In a pediatric patient with tachycardia and poor oxygenation, which ECG finding is most likely?

• A. Narrow QRS complex rhythm with absent P waves and HR > 220 bpm.
• B. Wide QRS complex tachycardia with delta waves
• C. Sinus rhythm with regular rate
• D. Ventricular fibrillation

Answer: (A)

Supraventricular tachycardia is a common cause of fast, regular heartbeats in children, and it fits a situation where a child is tachycardic with signs of poor oxygenation. On the ECG, this shows up as a narrow QRS complex tachycardia with a very high rate and P waves that are often hidden in or just after the QRS. The narrow QRS indicates the impulse is conducted through the normal His-Purkinje system, meaning the origin is above the ventricles. The extremely fast rate—typically over 220 beats per minute in infants—shortens diastole, reducing ventricular filling and coronary perfusion, which can lead to or worsen poor oxygenation. This combination of a rapid, narrow-complex rhythm with absent visible P waves is the classic signature of SVT in kids.

Patterns that would point away from this include a wide QRS tachycardia, which suggests ventricular origin or aberrant conduction, or a baseline finding of delta waves pointing to pre-excitation syndromes like WPW; a normal sinus rhythm with a regular rate would not be tachycardia, and chaotic activity such as ventricular fibrillation would look unlike a regular tachycardia at all.