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Age range (yrs)

Fig. 1. Age at clinical presentation of SVT in children. (Data adapted from Perry and Garson [2].)

school age. Age at clinical presentation of SVT in children has a bimodal distribution, with the first clinically apparent event usually occurring during early childhood, or even detected obstetrically during prenatal life, and a secondary peak occurring during the school-age years (Fig. 1). Among infants presenting with SVT, a significant fraction—possibly more than one-half of patients—will have no clinical tachycardias after initial presentation, may be noninducible during provocative electrophysiological study (EPS) or, in the case of children with WPW (4) may exhibit disappearance of preexcitation. This apparent resolution of the tachycardia symptoms and substrate may reflect age-related increases in accessory-pathway refractoriness (5). However, such patients may be vulnerable to recurrence later in childhood (2). The natural histories of incessantly recurring tachycardias such as ectopic atrial tachycardia and PJRT are less well known, but there is some clinical evidence to suggest that these tachycardias may also resolve spontaneously in a significant number of cases (6).

As with the adult population, the clinical and electrocardiographic presentations of SVTs are strongly influenced by the activities of the AV node and the specialized conduction systems. Sinus tachycardia may be frequently confused with SVT, especially in sick, febrile infants or children in respiratory distress, who may have sinus rates of 200-220 BPM. Mean resting heart rates and percentile ranges are presented by age in Fig. 2. Although orthodromic SVT associated with a narrow QRS complex is the most common electrocardiographic appearance of SVT in children, bundle-branch aberration—and occasionally, antidromic tachycardias—may result in a wide-complex tachycardia. It should be emphasized, however, that any wide-complex tachycardia should be considered to be a potentially hemodynamically unstable rhythm, and treated accordingly. An algorithm for the electrocardiographic diagnosis of narrow-complex paroxysmal tachycaridas in children is provided in Table 1.

In most cases, acute tachycardia events are well-tolerated by children who have otherwise normal cardiac anatomy and function, with syncope or other severe presentation very rare. Thus, the risks of SVT are low in this group, and clinical evolution is favorable. This suggests that it is preferable to avoid therapeutic choices with even a relatively small potential for major adverse side effects when treating infants and young children with clinically benign supraventricular arrhythmias. Thus, consideration of radiofrequency catheter ablation or treatment with certain classes of potent anti-arrhythmic agents, such as class IC and class III antiarrhythmic agents, would only be

Mean resting heart rate 200 i--

Mean resting heart rate 200 i--

Fig. 2. Mean resting heart rate by age, with 2-98% range bars. Normal heart rate at rest increases slightly in the first weeks and months of life, and then slowly declines. Heart rates greater than 180 BPM are rare in healthy, quiet children, but maximum heart rate may be as high as 200-220 BPM under stress caused by extreme exertion, fever, hypovolemia, or medication with adrenergic agents.

Fig. 2. Mean resting heart rate by age, with 2-98% range bars. Normal heart rate at rest increases slightly in the first weeks and months of life, and then slowly declines. Heart rates greater than 180 BPM are rare in healthy, quiet children, but maximum heart rate may be as high as 200-220 BPM under stress caused by extreme exertion, fever, hypovolemia, or medication with adrenergic agents.

considered suitable for children with unusually severe or unremitting tachycardia and for older children with established disease.

Although SVTs that occur in otherwise normal children are typically benign events, they may be associated with severe symptoms such as syncope and seizure under conditions of physiological stress, such as dehydration or intercurrent illness. Fatal outcomes associated with SVT are typically only seen in specific patient subpopulations: patients with severe structural congenital heart disease and hemodynamic derangement, patients with cardiomyopathies and significant abnormalities of ventricular function, and patients with WPW at risk for rapid conduction of atrial arrhythmias (Fig. 3). Of these groups, the latter is the most problematic from the perspective of clinical management, as patients with pre-excitation make up a relatively large proportion of the population of children with SVT and many are either minimally symptomatic or asymptomatic. The prevalence of cardiac arrest in patients with WPW may be similar to that observed in adult series (2.3%), with a mean age of this severe presentation of 11-12 yr (7). This small but real risk of sudden death is present although the incidence of AF, the electrophysiological risk factor for this event, is low in children. The antegrade conduction characteristics of accessory pathways tend to manifest longer refractory periods with increasing patient age; thus, it might be expected that the potential for rapid antegrade conduction in young patients is higher than in adults. Many pediatric electro-physiologists elect to assess the "risk" associated with pre-excitation by assessing the antegrade conduction characteristics of the accessory pathway directly by induction of AF and/or rapid atrial pacing, or indirectly by observing the response of pre-excitation to sinus tachycardia induced by exercise. The presence or absence of pre-excitation below a given RR interval is implicitly related to studies which have demonstrated that WPW patients with the shortest RR interval in AF of <220 ms constitute a group at greater risk of cardiac arrest (8). The presence of WPW also increases the likelihood of associated congenital heart disease, which may be clinically occult. The prevalence of pre-excitation has been particularly linked with Ebstein's malformation of the tricuspid valve (4,9), as well as congenitally corrected transposition of the great arteries

Electrocardiographic Diagnosis of Paroxysmal Narrow-Complex Tachycardias (SVT) in Children

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