What is the initial diagnostic tool for fetal Supraventricular Tachycardia (SVT)?

Diagnosing Fetal Supraventricular Tachycardia (SVT)

Fetal echocardiography is the initial and definitive diagnostic tool for fetal supraventricular tachycardia (SVT), allowing for accurate assessment of cardiac rhythm, rate, and potential hemodynamic compromise. 1

Diagnostic Approach

Initial Identification

• Fetal SVT is characterized by:
  • Extremely regular R-R intervals (after first 10-20 beats)
  • Rapid heart rate typically 260-300 beats per minute
  • 1:1 atrioventricular (AV) relationship 1

Echocardiographic Assessment

1. M-mode echocardiography:

   • Primary technique to document the relationship between atrial and ventricular contractions
   • Shows 1:1 AV relationship in most cases of fetal SVT
   • Allows measurement of atrial and ventricular rates 1, 2

2. Doppler techniques:

   • Simultaneous pulsed Doppler recording from the superior vena cava and ascending aorta
   • Helps distinguish SVT from other tachyarrhythmias 3
   • Can identify short AV interval and long ventriculo-atrial (VA) interval in specific types of SVT like PJRT 2

3. Comprehensive cardiac evaluation:

   • Assessment of cardiac structure to rule out congenital heart disease
   • Evaluation of cardiac function and signs of heart failure
   • Measurement of cardiothoracic ratio (normal: 0.25-0.35 using area, <0.5 using circumference) 1

Assessment for Hemodynamic Compromise

• Look for signs of cardiac decompensation:
  • Increased heart size (cardiothoracic ratio)
  • Valvular regurgitation
  • Abnormal Doppler waveforms
  • Decreased ventricular wall shortening (normal >28%)
  • Hydrops fetalis (pleural effusion, pericardial effusion, ascites, skin edema) 1

Distinguishing SVT from Other Arrhythmias

SVT vs. Ventricular Tachycardia (VT)

• In SVT:
  • QRS complex is narrow in >90% of cases
  • Only 3% have QRS complex different from underlying sinus rhythm 1
• In VT:
  • QRS complexes differ from normal sinus QRS
  • May have slight variation in R-R interval
  • Possible AV dissociation
  • Rates of 200-500 beats per minute in infants 1, 4

SVT vs. Sinus Tachycardia

• SVT typically has rates of 260-300 bpm
• Sinus tachycardia is almost always <230 bpm
• P wave morphology differs from sinus rhythm in SVT 1, 4

Management Considerations After Diagnosis

• Once diagnosed, assess for:

  • Presence of hydrops fetalis (indicates heart failure)
  • Duration of tachycardia (sustained vs. paroxysmal)
  • Gestational age 1, 3

• Treatment typically involves transplacental therapy:

  • Digoxin is most commonly used as first-line therapy
  • Additional antiarrhythmics may be needed for refractory cases 1, 3

Pitfalls to Avoid

1. Misdiagnosing blocked atrial bigeminy as sinus bradycardia

   • Blocked atrial bigeminy is benign
   • Severe sinus bradycardia may indicate systemic illness 1

2. Failing to perform a complete cardiac structural assessment

   • SVT may coexist with congenital heart disease
   • Comprehensive evaluation is essential 5, 6

3. Overlooking signs of hemodynamic compromise

   • Early detection of cardiac dysfunction allows for timely intervention
   • Serial monitoring is crucial for fetuses with SVT 1

4. Inadequate follow-up

   • Fetal SVT may progress or resolve during pregnancy
   • Regular monitoring is necessary to assess treatment response and detect deterioration 7

Fetal echocardiography has excellent diagnostic accuracy, with studies showing 88.5% complete concordance between fetal and postnatal echocardiography findings, making it the gold standard for diagnosing fetal arrhythmias 6.