Diagnosis: Supraventricular Tachycardia (SVT)
The most likely diagnosis is supraventricular tachycardia (SVT), given the narrow QRS complex, absent P waves, and regular rhythm in this 3-month-old infant. 1
Diagnostic Reasoning
Key ECG Features Supporting SVT
Narrow QRS complex: In over 90% of infants and children with SVT, the QRS complex is narrow, and only 3% have a QRS complex different from the underlying sinus rhythm 1
Absent P waves: In 40% of SVT cases in infants, P waves are not visible; when visible (60% of cases), they have a different morphology from sinus P waves 1
Regular rhythm: SVT in infants characteristically demonstrates an extremely regular R-R interval after the first 10-20 beats, typically at rates of 260-300 beats per minute 1
Why Not Atrial Flutter?
Atrial flutter would show characteristic flutter waves (best seen in leads II, III, aVF, and V1), which are not described in this case 1. The definition of SVT specifically excludes arrhythmias with the morphology of atrial flutter 1.
Why Not Ventricular Tachycardia?
Ventricular tachycardia is highly unlikely because the QRS complex is narrow. 1
VT in infants presents with QRS complexes that are different from the patient's normal sinus QRS, usually with prolonged duration (≥0.09 seconds in infants) 1
While infants may occasionally have VT with QRS duration <0.09 seconds, the complex would still be clearly different from the sinus complex 1
Persistent aberration of SVT in infants is exceedingly rare, meaning that in the majority of infants with a QRS complex different from sinus, the diagnosis would be VT—but this case describes a narrow complex 1
SVT with a different QRS beyond the first 10-20 beats is rare in infants; therefore, if the QRS were wide and different, VT should be strongly considered 1
Clinical Context
SVT is the most common symptomatic tachyarrhythmia in children 2, 3. In infants, it typically presents with signs of acute congestive heart failure rather than palpitations (which are more common in older children) 2.
Critical Next Steps
Immediate conversion to sinus rhythm should be attempted 1
Obtain an echocardiogram to determine ventricular function and assess for possible congenital heart disease 1
Adenosine is the drug of first choice for tachycardias involving the atrioventricular node in any age group 2
Common Pitfall to Avoid
Do not confuse narrow complex SVT with VT based solely on absent P waves—the QRS morphology and duration are the critical distinguishing features in infants 1.