What are the guidelines for managing fetal supraventricular tachycardia (SVT)?

Management of Fetal Supraventricular Tachycardia (SVT)

Transplacental antiarrhythmic therapy with flecainide should be considered first-line treatment for fetal SVT, particularly in the presence of hydrops fetalis, due to its superior conversion rates and impact on mortality compared to other agents. 1

Diagnosis and Assessment

• Fetal SVT is diagnosed by fetal echocardiography, which should evaluate:

  • Heart rate and rhythm pattern (sustained vs. intermittent)
  • Presence of hydrops fetalis (pleural effusion, pericardial effusion, ascites, skin edema)
  • Ventricular function
  • Structural cardiac abnormalities

• SVT mechanism classification using mechanical ventriculoatrial (VA) time intervals:

  • Short VA interval (most common, ~80% of cases)
  • Long VA interval (~20% of cases) 1

Treatment Algorithm

First-line Treatment Options:

1. Flecainide:

   • Preferred first-line agent, especially for hydropic fetuses
   • Conversion rates: 96% for short VA SVT, 89% for long VA SVT (including rate control)
   • Particularly effective in hydropic fetuses (86-100% success) 1
   • Significantly reduces mortality in hydropic fetuses compared to digoxin 1

2. Digoxin:

   • Alternative first-line agent for non-hydropic fetuses
   • Conversion rates: 69% for short VA SVT, 50% for long VA SVT
   • Less effective in hydropic fetuses (38% success rate) 1
   • Poor transplacental transfer in the presence of hydrops 2

Second-line Treatment Options:

1. Sotalol:

   • Effective for atrial flutter (superior to digoxin and flecainide for AF) 3
   • Can be used alone or combined with digoxin
   • 85% complete or partial response rate 2
   • Caution: potential proarrhythmic effects 4

2. Combination therapy:

   • Flecainide + digoxin for refractory SVT
   • Flecainide + amiodarone for severe hydrops with refractory SVT 1

3. Amiodarone:

   • Reserved for drug-refractory fetal tachycardia with hydrops 4
   • May be considered for potentially life-threatening SVT when other therapies are ineffective 5
   • Caution: risk of fetal hypothyroidism (reported in ~17% of cases) 5

Monitoring and Follow-up

• Regular fetal echocardiography to assess:

  • Heart rate and rhythm
  • Resolution of hydrops if present
  • Ventricular function

• Maternal monitoring:

  • ECG monitoring for QRS prolongation with flecainide
  • Serum drug levels when appropriate
  • Assessment for maternal side effects

Important Considerations and Pitfalls

1. Hydrops fetalis:

   • Critical prognostic factor - increases mortality risk
   • Reduces transplacental transfer of digoxin
   • Flecainide has superior outcomes in hydropic fetuses 1

2. Pattern of arrhythmia:

   • Incessant SVT has higher treatment failure rate than intermittent SVT (HR=3.1) 3
   • Conversion rates at 5 and 10 days are approximately 50% and 63% for SVT 3

3. Postnatal management:

   • Fetuses requiring second-line therapy have more complex postnatal courses (67% vs 23%) 6
   • Continued antiarrhythmic treatment is typically needed after birth due to high recurrence risk 4

4. Drug-specific considerations:

   • Avoid antiarrhythmic drugs in first trimester when possible due to risk of congenital malformations 5
   • Start with lowest recommended dose and adjust based on clinical response 5
   • Flecainide and propafenone should be avoided in patients with structural heart disease 5

5. Delivery planning:

   • Term delivery is preferred when possible
   • Early delivery may be necessary for refractory cases with worsening hydrops

By following this evidence-based approach to managing fetal SVT, with particular emphasis on early recognition of hydrops and appropriate selection of antiarrhythmic therapy, mortality and morbidity can be significantly reduced.