AVNRT, considered the second most common type of SVT in children, accounts for about 10% of pediatric cases. AVNRT is caused by an interaction between the two types of pathways within the AV node—one with a fast conduction time and a short refractory period, and the other with a slow conduction time and a long refractory period. AVNRT occurs when the antegrade conduction block in the fast pathway results in conduction over the slow pathway and back up the fast pathway, forming a microreentrant circuit.5
AT is the result of rapid depolarization from an automatic focus originating within the atria but outside the sinus node.3
Patient Presentation and History
The typical presentation of AVRT in children of school age includes palpitations, chest pain or tightness, dizziness, anxiety, decrease in exercise tolerance, easy fatigability, and/or shortness of breath.3 Onset is described as abrupt, while termination of SVT is described as slower because the catecholamine levels are typically elevated.4
The frequency and duration of SVT can vary greatly, from a few minutes to a few hours; it can occur as regularly as daily or as uncommonly as once or twice per year.4 Additionally, SVT symptoms can go unrecognized until a cardiac dysfunction develops. As for the patient in the case study, no apparent factor in her history was identified that may have induced SVT.
The differential diagnosis for SVT is broad, including sinus tachycardia, multifocal atrial tachycardia, and SVT with aberrancy.8 Additional considerations include stress, anxiety, hyperthyroidism, electrolyte abnormalities, and dehydration—any of which can present with a tachycardia response.4 Furthermore, clinicians are often unlikely to diagnose a child with any cardiac problem because chest pain is more commonly a presenting symptom of a pulmonary or musculoskeletal condition than a cardiac problem.3
Physical Examination
SVT can be diagnosed based on medical history and physical examination. During the physical examination, providers will assess the patient’s blood pressure and pulse, auscultate heart and lung sounds, assess the veins in the patient’s neck for different types of pulsations, and conduct cardiac maneuvers, including the Valsalva maneuver and carotid sinus massage.9,10
Laboratory Work-up and Diagnosis
Three specific tests help clinicians monitor and evaluate a patient’s conduction system. ECG is important to assess the heart rhythm both at baseline and when symptoms are occurring, if possible.3 Ambulatory ECG (ie, Holter monitoring, event recorders) record the patient’s heart rhythm on a continuous basis.
An EPS, which is performed to classify the mechanism of SVT, is conducted by inserting one or more electrocatheters into the heart by way of the femoral vein or other peripheral vessel.3 Pacing and sensing electrodes at the ends of the catheters record local intracardiac electrical activity and timing information, providing a detailed analysis of the heart’s electrical activity. The EPS is critical to determine the presence of one or more extra electrical pathways within the heart and to localize it by region.3,11 An ablation procedure may follow.
Management Options
SVT can be treated pharmacologically or nonpharmacologically. First-line pharmacologic options include certain beta-blockers (including atenolol and propranolol), digoxin, and calcium channel blockers. Second-line pharmacologic treatments include amiodarone, flecainide, and sotalol,4 all of which are contraindicated in children younger than 1 year because of these patients’ hemodynamic dependency on the heart and inability to generate stroke volume.3 Pharmacologic treatment of SVT is associated with a 68% success rate in children4 (see Table 14).
For children in whom pharmacologic treatment is ineffective, an ablation procedure may be performed. Radiofrequency catheter ablation is currently considered first-line therapy for AVRT and AVNRT.12 In this invasive procedure, intracardiac electrical mapping is performed and the initiating focus of the arrhythmia or the accessory electrical pathway that has been identified within the heart is destroyed by radiofrequency energy, delivered by electrocatheter. Ablations performed during the acute phase of SVT have a 95% success rate.3,13
Cryoablation is a relatively new treatment in which liquid nitrous oxide is used to cool the catheter to subfreezing temperatures, enabling it to destroy the myocardial tissue by freezing.3,14 The advantage of cryoablation is the option of reversible cooling, which allows the electrophysiologist to test the area first, confirming the accuracy of the apparent location accessory pathway.15
Noninvasive, nonpharmacologic interventions that increase the refractoriness of the AV node may be successful in terminating the tachyarrhythmia during episodes of SVT (see Table 23,9,13,16). They are used to terminate and diagnose tachydysrhythmias, increase parasympathetic tone, and slow conduction through the AV node.3
Patient Education
It is very important for health care providers to relieve parents’ and caregivers’ stress, anxiety, and uncertainty by educating them about the child’s cardiac condition of SVT. Information to convey include an understanding of what SVT is, what may cause it, what triggers the patient should avoid, what treatments are available and appropriate (including the maneuvers shown in Table 2), and what outcomes may be expected. An excellent patient/family education handout from the Children’s Hospitals and Clinics of Minnesota17 is available at www.childrensmn.org/Manuals/PFS/Condill/018303.pdf.