prevent episodes has been shown to be ineffective in more than 20% of these patients.117 In such instances, another pharmacologic agent, such as a β-blocker, a class III antiarrhythmic agent, or catheter ablation is warranted Ablation in infants is generally a last resort and should be reserved for arrhythmias that are incessant, sufficiently frequent to contribute to ventricular dysfunction, and cannot be controlled medically In general, catheter ablation for children with VT should be deferred, if possible, until the child is larger, especially since many resolve spontaneously For older children, the decision to proceed to ablation is determined by assessment of the VT burden and diagnosis, procedural risks and benefits, and patient and family considerations Although ablation of VT has been performed effectively in infants, risks are likely greater for children weighing less than 15 kg The risks of growth of the ablation lesion and injury to adjacent coronary arteries remain concerns The recent adoption of advanced three-dimensional mapping techniques has all but eliminated the use of fluoroscopy and its associated risks Polymorphic Ventricular Tachycardia Polymorphic VT is characterized by beat-to-beat variations in QRS morphology and/or axis It is uncommon in the pediatric population and carries a less favorable prognosis than does monomorphic VT This is rare in children with structurally normal hearts or a channelopathy Events are often hemodynamically unstable, and although they may terminate spontaneously, the potential for degeneration into VF exists Polymorphic VT can be divided into bidirectional VT and true polymorphic VT, a classic example of which is TdP Polymorphic VT in a young person without structural heart disease can occur in the setting of QT prolongation, either at baseline or precipitated by drugs, hypokalemia, or hypomagnesemia Although provocation of polymorphic VT by a noncardiac medication is less common than by antiarrhythmic medications, a number of noncardiovascular drugs have been withdrawn from the market because of unexpected sudden cardiac death associated with prolongation of the QT interval and TdP The hallmark mechanism of drug-induced QT prolongation and TdP is blockade of the cardiac delayed rectifier potassium channel.118 Incidence is difficult to estimate, and the occurrence of acquired QT prolongation and TdP is unpredictable Often multiple risk factors are present in a given individual, for example drug exposure coupled with electrolyte imbalance A female preponderance has been borne out in multiple studies.119 Cocaine, amphetamines, the weight-reducing substances phentermine and chlorpheniramine, and methadone have been associated with the occurrence of polymorphic VT; multiple agents are often present and interacting Methadone is mainly metabolized by the CYP3A4 isoenzyme of the hepatic cytochrome P450 system, which is used by numerous other QT-prolonging agents TdP (Fig 22.20) can also be seen in LQTS following a long pause and triggered by early afterdepolarizations, with repetitive ventricular beats leading to TdP.120 FIG 22.20 Rhythm strip demonstrating torsades de pointes (twisting around the points) in a patient with LQT type I The tachycardia is wide complex, irregular, and of different QRS morphologies and axes Complex ventricular arrhythmias have been documented in myocarditis, even without demonstrable ventricular dysfunction A series of 17 patients with otherwise clinically silent lymphocytic myocarditis presented with potentially life-threatening ventricular arrhythmias.121 Polymorphic and bidirectional VT are sufficiently rare in pediatric patients that this arrhythmia should alert one to the potential for underlying pathology, such as a channelopathy, drug toxicity including digoxin toxicity, and myocarditis (Fig 22.21) Therapy for hemodynamically significant events includes magnesium, β-blocking agents, and possibly lidocaine The use of other antiarrhythmic agents, with their tendency to prolong the QT interval owing to IKr (delayed rectifier potassium current) blockade, should be approached cautiously Correction of the underlying cause, including normalizing electrolyte abnormalities, is imperative Pacing at a relatively rapid rate may suppress the arrhythmia if it does not respond to magnesium FIG 22.21 A 12-lead electrocardiogram from a 5-year-old child with active viral myocarditis The electrocardiogram demonstrates salvos of nonsustained ventricular tachycardia Note the very low voltages in the lateral leads ... life-threatening ventricular arrhythmias.121 Polymorphic and bidirectional VT are sufficiently rare in pediatric patients that this arrhythmia should alert one to the potential for underlying pathology,