contractions. Sinus arrhythmia with varying cycle lengths may be present. Certain tachyarrhythmias that tend toward longer and more variable cycle lengths, such as persistent junctional reciprocating tachycardia and ectopic atrial tachycardia, are difficult to differentiate from sinus tachycardia using current ultrasound techniques.

In sinus tachycardia, Doppler tracing of atrioventricular valve inflow often demonstrates amalgamation of the E- and A-waves. The mechanical PR interval, as measured by gated Doppler ultrasound, should be constant and of normal duration.

Fetal sinus tachycardia is most often due to an underlying fetal or maternal stress such as drug exposure, hyperthyroidism, myocarditis, infection, hypoxia, or other causes of fetal distress. Treatment is directed at the primary cause of sinus tachycardia.

Fetal SVT can be sustained or intermittent. Typical rates are 240–250 bpm, with a range from 200 to 320 bpm. Detection is most common after 15 weeks gestation, although earlier presentation has been reported. Fetal tolerance of SVT depends on the duration and rate of arrhythmia; intermittent and slower (≤260 bpm) rhythms are less malignant. Mechanisms of SVT in the fetus are similar to those in neonates.

Atrioventricular reentry tachycardia, utilizing an accessory connection between the atria and ventricles, is most common when patients are examined postpartum. AV node reentry tachycardia, ectopic atrial foci, persistent junctional reciprocating tachycardias, and junctional tachycardias are less common. Although an abrupt onset and termination of the tachycardia, if observed during the fetal echocardiogram, would support the diagnosis of a reentry tachycardia, it is not possible to distinguish with certainty between these entities using current ultrasound techniques.

Diagnosis of SVT is consistent with an M-mode tracing through the atria and ventricle showing sequential 1:1 contractions at retrograde time intervals of 80–120 ms (Fig. 20.7). Similarly, Doppler ultrasound of ventricular inflow and outflow demonstrates sequential atrial and ventricular contractions. If measurable by gated Doppler, the PR interval will be constant in reentry tachyarrhythmias.

Treatment is predicated on the effect of the tachyarrhythmia on fetal well-being. If SVT is intermittent and late in pregnancy, fetal health is usually not jeopardized. In such patients, prognosis is generally excellent, and no treatment is indicated. If tachycardia is sustained at fast rates (>260 bpm), prenatal morbidity and even demise may be as high as 25 %. It is therefore important to frequently (every 3–5 days) assess all fetal patients who remain in SVT. Both obstetricians and cardiologists should be involved in the care of patients. Of note, patients with structural congenital heart disease are at greater risk for tachycardia-associated complications. One of the earliest signs of fetal compromise is an exaggerated systemic venous (inferior vena cava or hepatic) flow reversal (greater than 30 %) on the venous Doppler. This may progress to reversal in the ductus venosus, and eventual notching in the umbilical venous tracing in more advanced fetal heart failure. Other signs of fetal distress include cardiomegaly, decreased ventricular systolic function, atrioventricular valve regurgitation, and hydrops fetalis (pericardial effusion, pleural effusion, ascites, and/or skin edema).

Fetal treatment options include early delivery, transplacental (maternal) pharmacotherapy, or direct fetal pharmacotherapy. In addition, there is one report of fetal SVT conversion using trans-abdominal umbilical cord compression. Although not described in humans, there has been a single report of fetal SVT conversion using fetal transesophageal pacing in fetal sheep. Labor induction with delivery is the treatment of choice for term and near-term pregnancies with sustained fetal tachycardia or evidence of fetal compromise.

Although controversy exists, it is generally agreed upon that transplacental digoxin should be the first-line treatment of choice in pre-term pregnancies with sustained tachycardia without fetal compromise (Table 20.1). Digoxin treatment is safe and often effective. The drug can be administered to the mother in oral or IV form at relatively high maternal doses (up to 1 mg q day orally) in order to achieve an adequate level in the fetus. SVT cessation is achieved in approximately three-quarters of cases with maternal oral therapy. If conversion has not been achieved after 2 weeks of therapy with adequate maternal digoxin levels (1–2 ng/mL), a second antiarrhythmic agent may be added. Flecainide, along with other drug choices are reasonable (Table 20.1). When there are signs of fetal compromise, it is reasonable to begin dual therapy immediately (digoxin along with a second-line agent such as flecainide). Several studies in the literature as well as clinical experience have suggested that transplacental transfer of digoxin is limited in the hydropic fetus, whereas placental transfer of flecainide and sotalol are not as affected. Cardioversion with flecainide is generally achieved within 3–4 days. Other medications with reported efficacy include procainamide, verapamil, quinidine, amiodarone, or sotalol. There have been no definitive large, randomized published studies comparing fetal antiarrhythmic agents. Success with sotalol or the combination of digoxin with amiodarone has been described.