Progress in Pediatric Cardiology

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1 Progress in Pediatric Cardiology 35 (2013) 1 6 Contents lists available at SciVerse ScienceDirect Progress in Pediatric Cardiology journal homepage: The infant with supraventricular tachycardia: Diagnosis and management Susan P. Etheridge, Elizabeth V. Saarel 1 University of Utah and Primary Children's Medical Center, 100 North Mario Capecchi Drive, Salt Lake City, UT 84113, United States article info abstract Keywords: Supraventricular tachycardia Arrhythmia Congenital heart disease Pre-excitation 2012 Elsevier Ireland Ltd. All rights reserved. 1. Introduction and Clinical Features Supraventricular tachycardia (SVT) represents one of the most commonly encountered significant cardiac conditions in childhood occurring in 1 of children and is the most common sustained arrhythmia from intrauterine life onwards. The mechanism of SVT is age-related with atrioventricular reentry tachycardia (AVRT) representing the most common tachycardia substrate in infants and young children, atrioventricular nodal reentrant tachycardia (AVNRT) becoming more common in older children and young adults [1] and atrial fibrillation most common in adults. AVRT is due to a reentrant circuit that involves unidirectional conduction over the atrioventricular (AV) node, usually antegrade, and unidirectional conduction over an accessory AV connection, usually in the retrograde direction. For reasons that are not yet clear, some accessory AV connections are capable of only retrograde conduction, from the ventricles to the atria. In this setting SVT can occur, but the 12-lead electrocardiogram (ECG) during sinus rhythm is normal and the pathways are considered concealed. In others, antegrade conduction over the accessory connection is evident (manifest) on the surface ECG of these infants with Wolff Parkinson WhiteSyndrome(WPW). AVRT is the most common arrhythmia seen in WPW in which antegrade conduction over the accessory AV connection during sinus rhythm causes manifest changes on the surface ECG (Fig. 1). Because accessory pathways often have a more rapid rate of conduction from the atria to the ventricles than does the normal AV node, ventricular depolarization occurs before activation from the AV node and bundle of His (pre-excitation) causing a short PR interval and wide QRS complex with a slurred upstroke (delta wave) [2]. Corresponding author. Tel.: ; fax: address: pcsether@ihc.com (S.P. Etheridge). 1 Tel.: ; fax: The development of reentry SVT requires an anatomic substrate for abnormal conduction and the appropriate electrophysiologic environment. Essential for initiation of AVRT is a difference in the electrophysiological properties of 2 AV connections; this allows the occurrence of unidirectional block in one pathway while maintaining conduction in the other [3]. This difference can be exposed not only by critically timed atrial and ventricular premature beats but also by changes in the sinus rate as might occur with changes in sympathetic or vagal tone [4]. AVNRT is less common than AVRT in the neonate and infant occurring in 10 15% of patients with SVT under age 1 [5]. The ability to generate AVNRT implies dual AV node physiology likely a consequence of 2 (or more) distinct AV nodal pathways with different electrophysiological properties. Although in electrophysiology studies of children with AVNRT, this phenomenon of dual AV node physiology is not always demonstrable [6]. It has been speculated that the reentry circuit in these children may incorporate atrial muscle near but not within the AV node [7,8]. Both AVRT and AVNRT are usually manifested as a narrow QRS tachycardia with a 1:1 AV relationship. The ECG in tachycardia may distinguish the 2 SVT subtypes. In AVRT the retrograde limb of the tachycardia circuit involves passage through the ventricular myocardium and thus is longer. Therefore, the retrograde P wave is often visible, distinct from the preceding QRS, classically negative in the inferior leads, II, III and avf (Fig. 2). In typical AVNRT retrograde conduction is rapid as it does not traverse the ventricular myocardium and thus the retrograde P wave is masked by the larger QRS signal and therefore is not usually visible on the ECG during tachycardia. WidecomplexQRStachycardiacanoccurinAVRTandAVNRTasaresult of bundle branch aberrancy. This can be seen fairly commonly at SVT onset with resolution to a narrow complex over time. In AVRT this phenomenon can help determine the location of the accessory connection /$ see front matter 2012 Elsevier Ireland Ltd. All rights reserved.

2 2 S.P. Etheridge, E.V. Saarel / Progress in Pediatric Cardiology 35 (2013) 1 6 Fig. 1. Sinus rhythm with pre-excitation. If the rate of SVT increases as complexes change from wide to narrow, the accessory pathway is on the same side (ipsilateral) as the bundle branch block (e.g., left-sided accessory pathway if left bundle branch block). If the heart rate does not change, the accessory pathway is on the opposite side of the bundle branch block (Fig. 3). The presentation of patients with SVT during the first year of life is highly variable. Infants are often asymptomatic. In these cases SVT is diagnosed when tachycardia is observed by medical care providers or other caregivers. If the arrhythmia persists or is poorly tolerated the infants may develop nonspecific symptoms such as poor feeding, tachypnea, lethargy or pallor, symptoms characteristic of heart failure in this age population. In some cases SVT in the neonate can be life-threatening, especially in those with significant congenital heart disease [9]. 2. Developmental Aspects The peak occurrence of AVRT is in utero and during early infancy with a significant proportion of these patients experiencing no Fig. 2. Retrograde P waves visible during AVRT.

3 S.P. Etheridge, E.V. Saarel / Progress in Pediatric Cardiology 35 (2013) further arrhythmia beyond the first year of life. Some do not have a recurrence after initial presentation and approximately one third of those with AVRT show a disappearance of ventricular pre-excitation and noninducibility by transesophageal electrophysiological studies by 1 year of age [10]. The substrate for AVRT is abnormal electrical conduction through an accessory AV connection that allows for a circus movement between atria and ventricles. These muscular accessory connections are thought to be composed of normal myocardium, first identified in serial sections through the AV valve annulus in fetuses and infants [11]. These accessory connections represent defects in the annulus fibrosis, penetrating the conduction barrier formed by the electrically inert annulus fibrosis and they have been identified in infants less than 6 months post-natal age. Gradual disruption of these muscular Fig. 3. Infant SVT rate difference between narrow complex and LBBB proves left AP. A. Narrow complex AVRT. B. Same patient with AVRT LBBB.

4 4 S.P. Etheridge, E.V. Saarel / Progress in Pediatric Cardiology 35 (2013) 1 6 bridges occurs as part of normal post-natal development with their resolution as a part of the normal maturation of the annulus [12]. The development of the annulus fibrosis is complex, involving different processes including growth of the endocardial cushions on the luminal side of the primitive AV canal and the invagination of the epicardially located AV sulcus tissue [13 15]. Atrial and ventricular myocardia are noted to be continuous at the primitive AV canal at early stages of development with the beginning of separation of atrial and ventricular myocardium by the developing annulus fibrosus occurring at 7 weeks of gestation. Evaluation of fetal hearts throughout gestation and newborn hearts after birth demonstrates the presence and gradual disappearance of accessory AV connections that cross the developing annulus fibrosis in human embryonic, fetal and neonatal hearts [16]. This may explain the intriguing self-limiting character of AVRT in the fetus and the neonate as isolation of the AV junction is a continuing process that may not be complete at birth. The temporary presence of these accessory myocardial AV connections could serve as a substrate for perinatal AVRT. The reasons that AVRT can persist into childhood or adult life are not fully understood, nor have the cell types and instructive signaling routes responsible for normal annulus fibrosis formation been elucidated fully. In an interesting study in quail hearts, epicardium-derived cells were identified as essential for proper formation of the isolating annulus fibrosis and inhibition of their migration during cardiogenesis may result in imperfect annulus fibrosis development with persistence of accessory pathways [17]. The developmental aspects of AVNRT including mechanisms behind development of fast and slow AV node pathways are poorly understood. AVNRT is an uncommon SVT substrate in infancy and increases in frequency with age. The development of AVNRT in children as they age may be due to maturational changes in the electrophysiologic properties of the AV node. In an attempt to assess this, AV node function was evaluated by electrophysiology testing in two different pediatric age groups (approximately 4 months to 13 years and >13 to 20 years) [18]. The AV node in the younger patients was able to tolerate much faster pacing rates before developing antegrade AV block. In contrast, retrograde ventriculoatrial (VA) conduction was common and was not age-dependent. Dual AV node pathways occurred significantly more often in the older aged children. These observations indicate that the substrate for AVNRT (dual AV node pathways, with inducible antegrade block in one limb at moderately rapid rates) is more likely to be present in adolescents and is rare in children and infants. 3. Genetics There is little information on the genetics of SVT without preexcitation. The molecular basis of WPW is better understood and there are likely diverse genetic causes. In most cases of WPW there is no familial involvement; however, in some families WPW is inherited as a single-gene disorder or occurs as part of a syndrome with a strong genetic basis. Among those with the WPW, 3.4% have first-degree relatives with pre-excitation [19 21]. Clinicians have long recognized the occurrence of the WPW syndrome in some patients with the autosomal dominant disorder of familial hypertrophic cardiomyopathy [22]. Recently missense mutations have been implicated in familial WPW, often in association with cardiac hypertrophy [19,23]. The mutation, Arg531Gly in the gamma-2 regulatory subunit (PRKAG2) of AMPactivated protein kinase (AMPK) was found to be responsible for a syndrome associated with ventricular pre-excitation, early onset atrial fibrillation and conduction disease. 4. AVRT and Congenital Heart Disease Although most patients with AVRT have structurally normal hearts, concomitant congenital heart disease is described. Congenital heart disease was found in 37% of 140 children with WPW, and Ebstein anomaly of the tricuspid valve was the most common defect (Fig. 4) [24].Approximately 10 30% of patients with Ebstein anomaly have WPW syndrome and, as one would expect, right posterior or posteroseptal accessory connections are the most frequent location [25 27]. When Ebstein anomaly is seen in association with l-transposition of the great vessels, the accessory AV connections are on the left side, ipsilateral to the Ebsteinoid tricuspid valve. Downward displacement of the septal leaflet of the tricuspid valve, the hallmark of Ebstein anomaly, is associated with discontinuity of the central fibrous body and septal AV ring allowing for direct muscular connections from the atria to the ventricle, thus creating a potential substrate for accessory AV connections and pre-excitation [26]. Other congenital heart diseases associated with WPW syndrome include atrial and ventricular septal defects, coronary-sinus diverticula, and isolated l-transposition of the great vessels among others. In summary, the mechanisms leading to accessory AV connection formation and persistence after birth are not completely understood. The differences in location and specific electrophysiological properties of accessory AV connections, their genetic background as well as their association with structural and congenital heart disease, indicate that not all accessory connections share the same causative pathway. In many cases accessory AV connections are an embryonic or developmental defect in which processes that electrically insulate the atria from the ventricles go awry. The relative paucity of AVNRT in infants suggests that developmental processes leading to dual AV node pathways are very different from those leading to accessory AV connections. 5. Natural History AVRT that begins in the fetus or neonate may disappear entirely either due to the loss of the tachyarrhythmia substrate or a decrease in the burden of ectopy required for the initiation of the tachycardia [10]. Although in some the SVT resolves completely, in many the AVRT becomes temporarily quiescent during toddler years but symptoms recur later in childhood. When SVT is present after age 5 years it is likely to persist long term [24]. Longitudinal studies suggest that the electrophysiological properties of accessory AV connections become impaired over time and some will lose antegrade conduction leading to resolution of pre-excitation on the surface ECG [28,29]. Even though AV connection conduction properties are usually faster in younger patients with AVRT, the ability to induce SVT often persists in older patients. The electrophysiologic properties of the persistent form of junctional reciprocating tachycardia (PJRT) clearly demonstrate that slowly conducting accessory AV connections are not always benign. Clair et al. demonstrated that age was more important than other clinical variables in predicting the recurrence of SVT and that young patients with SVT could expect to experience more frequent events as they got older [30]. This suggests that while the conduction properties of accessory AV connections slow over time, the tachycardia symptom burden and frequency may not. Less common than AVRT in the neonate, AVNRT has a different natural history than AVRT and may be less likely to resolve spontaneously [5,31]. 6. Clinical Management When SVT is suspected an ECG should be obtained both in tachycardia and after its resolution. Typical rates for SVT are between 220 and 300 beats per minute in the first year of life. In most cases the RR interval is regular and the QRS duration is narrow. If the tachycardia is irregular, an atypical SVT, including ectopic atrial tachycardia, multifocal ectopic atrial tachycardia, atrial flutter or PJRT, should be suspected. The most likely time to document manifest ventricular pre-excitation pattern, indicating WPW syndrome, is immediately

5 S.P. Etheridge, E.V. Saarel / Progress in Pediatric Cardiology 35 (2013) Fig. 4. WPW in Ebstein anomaly patient. after tachycardia resolution when AV node conduction is temporarily suppressed. Since the presence of ventricular pre-excitation on ECG has important diagnostic and prognostic implications it is imperative to obtain a resting ECG in all infants after SVT has resolved (Fig. 5). In the majority of infants with SVT, symptoms are benign and tachycardia is well tolerated. These relatively healthy patients should be managed conservatively because any therapeutic intervention has potential side effects. If SVT does not resolve spontaneously and symptoms are present, cardioversion using vagal maneuvers, such as ice applied to the face for b20 s, pharyngeal suctioning, or measurement of the rectal temperature, should be attempted [7,32]. When vagal maneuvers fail, cardioversion using IV adenosine (given rapidly followed by a normal saline bolus in an I.V. as close to the central circulation as possible; starting dose μg/kg), IV esmolol (starting dose 100 μg/kg/min), transesophageal pacing or synchronized DC current may be used. Many infants will have a single episode of SVT prior to long-term resolution of symptoms. It is therefore prudent to take a watch and wait approach to healthy infants with a single episode of SVT. If SVT recurs, or if symptoms are severe, then chronic medication treatment may be warranted. The most common initial drugs used to control well-tolerated SVT in healthy infants are digoxin and/or propranolol [33]. These drugs have an excellent safety profile. Rapid loading of digoxin increases the risk for AV conduction block and sinus bradycardia, so this should be avoided in healthy infants. Parents and other caregivers should be educated regarding signs of SVT, how to count the infant heart rate, and how to safely perform vagal maneuvers. When infants present with severe symptoms, including shock due to decompensated heart failure, immediate DC cardioversion should be performed. Because of the excellent long-term safety profile it is worth trying beta-blockers, such as I.V. esmolol to prevent SVT recurrence unless the patient is hypotensive. A good choice for second line Fig. 5. Ventricular pre-excitation unmasked after adenosine administration during AVRT.

6 6 S.P. Etheridge, E.V. Saarel / Progress in Pediatric Cardiology 35 (2013) 1 6 drug treatment would be a sodium channel blocker such as flecainide, available in oral formulations, or procainamide, available in I.V. formulations. Second or third line drug options include sotalol and amiodarone, both available in oral and I.V. forms. SVT resolves in 30 80% of infants by 1 year of age. There are several options for management of medications until this time. If the patient is asymptomatic, one choice is maintenance of oral medication at the same dose for weight (increasing the absolute dose to account for weight gain) until about 1 year of age. Another choice is to let the infant wean off medication by keeping the same absolute dose as the baby gains weight over the first year. Ambulatory ECG monitoring using Holter and event monitors can be useful to detect subclinical episodes of SVT. Serial ECGs are useful for surveillance in patients with WPW syndrome. Some electrophysiologists advocate performance of an esophageal EP study at 1 year of age prior to withdrawal of antiarrhythmic medication. Others simply withdraw medication between 9 and 12 months and watch for clinical recurrence of SVT. Patients with congenital heart disease have a lower rate of spontaneous SVT resolution and are at increased risk for severe consequences from tachycardia so maintenance of antiarrhythmic medications beyond infancy may be prudent in this patient population. References [1] Ko JK, Deal BJ, Strasberger JF, Benson DW. Supraventricular tachycardia mechanisms and their age distribution in pediatric patients. Am J Cardiol 1992;69(12): [2] Basson CT. A molecular basis for Wolff Parkinson White syndrome. N Engl J Med 2001;344(24): [3] Wellens HJ. Value and limitations of programmed electrical stimulation of the heart in the study and treatment of tachycardias. Circulation 1978;57(5): [4] Doevendans PA, Wellens HJ. 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