In infants, a heart rate greater than 220 with a regular rhythm indicates which arrhythmia?

• A. Supraventricular tachycardia
• B. Ventricular tachycardia
• C. Atrial flutter
• D. Sinus tachycardia

Answer: (A)

When an infant’s heart rate is extremely fast and the rhythm is regular, think of a tachyarrhythmia that starts above the ventricles. In this age group, a rate exceeding about 220 beats per minute with a steady rhythm is most characteristic of supraventricular tachycardia. The heart is being driven rapidly by electrical activity from the atria and AV node, producing a very fast but regular ventricular response and usually a narrow QRS complex.

In contrast, sinus tachycardia is a normal physiologic response (fever, dehydration, activity, illness) and, even when fast, is less likely to stay so high and perfectly regular in a calm infant. Atrial flutter would typically show a sawtooth atrial pattern with a different AV conduction pattern (often not a simply regular very fast rate). Ventricular tachycardia would present with a wide, abnormal QRS and is less common in infants.

So the best-fitting explanation for a very fast, regular rate in an infant is supraventricular tachycardia.