The wide variation in children's heart rate with age and activity may lead to misinterpretation by those more used to adult electrocardiography. Systemic illness must be considered in any child presenting with an abnormal cardiac rate or rhythm. Sinus tachycardia in babies and infants can result in rates of up to 240 beats/min, and hypoxia, sepsis, acidosis, or intracranial lesions may cause bradycardia. Sinus arrhythmia is a common feature in children's electrocardiograms and is often quite marked. Its relation to breathing (slowing on expiration and speeding up on inspiration) allows diagnosis.
The approach to electrocardiographic diagnosis of tachyarrhythmias in children is similar to that used in adults. Most narrow complex tachycardias in children are due to atrioventricular re-entrant tachycardia secondary to an accessory pathway. If the pathway conducts only retrogradely, the electrocardiogram in sinus rhythm will be normal and the pathway is said to be "concealed". If the pathway conducts anterogradely in sinus rhythm, then the trace will show the typical features of the Wolff-Parkinson-White syndrome. AV nodal re-entrant tachycardia is rare in infants but may be seen in later childhood and adolescence.
Atrial flutter and fibrillation are rare in childhood and are usually associated with underlying structural heart disease or previous cardiac surgery. Atrial flutter can present as an uncommon arrhythmia in neonates with apparently otherwise normal hearts.
Although all forms of ventricular tachycardia are rare, broad complex tachycardia should be considered to be ventricular tachycardia until proved otherwise. Bundle branch block (usually right bundle) often occurs after cardiac surgery, and a previous electrocardiogram can be helpful. Monomorphic ventricular tachycardia may occur secondary to surgery for congenital heart disease. Polymorphic ventricular tachycardia, or torsades de pointes, is associated with the long QT syndrome.
Classification of atrioventricular block into first, second, and third degree follows the same principles as for adults, although a diagnosis of first degree heart block should take into account the variation of the PR interval with age. First degree heart block and the Wenckebach phenomenon may be a normal finding in otherwise healthy children. First or second degree block, however, can occur with rheumatic carditis, diphtheria, digoxin overdose, and congenital heart defects.
Aids for diagnosing tachycardias, such as atrioventricular dissociation and capture and fusion beats, are less common in children than in adults