describe the feeling that their hearts “flip-flop” or “stop.” Many patients with premature atrial or ventricular contractions notice the subsequent beat after the initial “short” beat because of the increased stroke volume ejected Other patients may complain of a choking or full sensation in the neck Jugular venous pulsation associated with right atrial contraction against a closed tricuspid valve (atrioventricular [AV] block with or without atrial tachycardia) can present in this way True cardiac arrhythmias arise from various mechanisms that are discussed in Chapter 86 Cardiac Emergencies DIFFERENTIAL DIAGNOSIS Many conditions may produce palpitations ( Table 63.1 ) Most children with palpitations not have significant cardiac pathology ( Table 63.2 ) However, there are some life-threatening conditions that may come to medical attention because of abnormal cardiac sensation ( Table 63.3 ) Wolff–Parkinson–White (WPW) syndrome and prolonged QT syndrome are two potentially lethal diseases that may be diagnosed on a resting electrocardiogram (EKG) A patient with palpitations during exercise should also raise concern for hypertrophic cardiomyopathy, SVT, VT, or myocardial ischemia In addition, palpitations in children with known congenital heart disease are more likely to be caused by a serious cardiac arrhythmia Diagnosis of noncardiac causes of life-threatening palpitations, including hypoxemia, hypoglycemia, hyperkalemia, and hypocalcemia, can be made by characteristic EKG changes, serum electrolyte determinations, rapid bedside glucose, and oxygen saturation measurements Hyperdynamic Cardiac Activity Increased HR and contractility are physiologic responses to catecholamine release, which may occur with exercise, emotional arousal, hypoglycemia, and pheochromocytoma Similarly, increased cardiac work accompanies conditions that increase the basal metabolic rate such as fever, anemia, and hyperthyroidism Sympathomimetic and anticholinergic drugs are groups of substances that directly modulate the autonomic nervous system, causing tachycardia, hyperdynamic cardiac activity, and palpitations ( Table 63.4 )