NeoQuest April 2025: A Fetus With an Abnormal Echocardiogram

A pregnant woman is referred to the pediatric cardiology unit due to concern for an abnormal fetal heart rhythm at 22 weeks’ gestation. Her fetal survey shows no evidence of hydrops fetalis. She undergoes a fetal echocardiogram with a 4-chamber B-mode view, which shows fetal tachycardia with normal ventricular size and function.  M-mode imaging from the echocardiogram is shown (Figure 1).¹

Figure 1: M-mode echocardiography image of the fetus in the vignette. The tracing shows tachycardia, with one ventricular contraction for each atrial contraction with a regular rhythm, suggestive of supraventricular tachycardia. From: Felimban A, Cavallé-Garrido T, and Altit G. Funny Beats in the Fetal Heart. Neoreviews. 2025;26(4): e291-296.¹

Based on the fetal echocardiogram findings, what is the most likely mechanism underlying this infant’s arrhythmia?

1. Anterograde atrioventricular conduction via the atrioventricular (AV) node with fast retrograde ventriculoatrial conduction via an accessory pathway
2. Anterograde atrioventricular conduction via the AV node with slow retrograde conduction via an accessory pathway
3. Enhanced automatism of an electric focus that overrides the sinus node
4. Macro re-entrant pathway within the atrial myocardium
5. Slow anterograde conduction down a pathway within the AV node with fast retrograde conduction via a second pathway within the AV node

Answer: A. Anterograde atrioventricular conduction via the atrioventricular (AV) node with fast retrograde ventriculoatrial conduction via an accessory pathway

Explanation

The fetal echocardiogram clip in this question shows a fetal tachycardia with normally sized ventricles and normal function (Video 1).¹ Normal fetal heart rates vary by gestational age, but a sustained heart rate of greater than 180 beats per minute is defined as fetal tachycardia.² There are many mechanisms that can result in an elevated heart rate, including sinus tachycardia and atrial or ventricular tachyarrhythmias. Most sustained fetal tachyarrhythmias are supraventricular tachycardias (SVTs).

Video 1: Four-chamber view echocardiography of the fetus in the vignette showing tachycardia with normal ventricular size and function From: Felimban A, Cavallé-Garrido T, and Altit G. Funny Beats in the Fetal Heart: Understanding Fetal Supraventricular Tachycardia. Neoreviews. 2025;26(4): e291-296.¹

A fetal echocardiogram is the preferred method for assessment of fetal tachyarrhythmias instead of an electrocardiogram (EKG), while EKG is used postnatally. Fetal EKG is highly susceptible to picking up noise and interference and therefore results in low amplitude QRS complexes, which can be inaccurate and difficult to interpret.³ Fetal echocardiography also allows for structural assessment of the heart. While most arrhythmias occur in structurally normal hearts, 5% of fetuses with SVT have a structural abnormality that can include Ebstein anomaly, AV canal defects, or hypoplastic left heart syndrome.⁴

M-mode (motion mode) echocardiography can be used to assess the fetal rhythm because the wall motion of the atria and ventricle serves as a surrogate for electrical systole.⁵ It enables comparison of atrial and ventricular myocardial contraction simultaneously.⁶  The M-mode cursor is placed across the atrium and ventricle and records their motion in a single plane, presenting any movement over time across this line as a two-dimensional static image. Fetal positioning may limit the use of this technique if there is shadowing of the cardiac structures.⁷ In this vignette, the M-mode shows a correlation between the atrial and ventricular contractions in a 1:1 ratio, which is indicative of SVT.

Pulsed-wave Doppler echocardiography is another technique used to assess AV time intervals on fetal echocardiography. The time interval between atrial and ventricular contraction represents a mechanical analog to the electrical PR interval of the EKG, which can be used to detect heart block⁷ as in Figure 2.⁸

Figure 2: Pulsed-wave Doppler fetal echocardiography of ectopic atrial tachycardia showing atrial (A) and ventricular (V) contraction. From: Pike JI, Anne Greene E. Fetal and neonatal supraventricular tachyarrhythmias. Neoreviews. 2012;13(10): 605-614.⁸

The most common mechanism of fetal SVT is anterograde atrioventricular conduction via the AV node with a fast retrograde ventriculoatrial conduction via an accessory pathway (Figure 3)⁹.  This is known as AV reentrant tachycardia (AVRT) (Option A). Seventy percent of fetal SVT cases are due to AVRT.¹⁰ AVRT typically develops between 24- and 32-weeks’ gestation. In most cases, the accessory pathway allows for the fast conduction of an ectopic beat, which results in the commencement of the tachyarrhythmia.⁷  AVRT that does not lead to fetal hydrops is observed without medical therapy, since the medications that are typically used for fetal tachyarrhythmias may have adverse effects such as a prolonged QT syndrome. These medication adverse effects may outweigh any small mortality benefit. It is important to evaluate the pregnant person for long QT syndrome prior to administering any medications for the fetus because administering flecainide and sotalol can cause the development of torsades de pointe in patients with a pre-existing long QT. If hydrops occurs, flecainide or sotalol, with or without digoxin, can be administered to the pregnant patient to attempt to slow the heart rate. The most efficacious in-utero treatment mechanism is not known, but a randomized controlled trial comparing flecainide to sotalol in combination with digoxin for fetuses with and without hydrops was recently completed, with the results not yet published.¹¹  If medical therapy is not effective, delivery in 48 to 72 hours is indicated due to the high risk of mortality associated with untreated hydrops.⁷

Figure 3: Mechanism of AVRT. The diagram shows the electrical activity passing through the AV node into the ventricle then re-entering the atria via the accessory pathway as in orthodromic AV reciprocating tachycardia (left) and antegrade conduction through the accessory pathway as in AVRT (right). From: Singh HR, Garekar S, Epstein ML, L’Ecuyer T. Neonatal supraventricular tachycardia (SVT). Neoreviews. 2005;6(7): e339–e350.⁹

In some clinical cases, there is slower conduction through the accessory pathway than through the AV node, which results in a rare form of AVRT called permanent junctional reciprocating tachycardia (PJRT)¹ (Option B). PJRT presents similarly to other types of fetal SVT and is difficult to distinguish from other forms of SVT in utero based on fetal echocardiography. This condition is difficult to control in both infants and children because there is similar conduction through the AV node and accessory pathway, making it difficult to selectively block one limb of the circuit. Fetal cases are typically refractory to transplacental treatment via medication administration to the pregnant person, and hydrops may develop in utero necessitating early delivery. Many affected patients develop end-stage cardiomyopathy in childhood and may even require cardiac transplantation¹².

A re-entrant pathway may exist within the AV node, with one fast and one slow conduction pathway (Figure 4).⁹ This causes AV node reentrant tachycardia (Option E). This is rare in infants, except after cardiac surgery¹³. The management of a fetus with AVNRT is similar to that of a fetus with AVRT, with the administration of digoxin if there is no hydrops or flecainide or sotalol with digoxin if hydrops is present.

Figure 4: Mechanism of AV nodal re-entry tachycardia. The left figure shows conduction down the fast conduction pathway in the AV node with re-entry into the slow conduction pathway in a normal infant, preventing arrhythmia. The right figure shows a re-entrant circuit through the slow pathway which causes AV node re-entrant tachycardia. From: Singh HR, Garekar S, Epstein ML, L’Ecuyer T. Neonatal supraventricular tachycardia (SVT). Neoreviews. 2005;6(7): e339–e350.⁹

Atrial ectopic tachycardia (AET) occurs due to an enhanced automatic focus within the atrium that overrides the sinoatrial node (Option D). This is rare in fetuses. The ectopic focus must generate a rhythm faster than 180 beats per minute to cause tachycardia. The tachycardia terminates with ventricular contraction. The rates are typically slower than that of AVRT, so if this rhythm is present in a fetus, it is less likely to cause hydrops.¹⁰ AET is also treated with the administration of flecainide or digoxin to the pregnant person in an attempt to decrease the fetal heart rate.¹⁴

Atrial flutter, the second most common fetal SVT⁵, is due to an intra-atrial macro reentrant circuit (Option D). It can be identified using M-mode echocardiography, which will show multiple atrial contractions for every ventricular contraction (Figure 4 and Figure 5).^5-6,8,9 The ventricular and atrial orientation in M-mode imaging depends on the probe’s orientation during the echocardiogram. The atrial rate in atrial flutter can be as high as 300-500 beats per minute, but not all beats are conducted at the AV node. This can result in 2:1, 3:1, or variable AV conduction. Atrial flutter is often persistent in a fetus after it develops and can cause hydrops fetalis.⁸ Sotalol administered to the pregnant person is currently the most effective therapy for fetal atrial flutter.¹⁰

Figure 5: M-mode echocardiography of a fetus with atrial flutter in a 2:1 ratio with an atrial rate of 400 bpm and ventricular rate of 200 bpm. From: Felimban A, Cavallé-Garrido T, and Altit G. Funny Beats in the Fetal Heart: Understanding Fetal Supraventricular Tachycardia. Neoreviews. 2025;26(4): e291-296.¹

Figure 6: M-mode echocardiography of a fetus in atrial flutter with an atrial rate of 375 bpm and a ventricular rate of 190 bpm. From: Pike JI, Anne Greene E. Fetal and neonatal supraventricular tachyarrhythmias. Neoreviews. 2012;13(10): 605-614.⁸

Did you know?
Fetal magnetocardiography is a new technique that, while not widely available, can be used to diagnose fetal rhythm disorders. It defines both repolarization and depolarization and records beat-to-beat recordings over time.¹⁵

SVT can be a presenting feature of a neurocutaneous syndrome.
To learn more, refer to the following article: Singh H, Garekar S, Epstein M, L’Ecuyer T. Neonatal Supraventricular Tachycardia (SVT). Neoreviews. 2005:6(7): e339-e350.⁹

NeoQuest April; 2025 Authors:
Andrew Beverstock, MBChB, FAAP, Baylor College of Medicine.
Allison N. J. Lyle, MD, MA, FAAP, University of Louisville,Norton Children’s Hospital.

References:

1. Felimban A, Cavallé-Garrido T, and Altit G. Funny beats in the fetal heart: understanding fetal supraventricular tachycardia. Neoreviews. 2025;26(4): e291-296
2. Zidere V, Vigneswaran T V., Syngelaki A, Charakida M, Allan LD et al.. Reference ranges for pulsed-wave doppler of the fetal cardiac inflow and outflow tracts from 13 to 36 weeks’ gestation. J A Soc Echocardiogr. 2021;34(9):1007-1016
3. Wacker-Gussmann A, Wakai RT, Strasburger JF. Complex fetal care: importance of fetal arrhythmias to the neonatologist and pediatrician. Neoreviews. 2016;17(10):e568-e578
4. Sethi N, Miller S, Hill KD. Prenatal diagnosis, management, and treatment of fetal cardiac disease. Neoreviews. 2023;24(5): e285-e295
5. Killen SAS, Fish FA. Fetal and neonatal arrhythmias. Neoreviews. 2008;9(6): e242-e252
6. Moon-Grady AJ, Donofrio MT, Gelehrter S, et al. Guidelines and recommendations for performance of the fetal echocardiogram: an update from the american society of echocardiography. J Am Soc Echocardiogr. 2023;36(7):679-723.
7. Veduta A, Panaitescu AM, Ciobanu AM, et al. Treatment of fetal arrhythmias. J Clin Med. 2021;10(11):2510
8. Pike JI, Anne Greene E. Fetal and neonatal supraventricular tachyarrhythmias. Neoreviews. 2012;13(10):605-614
9. Singh HR, Garekar S, Epstein ML, L’Ecuyer T. Neonatal supraventricular tachycardia (SVT). Neoreviews. 2005;6(7): e339–e350
10. Killen SAS, Strasburger JF. Diagnosis and management of fetal arrhythmias in the current era. J Cardiovasc Dev Dis. 2024;11(6):163.
11. Jaeggi E. Prospective randomized clinical trial of fetal atrial flutter & supraventricular tachycardia therapy (FAST RCT). Accessed Feb 3, 2025 at: https://clinicaltrials.gov/study/NCT02624765
12. Kylat RI, Samson RA. Permanent junctional reciprocating tachycardia in infants and Children. J Arrhythm. 2019;35(3):494-498:494-498
13. Baskar S, Czosek RJ. Evaluation and management of common neonatal arrhythmias. Neoreviews. 2020;21(9): e605-e615
14. Donofrio MT, Moon-Grady AJ, Hornberger LK, et al. Diagnosis and treatment of fetal cardiac disease. Circulation. 2014;129(21):2183-2242
15. Wacker-Gussmann A, Strasburger JF, Wakai RT. Contribution of fetal magnetocardiography to diagnosis, risk assessment, and treatment of fetal arrhythmia. J Am Heart Assoc. 2022;11(15):e025224