What is the most likely diagnosis for a 14-month-old patient presenting with recurrent syncope, worsening heart failure, poor growth, crackles on chest examination, palpitations, abnormal cardiac rhythm, and narrow complex tachycardia on electrocardiogram (ECG)?

Diagnosis: Supraventricular Tachycardia (SVT)

The most likely diagnosis is D. Supraventricular tachycardia (SVT), which is the most common symptomatic tachyarrhythmia in infants and young children, particularly presenting with narrow complex tachycardia at 250-300 bpm, heart failure, and syncope in this age group. 1, 2

Clinical Reasoning

ECG Characteristics Point to SVT

• Narrow complex tachycardia at 250-300 bpm is the hallmark of SVT in infants, distinguishing it from ventricular tachycardia which typically presents with wide QRS complexes 3, 1
• The heart rate range of 250-300 bpm is characteristic for SVT in the 14-month age group, whereas atrial flutter typically presents with atrial rates of 300-400 bpm with variable AV block 3, 4
• Atrial fibrillation is extremely rare in infants without structural heart disease and presents with irregularly irregular rhythm, not the regular narrow complex pattern described 1, 2

Age-Specific Epidemiology

• In infants under 1 year of age, AV reciprocating tachycardia accounts for 80% of SVT cases, making it by far the dominant mechanism 2
• SVT is observed in 0.1-0.4% of the pediatric population and represents the most common sustained arrhythmia in neonates and infants 5
• Atrial tachycardia accounts for only 15% and AV nodal re-entry for only 5% in this age group 2

Clinical Presentation Matches SVT in Infants

• Neonates and infants with paroxysmal SVT typically present with signs of acute congestive heart failure, exactly as described in this case with crackles throughout the chest 1
• Heart failure is more common in infants under 4 months of age (35% incidence) and can develop in older infants with prolonged episodes 2
• Poor growth (failure to thrive) occurs with chronic or recurrent SVT due to the metabolic demands of sustained tachycardia 1, 2
• Syncope in this context represents hemodynamic compromise from the sustained rapid heart rate, which is life-threatening 3

Why Other Options Are Less Likely

Atrial Flutter (Option A):

• Atrial flutter in infants typically shows characteristic "sawtooth" flutter waves and atrial rates of 300-400 bpm with 2:1 or variable AV block, resulting in ventricular rates of 150-200 bpm, not the 250-300 bpm described 3
• Much less common than SVT in this age group 1, 2

Atrial Fibrillation (Option B):

• Extremely rare in infants without structural heart disease 1
• Presents with irregularly irregular rhythm, not the regular narrow complex tachycardia pattern 3

Ventricular Tachycardia (Option C):

• Would present with wide complex tachycardia, not narrow complex as described in this case 3
• Much less common than SVT in infants without structural heart disease 1, 2

Critical Management Considerations

Immediate Risk Assessment

• This patient requires urgent treatment given the hemodynamic compromise evidenced by heart failure and syncope 1, 6
• The American Heart Association guidelines emphasize that in patients with malignant episodes of syncope, ruling out life-threatening arrhythmias like SVT takes priority 3

Diagnostic Workup Needed

• Echocardiogram to assess for structural heart disease and evaluate ventricular function given the heart failure presentation 3
• Consider evaluation for Wolff-Parkinson-White syndrome (accessory pathway) which is the most common mechanism in this age group 2
• Do not delay treatment to obtain additional testing if the patient is hemodynamically unstable 6

Common Pitfall to Avoid

• Never assume this is a benign neurocardiogenic syncope in a pediatric patient with documented tachycardia and heart failure—the American College of Cardiology emphasizes that underlying heart disease makes syncope potentially life-threatening in pediatric patients 3
• The presence of narrow complex tachycardia with heart failure in an infant mandates aggressive evaluation and treatment for SVT 1, 2