General

Fetal Arrhythmias (SVT and AV Block): Diagnosis and Intrauterine Treatment

Posted on by Prof. Dr. İbrahim Cansaran TANIDIR

The intrauterine management of fetal arrhythmias aims to preserve the hemodynamic stability of the fetus and prevent the development of hydrops, especially in the presence of supraventricular tachycardia (SVT) and atrioventricular (AV) block. Diagnosis is made by fetal echocardiography, and the treatment plan is determined according to the type of arrhythmia, its duration, and accompanying complications.

Treatment of fetal supraventricular tachycardia (SVT) includes transplacental antiarrhythmic drug administration in cases where persistent and high ventricular rate is detected. Agents such as digoxin, flecainide, or sotalol are administered maternally to reach the fetus and provide rhythm control. During the treatment process, the cardiac rhythm of both the mother and fetus is monitored regularly.

In congenital atrioventricular (AV) block cases, the intrauterine approach varies depending on the degree of block and ventricular rate. In complete AV block, low heart rate may increase the risk of heart failure. In selected cases, maternal steroid therapy and close fetal monitoring are recommended; the risk of progression is carefully evaluated.

Management strategies in fetal arrhythmias developing hydrops fetalis focus on urgent rhythm control and support of cardiac function. In the presence of fluid accumulation and signs of heart failure, early and effective treatment significantly affects prognosis. The timing and method of delivery are planned by a multidisciplinary team.

How Does the Baby’s Heart Development and Rhythm Progress in the Womb?

The cardiovascular system of the baby in the womb begins to take shape almost miraculously during the very early stages of pregnancy. By the sixth week of gestation, it becomes possible to observe the first movements and rhythmic beats of that tiny heart thanks to modern ultrasonography devices. However, the heart simply contracting and pumping blood is not sufficient on its own. There is a flawless electrical conduction system that ensures these contractions occur in an extremely regular, coordinated, and orderly manner. You can think of the heart as a smart house with its own internal wiring; if the electrical cables are correctly installed and the switches turn on and off at the right time, the system functions smoothly.

The maturation of this electrical network and the heart becoming controllable by the mechanism known as the autonomic nervous system is a continuous and dynamic developmental process that continues until birth. In a healthy developing baby, the heart rate is generally around one hundred twenty to one hundred sixty beats per minute around the twentieth week of pregnancy. As the due date approaches, this rate gradually settles into balance and decreases to around one hundred ten to one hundred fifty beats per minute. However, certain structural differences in this complex electrical system of the heart, fluctuations in the mother’s immune system, or temporary sensitivities in the heart muscle may disrupt this perfect order.

What Is the Rhythm Disorder Known as Fetal Arrhythmia?

The concept of fetal arrhythmia refers to situations where the baby’s heartbeat deviates from the normally accepted regular speed and rhythm range. Such heart rhythm irregularities are encountered in approximately half to two percent of pregnancies. When these irregularities are detected, there is no need to panic immediately because most of them consist of completely harmless premature beats that do not harm the baby in any way. These harmless beats usually resolve spontaneously during the later months of pregnancy or shortly after birth without the need for any treatment.

However, approximately ten percent of detected rhythm disorders consist of pathological conditions that may negatively affect the baby’s overall circulation, lead to heart failure, and require close monitoring. Rhythm disorders are classified into different categories depending on where the electrical impulse originates in the heart and how quickly it spreads.

The main rhythm disorder groups are as follows:

  • Tachyarrhythmia
  • Bradyarrhythmia
  • Premature beats
  • Atrial flutter
  • Atrial fibrillation

Tachyarrhythmia refers to the heart beating faster than one hundred eighty beats per minute. The heart works so rapidly that it tries to contract before it has time to fill with blood. Bradyarrhythmia is the exact opposite; it refers to the heart rate dropping below one hundred beats per minute (or even below 80). In this situation, the heart beats so slowly that it cannot pump enough blood to meet the body’s needs. Detecting these pathological conditions at an early stage and with great precision is the most important step in restoring the delicate harmony between the mechanical contraction force of the heart and its electrical impulses.

How Does Fetal SVT (Supraventricular Tachycardia) Affect the Baby?

Supraventricular Tachycardia, abbreviated as SVT in medical terminology, is the most commonly encountered tachyarrhythmia in the womb, where the heart beats at extraordinarily high rates of two hundred twenty to nearly three hundred times per minute. This condition constitutes the vast majority of all rapid rhythm disorders detected in the womb. In SVT, an abnormal and continuously recurring electrical loop forms between the upper chambers of the heart, known as the atria, and the lower chambers, known as the ventricles, almost like a short circuit. Due to this electrical storm that does not allow the heart to rest, the heart remains in a constant state of palpitations.

If SVT attacks last only a few minutes and resolve spontaneously, the baby can generally tolerate the condition without problems. However, if this relentless state of palpitations continues uninterrupted for more than twelve hours, very serious problems begin to emerge in the baby’s body. The heart beats so rapidly that it cannot relax and rest between contractions or fill with clean blood. A heart that cannot fill completely with blood also cannot send enough blood to the body when it contracts. This inefficient cycle rapidly reduces the pumping power of the heart, raises the blood pressure in the upper chambers to dangerous levels, and ultimately causes fluid accumulation in various parts of the baby’s body, resulting in signs of heart failure. The baby begins to tire and weaken even within the protected environment of the womb.

What Causes Fetal AV Block (Atrioventricular Block) and What Are Its Characteristics?

Atrioventricular Block, or AV Block for short, refers to the partial or complete interruption of the vital electrical conduction pathway between the upper chambers of the heart, called the atria, and the lower chambers, called the ventricles, which pump blood throughout the body. It can be thought of as a broken electrical cable or a damaged signal transmission station. In the most severe form, known as complete AV block, the upper chambers continue contracting at their normal rate, but the electrical signals they generate cannot reach the lower chambers. Since the ventricles do not receive instructions, they are forced to produce their own much slower internal rhythm and begin contracting independently at very low rates such as fifty to eighty beats per minute. This severe slowing is one of the most complex and demanding rhythm disorders to treat and monitor in the womb.

There are several fundamental factors underlying the development of this electrical interruption.

The main factors causing block development are as follows:

  • Structural heart differences
  • Maternal immune system diseases
  • Viral infections
  • Genetic predispositions
  • Fibrosis of the conduction tissue

One of the most common causes is certain antibodies in the mother’s immune system, often associated with rheumatologic diseases, passing through the placenta into the baby’s circulation. These antibodies directly attack the electrical conduction points in the baby’s heart, causing inflammation and permanent damage. If this inflammatory process can be detected at an early stage before it turns into permanent and irreversible tissue damage, it may be possible to stop the progression of the damage and protect the baby’s heart with special treatments.

Which Methods Are Used for Diagnosing Rhythm Disorders in Babies?

When a suspicion of arrhythmia arises in a baby in the womb, much more than routine obstetric ultrasonography examinations are required to establish a definitive diagnosis and identify the source of the problem. At this point, detailed fetal echocardiography techniques performed with high-resolution and advanced technology devices come into play. The golden period during which the structure of the heart, its chambers, valves, and especially conduction disorders can be evaluated in the finest detail and with the greatest clarity is between the eighteenth and twenty-second weeks of pregnancy.

During the diagnostic process, simultaneously examining the movements of the heart muscle and the flow of blood is of great importance.

The special techniques used during the diagnosis stage are as follows:

  • M-mode echocardiography
  • Pulsed-Wave Doppler
  • Color Doppler imaging
  • Tissue Doppler examination

The M-mode echocardiography method displays the contraction movements of different walls of the heart, the atria, and the ventricles with millisecond precision on a flowing timeline on the screen. In this way, it becomes possible to see exactly how long after the upper chamber contracts the lower chamber contracts and to detect the harmony or interruption of communication between them. Another technique called Pulsed-Wave Doppler uses sound waves to graphically measure the speed and direction of blood flow inside the heart and vessels. By carefully examining the timing of blood flow entering and leaving the heart under high pressure, it is possible to determine with complete accuracy where the electrical impulse first originates and which pathways it follows within the heart.

Why Is the Fetal Cardiovascular Profile Score (CVPS) Important?

A very special scoring system known as the Cardiovascular Profile Score is used to objectively and measurably evaluate the general health condition of a baby diagnosed with a rhythm disorder, its well-being in the womb, and whether the current arrhythmia is causing heart failure. This system is like a comprehensive report card evaluating the baby’s cardiac and circulatory health over ten points. During regular follow-ups, a decrease in this score below ten points is the clearest and earliest indicator that fatigue caused by the arrhythmia has begun to create losses in the baby’s organ functions.

The main body regions and functions evaluated in the scoring are as follows:

  • Subcutaneous tissue
  • Fluid status in the abdominal cavity
  • Heart size
  • Heart contraction strength
  • Umbilical cord blood flow

During this examination, doctors check whether there is abnormal fluid accumulation in the baby’s abdominal cavity, around the lungs, or in the membrane surrounding the heart. In addition, the speed and direction of blood flow in the arteries and veins of the umbilical cord are examined because when the heart begins to tire, blood flow in these vessels is also disrupted and pressure changes occur. Findings such as enlargement of the area occupied by the heart within the chest cavity and backward leakage of blood through the heart valves also cause point reductions in this scoring system, signaling to physicians that the time for urgent intervention has arrived.

Which Methods Are Applied in the Womb for SVT Treatment?

Since directly performing a physical intervention on the baby in the womb is extremely risky and technically very difficult, the primary strategy accepted worldwide and yielding the most successful results in the treatment of Supraventricular Tachycardia is transplacental therapy. This method is highly ingenious; the necessary heart medication is administered to the mother in appropriate doses. When the mother takes the medication orally or intravenously, the drug enters the mother’s circulation, reaches the placenta, known as the baby’s partner, and ultimately passes into the baby’s blood through the umbilical cord, treating the rhythm disorder inside that tiny heart from within.

The main medications used in SVT treatment are as follows:

  • Digoxin
  • Sotalol
  • Flecainide
  • Amiodarone

If fluid accumulation due to heart failure has not yet begun in the baby, the first treatment choice is generally a well-established and reliable medication called digoxin. This drug aims to break the dangerous palpitation cycle by slowing electrical conduction at the transition point between the upper and lower chambers of the heart. When treatment begins, the mother is given an intensive loading dose within the first twenty-four hours. Afterwards, the medication level in the mother’s blood is frequently measured with laboratory tests to determine the most appropriate maintenance dose that will be sufficient for the baby without harming the mother. However, if fluid accumulation has begun in the baby, the placenta also absorbs water and swells like a sponge, reducing the transfer rate of the medication from the mother to the baby by half. In these resistant situations, adding second-line medications such as sotalol or flecainide, which directly alter the electrical properties of the heart and are much stronger, becomes mandatory.

Which Methods Are Used in the Womb for AV Block Treatment?

The intrauterine management of Atrioventricular Block requires a much more difficult and delicate balance than palpitation treatment. The main reason for this is that it is often biologically impossible to completely restore damaged, inflamed, or fibrotic conduction tissue to its former healthy state. Especially in cases caused by destructive antibodies originating from the mother’s immune system, the primary and most vital aim of treatment is to suppress the current inflammation in the heart very rapidly and stop the existing block from progressing to the irreversible stage of complete heart block at all costs.

The medications administered to the mother in AV block management are as follows:

  • Dexamethasone
  • Terbutaline
  • Isoproterenol
  • Salbutamol

To stop and reduce this relentless damage caused by antibodies, special corticosteroid-derived medications that pass very effectively through the placenta are started in the mother. This treatment may calm the edema in the heart and the cellular-level inflammation. On the other hand, when the contraction rate of the baby’s lower heart chambers falls below fifty beats per minute, the amount of blood reaching vital organs such as the brain and kidneys decreases to dangerously low levels. To prevent this, support the heart’s contraction strength, and artificially increase the rhythm slightly, the mother may be given special agents with heart rate-increasing properties similar to asthma medications. During this process, both the mother and the baby are kept under extremely close follow-up with weekly or sometimes daily echocardiographic examinations.

What Is Fetal Hydrops and Why Is It Dangerous?

Fetal hydrops is the most feared complication of heart rhythm disorders and represents a critical condition indicating that the severity of the situation has reached its highest level. It is the condition in which excessive abnormal fluid accumulates in multiple cavities and tissue spaces of the baby’s tiny body where fluid should not normally be present. The development of hydrops due to fetal arrhythmias is the clearest and most painful proof that the heart can no longer tolerate the excessively fast or excessively slow rhythm, has become completely exhausted, and has entered heart failure due to collapse of its pumping function.

The main anatomical areas where fluid accumulation is observed are as follows:

  • Pleural membrane
  • Pericardial membrane
  • Abdominal cavity
  • Subcutaneous tissue
  • Intestinal surfaces

When this widespread edema develops in cases of SVT or AV Block, the condition enters a vicious cycle. This accumulated fluid acts like a physical barrier preventing the life-saving medications received from the mother from reaching the baby’s circulation and the target organ, the heart. In severe hydrops cases that remain untreated or fail to respond adequately and promptly to medications, the risk of fetal loss unfortunately increases significantly. Therefore, detecting the heart problem during the silent phase before fluid accumulation begins is the most important and only factor determining treatment success.

How Should the Delivery Process Be Planned in Babies with Fetal Arrhythmia?

In pregnancies closely monitored due to a diagnosis of fetal arrhythmia, deciding when, at which gestational week, and by which method the delivery should be performed is like walking on a fine line. This critical decision is determined according to the extremely delicate and vital balance between the success rate of the medication treatment administered in the womb and the risks of organ developmental delay and prematurity associated with the baby being born too early.

The main factors considered in delivery planning are as follows:

  • Current gestational week
  • Fluid accumulation status
  • Response to medication treatment
  • Baby’s developmental level
  • Quality of heart contractions

If the baby’s palpitations or rhythm disorder can be controlled with medications administered to the mother with great precision and there are no signs of fluid accumulation in the baby, the safest approach is to allow the baby to continue growing in the womb. In such successful cases, delivery can generally be postponed until after the thirty-seventh week, close to full term. However, if the palpitations cannot be stopped despite the strongest medication combinations, the baby’s heart muscle continues to weaken, and the amount of fluid in the body increases day by day, then intrauterine treatment is terminated to save the baby’s life. To help the baby’s lungs adapt to life outside the womb, lung-maturing injections are administered to the mother, and a planned, controlled, and urgent preterm delivery operation is performed.

What Awaits a Baby with a Rhythm Disorder in the Postnatal Period?

When babies who have been followed with great excitement and stress for nine months due to a diagnosis of rhythm disorder finally open their eyes to the world, the closed-box stage of treatment inside the womb ends, and the new challenges of the postnatal period begin. At this stage, a highly equipped team consisting of pediatric cardiology and neonatal intensive care specialists directly takes over the baby’s medical management. It should not be forgotten that everything does not end when the baby is born; on the contrary, a new monitoring process begins.

The main procedures planned for the baby after birth are as follows:

  • Continuous rhythm monitoring
  • Detailed echocardiography
  • Medication dose adjustment
  • Blood test analyses

Babies with a history of severe palpitations known as SVT in the womb have a high risk of recurrence of these palpitations, especially during the first few days of life, even if treatment was highly successful and they are born with a completely normal rhythm. Therefore, their heartbeats are monitored second by second with special monitors in the neonatal intensive care unit, the condition of the heart is evaluated with detailed ultrasound examinations, and the protective medication treatments started in the womb are usually continued orally in syrup form until around one year of age.

On the other hand, the situation is somewhat different for babies who developed complete AV block in the womb and whose hearts beat very slowly. If the baby’s heart rate remains too low to sustain life after birth and signs related to circulatory insufficiency such as paleness, weakness, and inability to feed appear, providing continuous and regular heart rate support from the outside becomes mandatory. These tiny fighters are usually fitted with pacemakers specially designed for their small bodies within the first days or sometimes even the first hours of life. Thanks to these pacemakers, implanted by expert hands at the right time and maintained regularly, these babies can live completely healthy, active, and long lives with no difference from their peers.

Güncellenme Tarihi: 13.10.2026

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