Supraventricular Tachycardia (SVT) is an abnormally fast heart rhythm, or arrhythmia, that originates in the upper chambers of the heart. It is the most frequently occurring serious rhythm disturbance diagnosed in infants and children. While an SVT episode can be alarming, it is highly treatable when recognized promptly by medical professionals.
Understanding the Heart’s Electrical Circuitry
The normal beating of the heart begins with an electrical impulse generated by the sinoatrial (SA) node, the natural pacemaker. This impulse travels through the atria before passing through the atrioventricular (AV) node, which slows the signal slightly. The signal then spreads rapidly to the ventricles, causing them to contract and pump blood out.
“Supraventricular” means “above the ventricles,” indicating the electrical problem starts in the atria or the AV node. The most common cause of SVT in infants is a re-entry circuit, which acts like an electrical short circuit. This occurs when an extra electrical connection, known as an accessory pathway, is present between the upper and lower chambers.
The electrical signal gets caught in a loop, bypassing the normal AV node pathway and the natural pacing system. This circular activation drives the heart rate to an extremely rapid, sustained pace, often 220 to 300 beats per minute in newborns. This excessive speed prevents the heart’s chambers from adequately filling, which reduces the amount of blood pumped out to the body.
Identifying SVT Symptoms in Infants
Since infants cannot verbalize feelings, SVT signs must be interpreted through behavioral changes and physical appearance. A sustained, abnormally fast heart rate lasting several hours can lead to poor cardiac output. The most immediate symptom is often extreme irritability or a sudden, unexplained change in the baby’s demeanor, indicating distress.
Parents may observe that their infant is breathing much faster than usual, sometimes described as panting or rapid, shallow breathing (tachypnea). The baby may show a sudden refusal to feed or be unable to complete a feeding due to fatigue. Signs of poor circulation include a pale or grayish skin tone, especially around the lips, and cool extremities.
A sustained episode can tire the heart, leading to signs of congestive heart failure, such as lethargy or unusual sleepiness. If a caregiver notices a combination of these symptoms and suspects a rapid heart rate, immediate medical attention is necessary.
Immediate Medical Interventions
Once an infant with suspected SVT is hospitalized, initial efforts focus on safely and quickly returning the heart to a normal rhythm. The first intervention involves vagal maneuvers, simple, non-invasive techniques that stimulate the vagus nerve. Stimulating this nerve temporarily slows conduction through the AV node, which can break the electrical re-entry circuit.
For infants, a common vagal maneuver is applying an ice-cold pack to the face or forehead, triggering the “diving reflex.” This reflex can cause a sudden, temporary drop in heart rate, potentially terminating the arrhythmia. If vagal maneuvers fail, the next step is administering the intravenous medication called Adenosine.
Adenosine is the standard pharmacological treatment because it acts instantaneously and has an extremely short half-life. The drug blocks electrical conduction through the AV node, stopping the circuit long enough for the heart’s natural pacemaker to reset. It must be administered as a rapid intravenous push into a large vein, immediately followed by a saline flush to ensure it reaches the heart before metabolism.
Long-Term Monitoring and Prognosis
After an SVT episode is terminated, the focus shifts to preventing future occurrences and monitoring the heart rhythm. Long-term management often involves maintenance antiarrhythmic medications, such as oral beta-blockers, to reduce the frequency and severity of future episodes. These medications slow the heart rate and increase the time it takes for the electrical signal to travel, preventing the re-entry circuit from forming.
Monitoring for recurrence is accomplished using devices like Holter monitors, which record the heart’s electrical activity continuously over 24 to 48 hours. The prognosis for infants with SVT is favorable, as the condition often resolves spontaneously as the child grows. Approximately 85% of infants who develop SVT in the first year of life will experience spontaneous resolution, often by age one or two.
For children who continue to have recurrent episodes past early childhood, other options, such as catheter ablation, may be considered later to permanently eliminate the accessory pathway. Ongoing follow-up with a pediatric cardiologist is necessary to determine when it is safe to discontinue preventative medication.