How is Atrial Septal Defect (ASD) size classified?

Atrial Septal Defect Size Classification

Atrial septal defects (ASDs) are classified as small to moderate when ≤2.0 cm and large when >2.0 cm in adults, according to the European Society of Cardiology and European Respiratory Society guidelines. 1

Anatomical Classification

ASDs are first categorized by anatomical type:

• Ostium secundum - Most common, located in the fossa ovalis region
• Sinus venosus - Located near the superior or inferior vena cava junction
• Ostium primum - Located in the lower portion of the atrial septum
• Coronary sinus defect - Unroofed coronary sinus creating communication with left atrium 1

Size Classification

Quantitative Size Classification

1. Small to moderate ASD: ≤2.0 cm in diameter in adults 1
2. Large ASD: >2.0 cm in diameter in adults 1

Qualitative Size Classification

The American College of Cardiology/American Heart Association/American Society of Echocardiography provides a functional classification:

• Small: Likely to be hemodynamically insignificant
• Moderate to large: Likely to be hemodynamically significant
• Present, but unable to characterize further 1

Clinical Significance of Size Classification

The size classification is clinically important for several reasons:

Risk of Pulmonary Arterial Hypertension (PAH)

• Small defects (<1 cm): Only 3% develop pulmonary hypertension
• Large defects (>2 cm): Up to 50% develop PAH with pulmonary vascular disease 1

Spontaneous Closure Rates

• ASDs <6 mm: High likelihood of spontaneous closure (79-89%)
• ASDs 6-8 mm: Moderate likelihood of spontaneous closure (16.6%)
• ASDs >8 mm: Very low likelihood of spontaneous closure (<5%) 2, 3

Intervention Decisions

• ASDs ≤2.0 cm: May be monitored if hemodynamically insignificant
• ASDs >2.0 cm: Generally require intervention due to higher risk of complications 4

Measurement Techniques

Accurate measurement is crucial for proper classification:

1. Transthoracic Echocardiography (TTE): Initial assessment but tends to underestimate size
2. Transesophageal Echocardiography (TEE): More accurate, especially 3D-TEE
3. Balloon Sizing: Gold standard for interventional procedures
   • Balloon Occlusive Diameter (BOD)
   • Stretched Balloon Diameter (SBD) 5, 6

Implications for Treatment

• Small ASDs (<2.0 cm): May be managed conservatively if no evidence of right heart enlargement
• Large ASDs (>2.0 cm): Generally require closure
• Closure method:
  • Percutaneous closure: Suitable for secundum ASDs with adequate rims, typically <38mm
  • Surgical repair: Required for sinus venosus, coronary sinus, primum ASDs, or large secundum ASDs (>38mm) 4

Pitfalls in ASD Size Assessment

• 2D imaging may underestimate true defect size
• Multiple defects can complicate measurement and intervention planning
• Defect shape (circular vs. elliptical) affects accurate sizing
• Dynamic changes in defect size throughout the cardiac cycle 5, 6

ASD size classification is critical for determining management strategy, predicting natural history, and planning appropriate interventions to prevent complications such as right ventricular volume overload, arrhythmias, and pulmonary hypertension.