A 7-month-old infant presents with listlessness, pallor, and increased work of breathing and has a rapid heart rate that is unresponsive to oxygen; which cardiac rhythm is most likely on monitor?

A 7-month-old infant presents with listlessness, pallor, and increased work of breathing and has a rapid heart rate that is unresponsive to oxygen; which cardiac rhythm is most likely on monitor?

• A. Narrow QRS complex rhythm with absent P waves and heart rate greater than 220 beats/min.
• B. Wide QRS complex rhythm with reentrant tachycardia
• C. Sinus tachycardia at a moderate rate
• D. Atrial flutter with regular rhythm

Answer: (A)

In infants who are listless, pale, and working hard to breathe, a very fast, regular, narrow-complex tachycardia is most concerning and classically points to supraventricular tachycardia caused by a reentrant circuit. The rapid rate often exceeds 220 beats per minute in this age group, and because the atrial activity is occurring so quickly, distinct P waves are not seen on the monitor. The ventricles are still being activated through the normal conduction system, which is why the QRS complexes remain narrow. This pattern explains the hemodynamic instability: the heart isn’t able to fill adequately between beats, reducing cardiac output and leading to poor perfusion despite supplemental oxygen. If you compared it to other rhythms, a wide QRS tachycardia would suggest ventricular involvement or aberrant conduction, which is less typical in this clinical scenario. Sinus tachycardia would show visible P waves and a more moderate rate, and atrial flutter would produce a sawtooth pattern with a different rhythm. The described narrow, rapid, P-wave–invisible tachycardia best fits infant SVT, the rhythm most likely responsible for these symptoms.

In infants who are listless, pale, and working hard to breathe, a very fast, regular, narrow-complex tachycardia is most concerning and classically points to supraventricular tachycardia caused by a reentrant circuit. The rapid rate often exceeds 220 beats per minute in this age group, and because the atrial activity is occurring so quickly, distinct P waves are not seen on the monitor. The ventricles are still being activated through the normal conduction system, which is why the QRS complexes remain narrow. This pattern explains the hemodynamic instability: the heart isn’t able to fill adequately between beats, reducing cardiac output and leading to poor perfusion despite supplemental oxygen.

If you compared it to other rhythms, a wide QRS tachycardia would suggest ventricular involvement or aberrant conduction, which is less typical in this clinical scenario. Sinus tachycardia would show visible P waves and a more moderate rate, and atrial flutter would produce a sawtooth pattern with a different rhythm. The described narrow, rapid, P-wave–invisible tachycardia best fits infant SVT, the rhythm most likely responsible for these symptoms.