1. Fetal echocardiography reveals situs solitus of the atria, levocardia, and left aortic arch.

2. Heart rate is 228 bpm, with equal atrial and ventricular rates (Fig. 22-1), suggesting 1:1 ventriculoatrial (VA) or atrioventricular (AV) conduction with a short VA interval (86 ms).

3. Cardiac size is normal (cardiothoracic ratio = 0.33), and cardiac axis and position are normal.

4. The four-chamber view is normal, and ventricles are of equal size.

5. There is no evidence of structural congenital heart disease.

6. The aortic and ductal arches are leftward and of similar size.

7. Outflow velocities are decreased (0.4 m/s) due to the decreased filling time in tachycardia.

8. The pulmonary venous flow pattern is abnormal (large a wave reversal).

9. Tricuspid and mitral valve holosystolic regurgitation is present (Fig. 22-2), with the additional finding of ascites fluid.

10. Tei indices (myocardial performance index) were not calculated due to the rapid heart rate.

11. The umbilical artery Doppler is normal in the free loop of the umbilical cord with persistent rapid, regular rate.

12. Other venous sites near the heart had large a wave reversal (hepatic vein, ductus venosus), with tachycardia, but the umbilical vein blood velocity has no pulsations.

13. Simultaneous superior vena cava (SVC) and ascending aortic Doppler confirm 1:1 VA association, with a shorter VA than AV interval.

1. The cardiovascular profile score is 6/10.

2. Points deducted were 2 for heart function (tricuspid regurgitation and mitral regurgitation), 1 for ascites, and 1 for abnormal venous Doppler.

1. Amniocentesis is not indicated. The risk from this procedure is higher than the chance of trisomy with SVT.

2. Initial management of the fetal SVT.

a. Hospital admission was advised for monitoring the fetal heart rate and the mother’s health during administration of antiarrhythmic medications.

b. The mother was fully evaluated by adult cardiology with a baseline electrocardiogram (ECG) and echocardiogram to exclude obvious cardiac pathology that would place her at risk in administering antiarrhythmic medications. Thyroid function testing was normal.

c. Management of the fetal SVT was discussed with the mother and her partner. It was explained that in more than 80% of patients, control of the dysrhythmia is possible, but occasionally this requires more than one medication and monitoring until the mother is on a sufficient dose of the medication that controls the fetal dysrhythmia.

d. With or without rhythm control, there is a small risk of a sudden fetal event or the development of fetal hydrops (particularly without control) as well as risks of other morbidities (e.g., stroke).

3. Daily fetal echo assessments included:

a. Progression of hydrops signs (pleural effusion, ascites, increasing pericardial fluid).

b. Increasing size indicating compromise of heart function.

c. Valve function for increasing degrees of tricuspid and mitral regurgitation. The dP/dt (change in pressure per change in time) can be assessed on AV valve regurgitation jets. Normal is greater than 1000 mm Hg per second, and a poor prognosis is associated with values less than 400 mm Hg per second.

d. Semilunar valve regurgitation, which is a sign of progression in myocardial dysfunction.

e. Myocardial function, which can be measured with the Tei index in sinus rhythm after conversion.

f. Venous Doppler assessment for signs of improvement.

4. Fetal therapy.

a. Baseline maternal 12-lead ECG was performed, which showed a prolonged PR interval of 0.25 seconds with no other abnormality and normal QTc.

b. The mother was tested (hematology) for hyperthyroidism and infection. Consultation with adult cardiology was obtained, and her exam and echocardiogram were normal.

c. Electrocardiogram.

d. Drug therapy.

e. Follow-up.

1. At term, a vaginal delivery followed spontaneous labor.

2. The cord blood digoxin level was found to be just subtherapeutic.

1. On admission, the neonate was active, with no obvious malformation and normal vital signs.

2. The baby was normal and had a normal ECG with preexcitation.

3. Endocrinology consultation was obtained, and thyroid studies were performed; results were normal.

4. A 24-hour Holter monitor was normal.

5. A two-dimensional Doppler echocardiogram was normal.

6. Liver function tests were normal.

1. Risk of recurrence of unconfirmed concealed WPW is difficult to estimate.

2. WPW is known to run in families. Autosomal dominant inheritance has been reported, which results in a 50% risk of recurrence if the family carries the gene.

a. The newborn had appropriate birth weight and Apgar scores of 9 at 1 minute and 9 at 5 minutes.

b. The baby was monitored in the intensive care unit (ICU) for 24 hours, and the rhythm was normal at 130 to 150 bpm.

c. Vital signs were normal.

d. Postnatal echocardiogram confirmed a structurally normal heart. The rhythm was normal sinus rhythm.

e. He was started on digoxin 5 mg/kg PO twice daily, and feeding was started.

f. He was discharged after 2 days.

g. Cord blood digoxin level was 80% for the newborn postnatal value.

h. The ECG was normal at all visits with no sign of a delta wave.

i. The baby was followed in cardiology every 2 months with ECG and Holter monitor, and digoxin dose adjustments were made based on weight increase.

j. At 1 year of age the digoxin was stopped.

k. No tachycardia occurred after birth.

l. He was seen yearly for 5 years.

1. The actual prevalence of fetal tachyarrhythmia is not known.

a. The most common fetal tachycardia is SVT, which accounts for 90% of fetal tachycardia.

b. The second most common form of fetal tachycardia is atrial flutter and is seen in 10% to 30% of patients. Atrial pulsations in the IVC at greater than 400 bpm are a clue to the diagnosis.

c. Ventricular tachycardia in the fetus is relatively rare and accounts for less than 5% of fetal tachycardia. One clue to this diagnosis is a tachycardia rate of less than 225 bpm with signs of congestive heart failure (CHF).

2. Fetal arrhythmia can occur at any gestational age, but it is most commonly seen after 24 weeks of pregnancy. Tachycardia before 20 weeks of gestation raises the likelihood of infection or maternal drug ingestion or hyperthyroidism.

3. The risk of tachycardia is highest in fetuses with multiple blocked atrial premature beats leading to a low ventricular rate (atrial bigeminy). The vast majority of fetuses with atrial and ventricular premature beats without tachycardia have uneventful prenatal and postnatal outcome. Intermittent SVT should be monitored to assess what portion of the whole day the fetus is in SVT (Fig. 22-3).

4. Fetal tachycardia is associated with structural cardiac anomalies in 1% to 5% of cases.

a. The most commonly described associations with SVT are Ebstein’s anomaly and myocardial tumors, particularly rhabdomyoma.

b. Myocardial tumors and cardiomyopathy may be present in fetal ventricular tachycardia.

1. In fetuses with SVT who were treated in utero, the postnatal outcome is complicated by recurrence of tachyarrhythmia in about 50% of the neonates.

2. In only 10% to 20% of infants does the tachycardia persist beyond the first year of life.

3. Fetal atrial ectopic (AET) and permanent (paroxysmal) junctional reciprocal tachycardia (PJRT) often persist after birth, and long-term antiarrhythmic therapy or electrophysiologic intervention may be necessary.

4. At 1 year of age, roughly one third of fetal SVT patients remain clinically symptomatic. One third do not have clinical tachycardia, but tachycardia can be induced using esophageal stimulation. In one third, tachycardia can no longer be induced, and presumably the bypass tract no longer conducts.

1. The majority of fetal tachyarrhythmias are detected during Doppler assessment of the fetal heart beat at routine obstetrics clinic visits in the second and third trimesters of pregnancy.

2. If monitoring of the fetal heart rate by ultrasound, continuous wave Doppler, or cardiotocography reveals, at any point, fetal arrhythmia (an irregular, rapid, or slow fetal heart rate), one should consider referring the mother for further evaluation by fetal ultrasound and fetal echocardiography. For persistently high or low rates at assessment, the referral should be immediate, given the potential risk of compromise and need for accurate diagnosis to plan management and intervention where possible.

3. Polyhydramnios and fetal hydrops can lead to detection of an underlying fetal tachyarrhythmia and should be considered in the setting where there is unexplained fetal hydrops. One might even consider continuous fetal heart rate monitoring or frequent Doppler assessment every day to exclude or confirm fetal tachyarrhythmia as a primary cause.

1. The definition of an abnormally fast heart rate depends on the gestational age of the fetus.

a. The fetal heart rates that have been observed in normal pregnancy are 90 bpm at 6 weeks, increasing to 160 to 180 bpm by 9 weeks.

b. The heart rate falls to 140 bpm ± 20 bpm by 20 weeks, and again to 130 bpm ± 20 bpm near term.

c. A fetal heart rate in excess of 180 bpm is abnormal.

2. The high incidence of atrioventricular reentry tachycardia (AVRT) or, similarly, unidirectional reciprocating accessory pathway tachycardia (URAP) during fetal and neonatal life and its decrease due to the spontaneous disappearance of SVT can suggest immaturity of the myocardium. In particular, these tachycardias suggest a delayed development of the annulus fibrosus and/or prolonged persistence of accessory AV pathways.

3. Fetal atrial flutter is rare, and atrial fibrillation to date has not been reported in a fetus.

a. Atrial flutter is generated within the atria by an atrial reentrant circuit and is observed mainly during the third trimester. This supports the hypotheses of an atrial macroreentry as an underlying mechanism of fetal atrial flutter.

b. It might resolve spontaneously or progress to hydrops and death.

c. Incessant heart rates as high as 300 bpm can lead to hydrops in 2 to 3 days (Fig. 22-4) and manifest umbilical venous pulsations.

4. The pathophysiology in SVT of the human fetus includes an impeded ventricular filling due to a short, inadequate diastolic period. This can alter the ventricular filling either directly or by changes in diastolic ventricular function due to inadequate oxygen supply by the reduced myocardial flow. Both mechanisms can elevate the venous pressure, resulting in increased rates of transcapillary fluid filtration into the interstitial space and significant reduction of lymphatic flow. This inadequate reduction in lymphatic drainage results in interstitial edema and finally hydrops (Box 22-1).

1. Irregular fetal heart rhythms.

a. Irregular fetal heart rhythms are usually detected during routine auscultation of the fetal heart. Mothers most of the time are asymptomatic.

b. The auscultatory findings are usually reported as “missed beats” or “extra beats.”

c. An irregular fetal rhythm is usually due to the presence of premature atrial or ventricular beats that are intermittent. With premature atrial contractions (PACs) there may be conduction or lack of conduction to the ventricle.

d. Premature ventricular contractions (PVCs) are much less common. A premature atrial contraction may be followed by a premature ventricular contraction.

2. Tachycardia.

a. The definition of an abnormally fast heart rate depends on the gestational age of the fetus.

b. The fetal heart rates that have been observed in normal pregnancy are:

3. Bradycardia.

a. Bradycardia is defined as rhythms that are slower than normal for gestation.

b. In general, these rhythms may be intermittent, as is true for transient sinus bradycardia or sustained bradycardia (e.g., heart block) (see Chapter 23).

c. Fetal heart rate less than 110 to 120 bpm is abnormal.

1. An irregular fetal heart rate usually represents PACs or, less commonly, PVCs, which are beats that originate from the atrium or ventricle, respectively, from a site other than the normal central conduction system (e.g., sinus node).

2. PACs might or might not be conducted to the ventricle through the AV node.

a. When there is conduction through the AV node following a PAC, the pause that seems to occur between the PAC and the next sinus beat has the same time interval as two sequential sinus beats.

b. The PAC is thought to reset the pace of the sinus rhythm. Alternatively, the PAC may be blocked at the AV node as a result of the AV node’s having not fully recovered from the previous beat.

c. PACs can occur frequently, every two or three beats (atrial bigeminy or trigeminy, respectively).

d. Although there is beat-to-beat variability in sinus rhythm, it does not have such a distinct pattern as observed in atrial bigeminy or trigeminy.

3. PVCs are occasional extra systoles of ventricular origin in the fetus.

a. In premature ventricular beats, the ventricular contraction occurs before the atrial contraction by M-mode and can be easily documented.

b. There might or might not be ventriculoatrial conduction.

1. General classification.

a. In extrauterine life, narrow QRS supraventricular tachycardia (SVT) is classified according to the relation of P waves to QRS complexes.

b. During intrauterine life, such classification is not possible due to the very low voltage of P waves on the currently available transmaternal fetal ECG. Thus, we rely on flow signals and mechanical action of the atria and ventricles to provide indirect clues to the arrhythmia mechanism.

c. Tachyarrhythmias can be divided into supraventricular (short and long VA tachycardias and atrial flutter), junctional (junctional ectopic tachycardia), and ventricular types.

2. Supraventricular tachyarrhythmias.

a. Fouron and colleagues (2003) suggest an echocardiographic classification using the measurement of AV and VA time intervals (Doppler velocimetry) (Fig. 22-5). This measurement shows the relationship between the atrial (beginning of the a wave on the SVC or mitral inflow Doppler) and ventricular contractions (beginning of the aortic wave).

b. Classification.

       (a) Concealed accessory pathway.

         (i) This is the most common cause of fetal tachycardia and the most common form of SVT in infancy.

         (ii) Involves an accessory pathway along the AV groove that allows retrograde conduction from the ventricle to the atrium. There is usually a shorter VA interval than AV interval due to the very fast retrograde conduction from the ventricle through the pathway to the atrium compared to the antegrade AV node conduction that must also occur in the circuit. There is 1:1 AV and VA conduction.

         (iii) This is a type of reentrant tachycardia with sudden onset and offset, typically initiated by a PAC and without beat-to-beat variability.

         (iv) The atrial contractions occur shortly after onset of the aortic ejection wave while the tricuspid valve is still closed. As a consequence, there is often a tall retrograde a wave observed in the systemic veins close to the heart, including the ductus venosus.

         (v) Heart rate usually ranges from 220 to 280 bpm.

         (vi) The accessory pathway may be able to conduct antegrade as well. This type of pathway is referred to as WPW, which postnatally by ECG is manifested by preexcitation.

       (b) AV node reentry tachycardia (AVNRT).

         (i) This is the most common SVT of childhood but is very unusual in the fetus and neonate.

         (ii) This short VA tachyarrhythmia involves a fast and slow pathway within the AV node itself, and the VA interval is often shorter than that found in concealed pathway SVT.

         (iii) The typical rates are 160 to 220 bpm.

    (2) Long-VA tachycardia is diagnosed when the ratio of VA to AV time interval measured by Doppler and M-mode techniques is greater than 1.

       (a) Sinus tachycardia is a long VA tachyarrhythmia.

         (i) It is associated with fetal heart rates of 160 to 220 bpm in general and 1:1 AV conduction.

         (ii) Because the sinus node is an automatic rather than reentry focus, there is a period of warm-up and slow-down.

         (iii) Maternal hyperthyroidism should be considered in the etiology of sinus tachycardia in the fetus.

       (b) Other long VA tachycardias: Ectopic atrial tachycardia (EAT) and permanent junctional reciprocating tachycardia (PJRT) are two forms of long VA tachycardia in which the VA interval is longer than the AV interval.

         (i) The rates of these two rare forms of long-VA tachycardia are in general slower than the typical rates of concealed pathway SVT.

         (ii) EAT and PJRT are rare; however, differentiation of these from short-VA tachycardias is important given that they are much more difficult to treat and are in particular less likely to respond to digoxin.

    (3) Atrial flutter.

       (a) Atrial flutter is the second most common form of fetal tachycardia.

       (b) Atrial flutter involves a reentrant mechanism within the atria, with variable conduction to the ventricles through the slower AV node.

       (c) The atria rate is typically 300 to 500 bpm. Fortunately, as a result of the slower AV node, 1:1 AV conduction is extremely unusual. There is usually some degree of AV block that only allows one half or one third of the atrial signals to conduct to the ventricles. As a consequence, the ventricular rates may be one half or one third the rate of the atrial rates.

       (d) As with other forms of SVT, atrial flutter is often initiated with an atrial premature beat.

3. Junctional ectopic tachycardia.

a. In this tachyarrhythmia, the atrial contractions can occur simultaneously with the ventricular contractions.

b. There is usually a warm-up and slow-down period.

c. This is a very rare form of fetal SVT.

4. Ventricular tachycardia.

a. Prenatal ventricular tachycardia is very rare but occasionally may be seen in isolation, in association with cardiomyopathies and tumors, long QT syndrome, and in myocardial ischemia.

b. It may be a manifestation of long-QT syndrome, although this diagnosis is more often associated before birth with persistent fetal bradycardia.

c. There may or may not be 1:1 conduction in ventricular tachycardia. If there is not, which is most common, the atrial rates are slower than the ventricular rates.

Fig. 22-5 Ascending aortic and superior vena caval flow patterns for rhythm diagnosis. Here is an example of supraventricular tachycardia (SVT) with short ventriculoatrial (VA) interval. AV and VA intervals can be measured. a, atrial contraction; v, ventricular contraction.

Regular, slow ventricular ejection with atrial rate determined by atrial wall or hepatic vein sampling

SVC-Ao Doppler shows regular, slow ventricular (v) waves with faster unrelated atrial (a) wave signal.

1. M-mode and Doppler interrogation can indirectly assist in defining the mechanism of the dysrhythmia.

2. The most common cause of fetal tachyarrhythmia is SVT, usually due to URAP or concealed accessory pathway with a shorter VA interval than AV interval.

3. Atrial flutter is the second most common fetal tachyarrhythmia encountered.

4. Consider serial assessment of pregnancies with frequent fetal PACs given the risk of SVT.

5. SVT can occur with Ebstein’s anomaly and other causes of significant AV valve regurgitation or in any situation in which there is atrial enlargement and tumors.

6. In unexplained fetal heart failure or nonimmune hydrops, fetal heart rate monitoring over a prolonged period can assist in confirming this potentially treatable diagnosis.

7. Venous Doppler velocimetry should be assessed with caution in the fetus with SVT or atrial flutter because often there is significant a wave reversal even to the ductus venosus. Umbilical vein pulsations, however, may be most consistent with significantly elevated central venous pressures and impending hydrops or heart failure.

8. After cardioversion to sinus rhythm, assessment of venous Doppler indices is the best method to assess cardiac function.

1. Transplacental therapy is effective and converts SVT (even with hydrops) in more than 80% of cases.

2. Given the success of maternal–placental therapy even in the presence of hydrops, an aggressive trial of therapy for fetal SVT, particularly when the fetus is preterm, is warranted.

3. There are several mechanisms of fetal SVT. Treatment strategies should take into consideration the likelihood of success for a given mechanism and the degree of fetal compromise. Although digoxin is an easily administered medication, it might not be effective for every case and might not act rapidly enough in isolation for the grossly hydropic fetus.

4. In the treatment of fetal SVT, the mother’s health should be monitored very closely.