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