The objective of this PMCF study is to:

• confirm the safety and performance of the AcuMarkTM Sizing Balloon
• identify previously unknown side-effects
• monitor the identified side-effects (related to the procedure or to the medical devices)

Inclusion Criteria:

1. Patients who have been confirmed with single secundum atrial septal defect (ASD) by echocardiography;
2. The anatomy of ASD is suitable for percutaneous closure;
3. Patients are scheduled to perform ASD closure;
4. Patients or legally authorized representative(s) who are willing and capable of providing informed consent.

Exclusion Criteria:

1. Any contraindication for ASD closure;
2. Patients have ostium primum ASD or coronary sinus ASD;
3. Patients who are pregnant or breastfeeding;
4. Patient is currently participating in another clinical trial that has not yet completed its primary endpoint.

Patients with echocardiographic confirmation of a single secundum atrial septal defect (ASD) who are scheduled to undergo ASD closure.