The aim of this study is to determine whether three-dimensional tranesophageal echocardiography (3D TEE) assessment of ostium secundum atrial septal defect (ASD II) may be as efficient as two-dimensional (2D) TEE assessment complemented by the balloon-sizing during transcatheter closure of the defect.

A total of 50 consecutive patients eligible to percutaneous ASD II closure will be enrolled in the study. 3D-TEE will be performed in the diagnostic process; the images will be recorded for further investigation, however, measurments of the defect will not be taken until after the procedure. Traditional 2D-TEE together with balloon-sizing assessment of the defect will be performed in enrolled patients in order to chose a device. Control transthoracic echocardiography (TTE) will be provided twice to evaluate the outcome of transcatheter treatment: 1 day after the procedure and 6-12 months later. The previously obtained 3D images will be retrospectively assessed by 2 independent investigators excluded from the interventionalists' team. Maximum and minimum diameter, perimeter and area of the defect will be measured to propose the assumed optimal occluder size. Correlation between proposed occluder size and the device used during the procedure will be statistically analyzed.

Inclusion Criteria:

• ASD II suitable for percutaneous treatment
• free and informed consent to analyze the medical data of the person concerned
• age: patients over 16

Exclusion Criteria:

• ASD II not suitable for percutaneous treatment
• any valvular heart disease requiring cardiac surgery
• no free and informed consent to analyze the medical data of the person concerned
• age: under 16

50 consecutive adult patients with ASD II closed percutaneously in our center in the study time frame i.e.