TACHYCARDIA

JAMES F. WILEY II, MD, MPH AND STEVEN C. ROGERS, MD

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 caregiver who notes a rapid heart rate while holding the child or observes rapid jugular venous pulsations, increased apical heart rate, or pulse rate. The definition of tachycardia varies by age (callout to the table of normal vital signs in triage Chapter 73 Triage). 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 94 Cardiac Emergencies).

DIFFERENTIAL DIAGNOSIS

Many conditions may produce tachycardia (Table 72.1). Most tachycardic children exhibit sinus tachycardia without significant cardiac pathology (Table 72.2). However, life-threatening conditions frequently come to medical attention because of fast heart rate and may reflect cardiac and noncardiac origins (Table 72.3).

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 SA node firing rate (Table 72.2). In addition, 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 drugs (cocaine, amphetamines, methcathinones [e.g., bath salts], or synthetic cannabinoids [e.g., K2, Spice]) are frequently implicated (see Table 58.4).

Shock is a life-threatening cause of sinus tachycardia that requires rapid recognition and reversal to prevent permanent organ damage or death (see Chapter 5 Shock). Circulatory shock may result from intravascular volume loss, inadequate cardiac contractility, a marked drop in systemic vascular resistance, or a combination of these mechanisms. Physical findings help differentiate the different forms of shock (hypovolemic, cardiogenic, septic, and distributive) and identify the underlying cause.