Funny Beats in the Fetal Heart: Understanding Fetal Supraventricular Tachycardia

At 20 weeks’ gestation, an obstetrician performs a routine fetal ultrasonogram that shows normal intracardiac anatomy and function but raises concerns about an abnormal fetal heart rhythm. The rest of the fetal ultrasonogram survey reveals no abnormalities, with no signs of pericardial effusion, pleural effusion, ascites, or skin edema, and a normal quantity of amniotic fluid. At 22 weeks’ gestation, the pregnant patient is referred to fetal cardiology, where a fetal echocardiogram is performed. (Video 1 and Figures 1 and 2A).Question 1: What is the most common fetal tachyarrhythmia?Atrial flutterComplete heart blockSinus tachycardiaSupraventricular tachycardia (SVT)Ventricular tachycardia (VT)Question 2: Which congenital heart disease should be excluded in a fetus with SVT?Atrioventricular septal defectCoarctation of the aortaTransposition of the great arteriesEbstein anomalyVentricular septal defectThe fetal echocardiogram clip from the 4-chamber view of the patient in this scenario demonstrates fetal tachycardia with normal-sized ventricles and normal ventricular function (Video 1; still image annotated in Figure 1). M-mode is performed to correlate the atrial and ventricular contractions (Figure 2A) demonstrating 1:1 conduction, where each atrial contraction is followed by a ventricular contraction.Fetal tachyarrhythmias manifest with an abnormally high fetal heart rate. Although rare, at the most severe end of the spectrum, these abnormal rhythms can cause fetal heart failure, which can lead to intrauterine fetal demise.1 Typically, fetal tachyarrhythmias occur in an anatomically normal heart. However, about 5% of fetuses with these arrhythmias may also have a congenital heart defect (CHD) such as Ebstein anomaly (the most common CHD associated with fetal SVT), intracardiac tumors, or other CHD.2 Up to 30% of patients with Ebstein anomaly have one or more accessory pathways, which put them at risk of SVT independently of the presence of right atrial dilation.There are several types of fetal tachyarrhythmias. Therefore, it is critical to make the correct diagnosis to guide the management plan and provide appropriate counseling. It is also important to rule out sinus tachycardia, which can be secondary to fetal distress, anemia, infection, thyrotoxicosis, or the use of beta-stimulant drugs in the pregnant person.Fetal tachyarrhythmias may originate in the ventricles (ventricular tachycardia) or above the ventricles (supraventricular arrhythmias). The latter is by far the most common. They are classified according to the arrhythmia mechanism as reentrant or automatic. The reentrant circuit may occur between the atria and the ventricles via an accessory pathway (atrioventricular [AV] reentry tachycardia or AVRT) or may be confined to the atria, causing atrial flutter (intra-atrial macro reentrant circuit). Atrial ectopic tachycardia (AET) is triggered by enhanced automatism of an electric atrial focus that beats faster than and overrides the sinus node. In AVRT, the AV reentrant circuit usually conducts from the atria to the ventricles via the AV node (antegrade AV conduction) and from the ventricles to the atria via the accessory pathway (retrograde ventriculoatrial [VA] conduction). Most accessory pathways conduct electricity much faster than the AV node. This means that, in tachycardia, the electricity travels faster from the ventricles to the atria than from the atria to the ventricles. There is a rare type of AV reentrant arrhythmia known as permanent junctional reciprocating tachycardia (PJRT) caused by slow retrograde accessory pathway conduction, where the antegrade conduction is faster than the retrograde. These tend to be incessant and difficult to control. SVT is a generic term that encompasses AVRT, PJRT, and AET but does not include atrial flutter. SVT is the most common fetal tachyarrhythmia, accounting for 66% to 90% of cases3; of those, 90% are AVRT. The second most common cause is atrial flutter (Figure 2B), representing 10% to 30% of all tachyarrhythmias.1The following parameters are helpful to make a concrete arrhythmia diagnosis: (1) heart rate, (2) beat-to-beat variability, (3) relationship between the atrial and ventricular beats, and (4) time between atrial and ventricular conduction (AV interval) versus time between ventricular and atrial contraction (VA interval) (Figure 2).Sinus tachycardia occurs at lower rates than the supraventricular arrhythmias. In AV reentry, there is no significant beat-to-beat variability, whereas it is present in sinus tachycardia and AET. The latter is characterized by a distinct “warm-up” phenomenon, where at the beginning of a run of tachycardia, the heart rate gradually increases. In sinus tachycardia and AV reentry, there is a 1:1 AV relationship, meaning that there is 1 ventricular beat for each atrial beat. In atrial flutter with atrial rates as high as 350 to 500 beats per minute (bpm), and sometimes in AET, some of the atrial beats are blocked at the AV node, resulting in 2:1, 3:1, or variable AV conduction. Finally, in AVRT, the VA is shorter than the AV interval, whereas in sinus tachycardia, PJRT, and AET the opposite is true.Since fetal electrocardiograms are not available in routine clinical practice, cardiologists use fetal echocardiography (FE) instead. For diagnosis, FE analyzes mechanical events, such as atrial contraction, ventricular contraction, and intracardiac flows (Figures 3 and 4), as surrogates of the electrical events that cause the arrhythmia. In addition, FE provides an assessment of ventricular function and other indicators of fetal heart failure—namely, the presence of pericardial or pleural effusions, ascites, or hydrops.The first step for the evaluation of the fetal heart rhythm involves analyzing the sequence of mechanical events during the atrial and ventricular contraction by M-mode. M-mode (“motion-mode”) imaging records the motions of the atrial and ventricular walls across the ultrasonographic line of interrogation (y-axis) and time (x-axis) (Figure 2). Additionally, Doppler spectral displays of the aortic and pulmonary flow patterns are used to determine the ventricular heart rate (Figure 3). The sequence and timing of blood flow events resulting from atrial and ventricular contractions are analyzed using simultaneous pulse wave Doppler evaluation of the mitral inflow and aortic outflow (mitral/aorta) or of the superior vena cava (SVC) and the ascending aorta (SVC/aorta Doppler).4 The time between the onset of the mitral “A wave” or the retrograde SVC “a wave,” both representing atrial contraction, and the onset of aortic forward flow is referred to as the AV time (Figure 4). The period from the onset of aortic forward flow to the onset of the mitral “A wave” or the SVC “a wave” is known as the ventriculoatrial (VA) time.5 The AV interval is the mechanical surrogate of the PR interval in an electrocardiogram. A diagnosis of a regular atrial and ventricular rhythm with a normal rate for gestational age is necessary to recognize a normal fetal heart rhythm. Appropriate 1-to-1 AV conduction is confirmed when each atrial systole is followed by a ventricular systole within the appropriate AV time interval as per established normal values for gestational age.4In fetal AVRT, there is a 1-to-1 AV conduction and a short VA time interval. The rate is usually above 250 bpm, regular, and shows minimal variability between beats (Video 1 and Figures 3 and 4).2 Atrial flutter diagnosis is established when there is a consistently rapid atrial rate (usually around 400 bpm), with variable AV conduction, usually 2:1 or 3:1 block, or occasionally showing intermittent 1-to-1 conduction.Although most tachyarrhythmias are intermittent, persistent tachyarrhythmias can result in fetal heart failure or nonimmune hydrops fetalis.1,2,6 Hydrops occurs usually at higher ventricular rates and when the tachyarrhythmia persists for long periods of time. Initial echocardiographic signs of hemodynamic compromise include enlargement of both atria and AV valve insufficiency. Later indicators are global cardiomegaly, reduced ventricular systolic function, and the development of effusions. The prognosis for hydrops related to fetal tachyarrhythmia is of concern, with high mortality rates. In contrast, the mortality rate in cases without significant heart failure is between 0% and 4%.2 The overall mortality rate from atrial flutter is 8%, but it can increase to as high as 30% in hydropic fetuses.6Not all fetuses with supraventricular arrhythmia require pharmacological treatment. If the tachycardia is intermittent and there are no signs of hemodynamic compromise, close monitoring of the fetus may be sufficient. If active treatment is required, drug therapy for the pregnant person or fetal delivery can be used, depending on gestational age, factors in the fetus and pregnant person, and institutional practices. Transplacental delivery of antiarrhythmic drugs is the cornerstone of treatment for fetal arrhythmias. Although multiple antiarrhythmic drugs may be required, effective conversion to sinus rhythm or sufficient rate control is typically achievable, often with a single drug.1 The goal of prenatal medical treatment is to avoid adverse outcomes such as fetal hydrops, fetal demise, premature delivery, the need for high-risk surgical delivery, drug toxicity, and pro-arrhythmic side effects. Antiarrhythmic medications should be administered based on a multidisciplinary approach that includes fetal cardiology, maternal-fetal medicine, neonatology, and pharmacy. The therapeutic approach must be tailored to the most likely underlying diagnosis of the abnormal fetal rhythm, primarily determined by fetal echocardiography. The expectant parent should undergo a cardiac assessment and surveillance by an adult cardiologist to ensure safety prior to commencing drug therapy. Flecainide and digoxin, as well as sotalol, are often used as first-line therapies. For refractory fetal tachycardias or hydropic fetuses, which carry higher mortality, combination treatments are considered. The Fetal Atrial Flutter and Supraventricular Tachycardia (FAST) Therapy Trial (ClinicalTrials.gov: NCT02624765), conducted by Dr Edgar Jaeggi and collaborators, will soon shed more light on the optimal pharmacological treatment of fetal tachyarrhythmias. Counseling should cover potential side effects in the pregnant person, and a plan for ongoing monitoring of the pregnancy and fetus should be established. This plan should include regular follow-ups with maternal-fetal medicine and fetal cardiology for therapy titration and decisions regarding the timing of delivery based on fetal well-being. Fetuses in sinus rhythm may be delivered by spontaneous vaginal delivery. Fetuses with persistent tachyarrhythmia, cardiac dysfunction, hydrops, or fetal distress are delivered by Caesarean section in a tertiary care center with a neonatal intensive care unit and pediatric cardiology services, as medical or electrical therapies may be required as early as in the delivery room. Delivery of stable fetuses converted to sinus rhythm should take place close to or in an institution with pediatric cardiology services, as the newborn will need to be admitted to such an institution to monitor for recurrence of SVT and further management immediately after birth.Infants who have experienced fetal SVT remain susceptible to a recurrence of SVT in the postnatal period, even with appropriate medical care during pregnancy. However, parents and caregivers can take comfort in knowing that recurrence after 2 weeks of age is rare, and if there is no recurrence within 48 hours of postnatal surveillance, it is less likely to occur later.1 A baseline postnatal 12-lead electrocardiogram should be obtained for all these newborns, and follow-up care should be coordinated with cardiology. Additionally, a 48-hour postnatal observation period provides an opportunity for families to learn how to recognize SVT and to seek immediate medical evaluation if a recurrence occurs, thereby reducing the risk of tachycardia-induced cardiomyopathy.1 Fetal atrial flutter rarely recurs after birth.Question 1: D. Supraventricular Tachycardia (SVT)Question 2: D Ebstein’s anomalyAmerican Board of Pediatrics Neonatal-Perinatal Content SpecificationDifferentiate normal from common abnormal electrocardiographic patterns and rhythms in the fetus and newborn infant.Know the physiologic consequences of an arrhythmia in a fetus or newborn infant.Know appropriate management of common arrhythmias in the fetus and newborn infant and understand the potential complications or adverse effects of approaches and drugs used.

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