Transthoracic echocardiography measurement versus balloon occlusive diameter for atrial septal defect: experience from a Vietnamese cohort (Vietnamese cohort: TTE vs balloon ASD sizing).

Accurate sizing of atrial septal defects (ASDs) is essential for safe transcatheter closure. Transthoracic echocardiography (TTE) is widely available but often considered to underestimate ASD size compared with balloon occlusive diameter (BOD). This study aimed to assess agreement between TTE and BOD in a large Vietnamese cohort using standardized imaging protocols.We conducted a cross-sectional study of 383 patients with secundum ASD undergoing transcatheter closure at Da Nang Hospital (2015–2022). ASD size was measured by TTE and BOD (stop-flow technique). Agreement was assessed by Bland–Altman plots and the intraclass correlation coefficient (ICC). Correlations with ASD closing device size were analyzed using Pearson’s coefficient.The mean ASD diameter was 15.3 ± 7.5 mm by TTE and 16.9 ± 8.4 mm by BOD (p < 0.001). Agreement was strong overall (ICC = 0.87), higher in patients < 18 years (ICC = 0.83) than in adults (ICC = 0.72). Bland–Altman analysis showed minimal bias, though discrepancies increased with larger ASDs. ASD closing device size correlated almost perfectly with BOD (r = 0.99, p < 0.001) and strongly with TTE (r = 0.89, p < 0.001). TOST confirmed equivalence for defects < 20 mm within ± 2 mm (Δ = 0.62 mm, 90% CI 0.30–0.94, p < 0.001), whereas larger defects did not meet equivalence at ± 10% but achieved it under hybrid margins (± 2 mm for < 10 mm, ± 20% for ≥ 10 mm). No proportional bias was detected, and concordance remained good (Lin’s CCC = 0.857).TTE slightly underestimates ASD size compared with BOD but demonstrates high concordance, particularly in pediatric and small-to-moderate defects. TTE can reliably guide device selection, reducing the need for routine balloon sizing. Selective use of BOD or advanced imaging is advisable for large defects.