Pediatric emergency medicine trisk 1521 1521
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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
