Supraventricular Tachycardia (SVT) is the most common serious heart rhythm disorder experienced by infants and children. It is defined as an abnormally fast heart rate that originates in the upper chambers (atria) or the connection point between the atria and the lower chambers (ventricles). While the sudden onset of a rapid heart rate can be alarming, SVT is a highly treatable condition with a favorable outcome in babies. During an episode, the heart rate is extremely fast, often exceeding 220 beats per minute. This rapid rate prevents the heart from filling properly, which reduces the heart’s pumping efficiency if the episode lasts for a prolonged time.
The Electrical Basis of SVT
The normal heart rhythm, known as sinus rhythm, begins with an electrical impulse from the heart’s natural pacemaker, the sinus node, located in the right atrium. This signal travels across the atria and passes through the atrioventricular (AV) node before activating the ventricles to pump blood. In SVT, this organized electrical process is disrupted, causing the heart to suddenly race.
The most common cause of SVT in infants is re-entry tachycardia, which creates an electrical short circuit. This short circuit typically involves an extra electrical connection, or accessory pathway, between the atria and ventricles that bypasses the AV node. The electrical signal travels down the normal pathway and then immediately loops back up the extra pathway, creating a continuous, rapid circular movement of impulses.
This abnormal pathway is a structural feature developed while the baby was in the womb, present in approximately 90% of SVT cases in healthy newborns. Other, less frequent forms of SVT include atrial tachycardia and atrial flutter, which involve a rapid signal originating from a single, isolated spot within the atria.
Recognizing the Observable Signs
Recognizing SVT in a baby is challenging because infants cannot communicate the sensation of a racing heart or palpitations. Parents and caregivers must look for external signs suggesting acute distress or compromised heart function. The onset of an SVT episode is often abrupt, and symptoms can be intermittent, making them easy to miss.
Acute signs often include a sudden change in disposition, such as extreme irritability or unusual lethargy. The baby may appear pale or dusky (pallor or cyanosis) due to poor circulation caused by inefficient heart pumping. Rapid breathing (tachypnea) may also be observed as the body tries to compensate for reduced blood flow.
Poor feeding is a common and important sign, as the baby may tire quickly while eating or refuse to feed altogether. If an SVT episode continues for many hours or days without treatment, the infant can develop signs of congestive heart failure, including vomiting and significant shortness of breath. Because these symptoms can mimic other childhood illnesses, a very rapid, persistent heart rate should prompt immediate medical attention.
Immediate Interventions and Medical Treatments
The goal of immediate treatment for SVT is to interrupt the electrical short circuit and restore the normal sinus rhythm. For a baby who is alert and stable, medical professionals may first attempt non-invasive techniques called vagal maneuvers. These maneuvers stimulate the vagus nerve, which helps to slow the heart rate.
In infants, the most common vagal maneuver is eliciting the “diving reflex” by applying an ice-cold, wet towel or ice pack to the face for five to fifteen seconds. Parents should only attempt vagal maneuvers if they have been specifically instructed and trained by a healthcare professional. If the SVT episode does not stop on its own or with vagal maneuvers, the baby requires definitive medical treatment in a hospital setting.
The primary medication used to stop an acute SVT episode is Adenosine, administered rapidly through an intravenous line. Adenosine works by temporarily blocking electrical conduction through the AV node, which often stops the re-entry circuit and allows the heart’s natural pacemaker to take over. Because Adenosine has a very short half-life, its effect is quick but transient.
If the baby shows signs of hemodynamic instability, such as very low blood pressure or severe respiratory distress, or if Adenosine is ineffective, a more immediate intervention is required. A synchronized cardioversion may be performed, which involves delivering a controlled electrical shock to the chest to reset the heart’s electrical system. For SVT episodes resistant to initial treatments, other anti-arrhythmic medications like Procainamide or Amiodarone may be used. The choice of treatment is guided by the baby’s stability and the specific type of SVT, and it is usually managed by a pediatric cardiologist.
Long-Term Management and Recurrence
For many infants, the long-term outlook following an SVT diagnosis is very positive, as the condition often resolves spontaneously with age. Many babies, especially those with an accessory pathway, “outgrow” the condition as the heart’s electrical properties change and the pathway becomes nonfunctional, often by one year of age. For this reason, infants are often placed on daily prophylactic oral anti-arrhythmic medications to prevent recurrence until they reach that age.
The most common long-term medication is a beta-blocker, such as Propranolol, which helps keep the heart rate from climbing too high. Recurrence occurs in approximately one-third of infants, with higher risk in babies diagnosed with Wolff-Parkinson-White (WPW) syndrome. Ongoing management involves regular follow-up with a pediatric cardiologist, including periodic electrocardiograms (ECGs). In a small proportion of children who do not outgrow the condition, a procedure called catheter ablation may be considered. This procedure uses thin wires to eliminate the extra electrical pathway using heat or cold energy, offering a potential cure.