include drug exposure, congenital heart disease, and WPW syndrome (see Chapter 86 Cardiac Emergencies ) Approximately 50% of children with SVT have neither physical findings nor EKG abnormalities between episodes In these patients, descriptions of abrupt onset and rapid termination of palpitations (“like a light switch”) can often be elicited VT may also present with palpitations and may be associated with infections, drug exposure, or even exercise Infections, especially viral myocarditis and acute rheumatic fever are some of the most common causes of acquired VT in children with normal cardiac anatomy Similarly, ingestion of drugs that block fast sodium channels and/or potassium channels (e.g., tricyclic antidepressants, phenothiazines, and antiarrhythmic agents) is a preventable cause of torsades de pointes (polymorphic VT) and unstable VT in the otherwise normal child ( Table 63.4 ) Palpitations associated with exercise may be caused by VT that occurs in conjunction with hypertrophic cardiomyopathy or myocardial ischemia (see Chapter 86 Cardiac Emergencies ) Patients with the prolonged QT syndrome have a genetically determined predisposition to fatal VT or have an acquired long QT syndrome (LQTS) from drugs, hypokalemia, or hypomagnesemia LQTS may present with palpitations, presyncope, syncope, cardiac arrest, and/or seizures (see Chapter 86 Cardiac Emergencies ) Patients who have undergone ventriculotomy for tetralogy of Fallot comprise another group who are at high risk for VT as a result of the postoperative development of scarring in the right ventricular outflow tract Finally, electrolyte disturbances, particularly hyperkalemia, hypocalcemia, and hypomagnesemia, may be causative in a child with palpitations and VT (see Chapter 100 Renal and Electrolyte Emergencies )