Tachycardia in children is generally defined as a heart rate exceeding 120 beats per minute in older children and adolescents, and 180 beats per minute in infants and young children. The key to effective management of tachycardia is identifying its site of origin and understanding the underlying mechanism.
Thus, one of the initial steps in the evaluation of a child with tachycardia includes examination of the standard electrocardiogram (ECG), because assessing the width of the QRS complex aids in determining the origin of the arrhythmia and guides management decisions (algorithm 1).
●Narrow QRS complex tachycardia — A narrow QRS complex (QRS duration ≤80 msec) indicates that the origin of the arrhythmia is above or within the atrioventricular (AV) node. The short QRS duration indicates rapid activation of the ventricles through the AV node and the His-Purkinje system, and eliminates ventricular tachycardia (VT) as an etiology of the increased heart rate. Causes of narrow QRS complex tachycardia include sinus tachycardia, atrial fibrillation and flutter, and most cases of supraventricular tachycardia (SVT). These conditions are discussed in detail separately. (See "Sinus tachycardia" and "Epidemiology of and risk factors for atrial fibrillation" and "Supraventricular tachycardia in children: AV reentrant tachycardia (including WPW) and AV nodal reentrant tachycardia" and "Overview of atrial flutter".)
●Wide QRS complex tachycardia — With wide QRS complex (QRS duration >80 msec) tachycardia, VT must be first considered, because VT may rapidly degenerate to ventricular fibrillation and result in cardiac arrest. Wide QRS complex tachycardia may also be seen in patients with supraventricular tachycardia with an aberrant conduction system or antidromic AV reentrant tachycardia. (See 'Supraventricular tachycardia' below and 'Ventricular tachycardia' below.)
Causes of wide QRS complex tachycardia in children will be reviewed here. The evaluation and management of wide QRS complex tachycardia in children are discussed separately. (See "Management and evaluation of wide QRS complex tachycardia in children".)