Heart rate alone is not always enough to establish the diagnosis of a pathologic tachycardia. Infants can have a sinus tachycardia with rates up to at least 230 beats/min in response to serious illnesses, fever, hypovolemia, anemia, pain or infusion of inotropic/chronotropic agents. Additionally, some unusual pathologic supraventricular tachycardias can have rates less than 180 beats/min. In assessing a child with a fast heart rate, one should determine if the
QRS complex is narrow or wide during tachycardia, if the rate is fixed or variable, if there is a visible P wave and if so, the P wave axis. Supraventricular tachycardias typically have a narrow (normal) QRS complex. In general, if the QRS complex in tachycardia remains wide, the rhythm should be considered ventricular tachycardia. However, it is not uncommon for the first few beats of SVT to be wide because of aberrant conduction (right or left bundle branch block) before changing to a narrow QRS complex.

A patient with an apparently fixed high heart rate should be carefully assessed. The fixed heart rate could represent a sinus tachycardia secondary to a high-catecholamine state in an otherwise sick infant. An electrocardiogram should be obtained, and the P wave morphology clearly established. If there is no clear P wave, or the P wave does not have a sinus morphology (positive in leads I, II, and aVF; negative in lead aVR) one should strongly consider a pathologic tachycardia or structural heart disease with heterotaxy (see Table 33-17).

All neonates with documented tachyarrhythmias should have a complete cardiac evaluation, including 12-lead ECG (during the tachycardia if hemodynamically stable, and later in sinus rhythm), and echocardiography, to assess cardiac structure and function. It is estimated that between 8% and 25% of infants with SVT have structural heart disease, most often Ebstein malformation of the tricuspid valve, corrected transposition of the great arteries, or hypertrophic cardiomyopathy (100). Cardiac tumors and myocarditis rarely are predisposing causes for ventricular arrhythmias.

The clinical status of infants with tachyarrhythmias depends on the ventricular rate, duration of tachycardia, presence of underlying structural or functional heart disease, and other clinical problems. Patients may be completely asymptomatic, with the arrhythmia noted during an otherwise routine evaluation or although monitored for other reasons. The infant may not have been appearing well, with irritability, poor feeding, restlessness, or respiratory difficulty with tachypnea, retractions and wheezing. With persistent tachyarrhythmia, the child may develop signs and symptoms of congestive heart failure or acidosis, becoming pale and listless. If the tachycardia persists for long enough, heart failure and a secondary dilated cardiomyopathy may develop. In the fetus with persistent or recurrent tachycardia, this is manifest as nonimmune hydrops. Whether a tachycardia related cardiomyopathy develops depends on the ventricular rate, whether the tachyarrhythmia is intermittent or incessant, the frequency of recurrences if intermittent, and the presence of structural heart disease.