CHAPTER 77 ■ TACHYCARDIA STEVEN C ROGERS, V MATT LAURICH INTRODUCTION Fast heart rate or tachycardia is a common sign in children receiving emergency care It may be noticed on initial evaluation by the emergency provider or may be raised as a concern by the patient or a caregiver who notes a rapid heart rate while holding the child or observes rapid jugular venous pulsations, increased apical heart rate, or pulse rate Normal heart rate varies by age, but there is no universally accepted definition for tachycardia for any given age Published normal ranges by age exist, and even commonly used guidelines and definitions vary In infants and young children, the higher resting heart rate, relative to older children, adolescents, and adults, reflects higher tissue oxygen utilization and metabolic rate In most instances, the underlying cause for tachycardia in children is benign However, children with a life-threatening etiology for their tachycardia require prompt recognition and treatment PATHOPHYSIOLOGY Cardiac muscle has intrinsic automaticity that allows it to beat without any external stimulus Resting heart rate typically reflects a balance of input from the vagus nerve (cranial nerve X) and the thoracic sympathetic ganglion (levels T1 to T4) Vagal stimulation results in slowing of the heart rate mediated by cholinergic receptors and has a greater impact on resting heart rate than on the sympathetic nervous system Thus, medications with anticholinergic receptor effects (e.g., antihistamines, atropine) may cause tachycardia Sympathetic stimulation results in increased heart rate and force of contraction primarily through the β1 adrenergic receptors These receptors may also be stimulated by circulating endogenous substances (e.g., epinephrine, increased carbon dioxide tension, hypoxemia) and by exogenous agents (e.g., sympathomimetic drugs) Life-threatening cardiac tachyarrhythmias (e.g., supraventricular tachycardia [SVT], ventricular tachycardia) arise from various mechanisms that disrupt normal electrical conduction in the heart The pathophysiology of these arrhythmias is discussed separately (see Chapter 86 Cardiac Emergencies ) DIFFERENTIAL DIAGNOSIS Many conditions may produce tachycardia ( Table 77.1 ) Most tachycardic children exhibit sinus tachycardia without significant cardiac pathology ( Table 77.2 ) However, life-threatening conditions frequently come to medical attention because of fast heart rate and may reflect cardiac and noncardiac origins ( Table 77.3 ) Sinus Tachycardia Fever, pain, and emotional arousal (e.g., crying, anxiety) are the most frequent causes of sinus tachycardia in children Sympathetic stimulation from other conditions such as hypoxemia, hypoglycemia, hypercarbia, anemia, and excess circulating catecholamines (e.g., hyperthyroidism, pheochromocytoma) also increases sinoatrial node firing rate ( Table 77.2 ) Exogenous sympathomimetic or anticholinergic substances may cause sinus tachycardia Over-the-counter medications that contain antihistamines or pseudoephedrine, “energy” drinks and diet pills that have high concentrations of caffeine, and commonly abused substances nicotine (e.g., cigarettes, e-cigarettes/vaping products, gums, patches), cocaine, amphetamines, methcathinones (e.g., bath salts), or synthetic cannabinoids (e.g., K2, Spice) are frequently implicated (see Table 63.4 ) Conversely tachycardia can be a sign of withdrawal from alcohol, benzodiazepines, or opiates Shock is a life-threatening cause of sinus tachycardia that requires rapid recognition and reversal to prevent permanent organ damage or death (see Chapter 10 Shock ) Shock may result from intravascular volume loss, inadequate cardiac contractility, a marked drop in systemic vascular resistance, or a combination of these mechanisms History and physical findings help differentiate the various forms of shock (hypovolemic, cardiogenic, septic, and distributive) and identify the underlying cause Cardiac inflammation associated with viral myocarditis, acute rheumatic fever, or Kawasaki syndrome frequently presents with sinus tachycardia (see Chapter 86 Cardiac Emergencies ) Patients with these conditions, especially myocarditis, are also at risk for life-threatening arrhythmias, myocardial ischemia, congestive heart failure, and/or cardiogenic shock For patients with pericardial effusion, sinus tachycardia is a physiologic response to impaired cardiac outflow in order to maintain cardiac output (see Chapter 86 Cardiac Emergencies ) Pericardial effusion with tamponade may complicate pericarditis, blunt chest trauma, or recent cardiac surgery and results in decreased cardiac output with significant impairment of systemic circulation In this setting, pericardiocentesis or surgical pericardiotomy is lifesaving (see Chapter 115 Thoracic Trauma ) Tachyarrhythmias SVT represents the most common tachyarrhythmia of childhood (see Chapter 86 Cardiac Emergencies ) The typical heart rate in infants with SVT exceeds 220 beats per minute, whereas older children usually have a heart rate in excess of 180 beats per minute Infants and children with SVT demonstrate a range of physical signs including no symptoms, palpitations, chest pain, tachypnea (often with feeding in infants), diaphoresis, and severe cardiogenic shock TABLE 77.1 DIFFERENTIAL DIAGNOSIS OF TACHYCARDIA Sinus tachycardia Fever Crying Pain Hypoglycemia Hypoxemia Hypercarbia Shock Anemia Poisoning (see Table 63.4 ) Sepsis Anaphylaxis Hyperthyroidism Pheochromocytoma Drug induced (e.g., antihistamines, caffeine, dietary supplements) Withdrawal (e.g., alcohol, benzodiazepines, opiates) Anxiety Myocarditis Acute rheumatic fever Kawasaki disease Pericardial effusion with tamponade Tachyarrhythmias Supraventricular tachycardia Atrial flutter Ventricular tachycardia (monomorphic and polymorphic/torsades de pointes) The most common form of SVT involves an accessory atrioventricular (AV) pathway Additional etiologies include drug exposure, congenital heart disease, and Wolff–Parkinson–White syndrome Sympathomimetics in cough and cold preparations are the most common drugs to incite SVT in children Unregulated dietary supplements such as ephedra (and its congeners, often advertised as “ephedra-free” products) and high-caffeine energy drinks also have the potential to precipitate SVT Cardiac lesions associated with SVT include Ebstein anomaly, repaired dextrotransposition of the great arteries, and single-ventricle lesions status post-Fontan operation Ventricular tachycardia (monomorphic or polymorphic/torsades de pointes) and atrial flutter rarely occur in children (see Chapter 86 Cardiac Emergencies ) Congenital heart disease, electrolyte disturbance (e.g., hyperkalemia, hypocalcemia, hypomagnesemia), genetic predisposition (e.g., long QT syndromes), or poisoning accounts for most cases of ventricular tachycardia in children Atrial flutter usually arises from an intra-atrial reentry circuit Most children with atrial flutter have congenital heart disease Although rare, atrial flutter carries a significant risk of sudden death if not controlled by medications or surgical intervention TABLE 77.2 COMMON CAUSES OF TACHYCARDIA Fever Pain Crying Anxiety Anemia Drug induced (e.g., caffeine, herbal medications, dietary supplements, illicit drugs) Hypovolemic shock