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and very young children may tolerate long periods of VT quite well; occasionally they may present with incessant VT and associated tachycardia-mediated cardiomyopathy.

Long QT Syndrome

As is the case with adults, children with a prolonged corrected QT interval are at risk of arrhythmic sudden death. LQTS in children may present in a variety of ways, including aborted sudden death, unexplained syncope, and 2:1 AV block, especially in infancy (32). In recent years, it has been increasingly common to discover this diagnosis as an asymptomatic observation made in the course of evaluating the family members of another patient diagnosed with long QT, or serendipitously on a random ECG obtained for an unrelated reason. Patients with deafness of undetermined etiology often have ECGs checked to assess whether they may have the associated recessive form of LQTS, Jervell-Lange-Nielsen syndrome. Patients may also be more likely to present at times when they are prescribed other medications which may increase QT interval, such as certain antibiotic, antifungal, and GI motility agents. In some patients, specific genetic diagnosis of a variety of point mutations affecting membrane-channel proteins for potassium and/or sodium transport is now possible. As the varied genetic picture of this disease becomes more fully understood and related to the phenotypic variability in clinical presentation (33), it is likely that specific therapies may be developed and improved tools made available for risk assessment in individual patients.

VT Associated with Congenital Heart Disease

Late cardiac mortality caused by acute atrial tachycardios and VTs is seen in a significant number of children and young adults with tetralogy of Fallot (34), transposition of the great arteries (35), aortic stenosis (36), and single ventricle (37). Certain congenital anomalies, most notably tetralogy of Fallot and other malformations that result in chronic hypertrophy and/or surgical injury to the ventricles, seem to pose particular risk of ventricular arrhythmias. However, the overall mortality caused by sudden cardiac death even in these "high-risk" subgroups is still quite low—on the order of 3 of 1000 patient-years (38). Simple formulas for predicting the specific risk of individual patients in this group do not exist at present, and several retrospective studies have come to sometimes conflicting conclusions with respect to factors associated with poor outcomes in this group. Associations which have been proposed and seem to have some relevance include: age (39), occurrence of spontaneous VT (39,40), positive ventricular stimulation study (i.e., EPS) (39), and prolonged QRS duration on a 12-lead ECG (41).

IVT in Childhood

Premature ventricular contractions (PVCs) are a frequent observation in childhood. Ambulatory ECG monitoring studies of children and young adults have demonstrated that higher grades of ventricular ectopy (such as ventricular bigeminy or couplets) may also be observed in children with normal hearts, recognizing that the more abnormal the monitoring study, the more a focused cardiac evaluation is needed before the heart is pronounced "normal." More problematic are those patients who present with sustained VT, either with symptoms (palpitations, dizziness and/or fainting) or without, and on evaluation have no other detectable abnormalities of cardiac structure or ventricular function. VT in the anatomically normal heart occurs in young patients in the same electrocardiographic patterns as the adult population: a common left bundle-branch block (LBBB) morphology which generally originates from a focus in the outflow tract of the right ventricle, and a less common right bundle-branch block (RBBB) morphology which originates in the ventricular septum (Fig. 4).

Concern has been expressed in the past that such VTs may be the initial signs of subtle underlying abnormalities of the ventricular myocardium. Although some forms of IVT that occur in an otherwise normal heart may be quite benign, in cases where overt clinical abnormalities are known or discovered in the course of diagnostic evalua-

Fig. 4. Examples of idiopathic ventricular tachycardias in children with anatomically normal hearts. (Top frame) Superior axis and right BBB pattern consistent with origin from left ventricular septal surface. (Bottom frame) Inferior axis and left BBB pattern consistent with origin from right ventricular overflow tract area.

Fig. 4. Examples of idiopathic ventricular tachycardias in children with anatomically normal hearts. (Top frame) Superior axis and right BBB pattern consistent with origin from left ventricular septal surface. (Bottom frame) Inferior axis and left BBB pattern consistent with origin from right ventricular overflow tract area.

tion, patients may be at increased risk for cardiac sudden death (42). More malignant etiologies of VT in an apparently normal heart include arrhythmogenic right ventricular dysplasia, acquired or congenital abnormalities of the coronary arteries, and catechol-aminergic VT. Thus any child who presents with an episode of sustained VT of uncertain etiology should be subject to a comprehensive cardiac evaluation, which may sometimes include magnetic resonance imaging (MRI) imaging and endomyocardial biopsy to rule out more malignant causes of VT. Among patients with no evidence of myocardial abnormalities, these "idiopathic" VTs (IVTs) are generally considered to be benign because they appear to have a very low association with either progressive ventricular dysfunction or with cardiac arrest and sudden death, and have a high likelihood of spontaneous regression (43). Nonetheless, they are often symptomatic enough to demand either pharmcological or ablative therapy.

Emergency Management of VTs

Emergency management of VT in children and young adults begins with the ABCs of basic life support, with immediate institution of cardiopulmonary resuscitation (CPR) and the advanced cardiac life-support/pediatric advanced life-support sequences in the event of cardiac arrest. Recommended initial drug doses and cardioversion energies for therapy of VT and VF are listed in Table 4.

A more difficult decision-making process must be applied for those patients who present in sustained, wide-complex tachycardias that are hemodynamically stable. In general, it is safest to make the presumption that all such rhythms represent VT until proven otherwise. However, as with adults, a wide-complex tachycardia in such patients will occasionally be revealed to be supraventricular in origin—either atrial, AV-nodal, or AV reciprocating—with aberrant ventricular conduction. Some patients in VT may spontaneously revert to sinus rhythm or break in response to interventions such as infusion of lidocaine, other antiarrhythmic agents, or, in the case of recurrent torsades des pointes, magnesium. If external cardioversion is necessary in these patients, it should be performed if possible under conditions that maximize patient safety and comfort, with adequate iv access and monitoring and an appropriate level of sedation monitored by an anesthesiologist. Extra care must be taken with these patients to monitor the evolution of their vascular status while they await cardioversion, and especially when they are sedated for cardioversion, as abrupt hemodynamic collapse may occur.

Chronic management of patients with VT is often problematic, although recent innovations in the design and utilization of implantable cardioverter defibrillators (ICDs) have allowed this therapy to be used with increasing freedom in the pediatric age group. As mentioned previously, a variety of retrospective studies have allowed identification of empiric approaches to risk stratification in certain categories of children with VT. However, with the exception of utilization of beta-blocker therapy for the prevention of sudden death in patients with LQTS, there are no available studies of pediatric patients, which clearly suggests a specific benefit of any antiarrhythmic agent in the prevention of VT and sudden death. Similarly, the value of serial testing of anti-arrhythmic agents for their ability to suppress inducible VT in pediatric patients has not been investigated. To a significant degree, the choice of antiarrhythmic agents in this special population has been extrapolated from the results of studies in adult populations of VT occurring in patients with acquired cardiomyopathy with or without coronary atherosclerosis. At present, standard therapeutic choices include beta-blockers and class III agents, such as sotalol and amiodarone. As in the adult population, the chronic use of amiodarone has been associated with significant extracardiac morbidity.

Device Therapy in Children

ICD therapy for prophylaxis against cardiac sudden death is now being used with increasing frequency among patients in the pediatric age group who have recurrent, potentially lethal ventricular arrhythmias. In prospective, controlled adult studies, a clear survival benefit has been demonstrated, comparing ICD therapy to chronic therapy with antiarrhythmic medications. Although sudden cardiac death does occur in children,

Table 4

Initial Drug Doses and Cardioversion Energies for Therapy of VT and VF

Rhythm

Assessment

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