AbstractPersisting iatrogenic atrial septal defects (iASD) after transcatheter mitral edge-to-edge repair (M-TEER) are associated with impaired outcomes. We investigated the natural history of relevant iASDs with left-to-right shunting post-M-TEER, predictors of spontaneous closure of iASD between 1 and 6 months post-M-TEER, and outcomes (heart failure [HF] hospitalization) in patients with spontaneous closure versus those with persistent iASD 6 months post-M-TEER. Patients with a relevant iASD 1-month post-M-TEER, who were treated conservatively in the randomized controlled MITHRAS trial, underwent clinical follow-up including transesophageal echocardiography 6 months post-M-TEER. Overall, 36 patients (median 77 [interquartile range 65–81] years; 36% women) completed the 6-months follow-up. Six (17%) patients had a spontaneous closure of the iASD. The eccentricity index of the iASD 1-month after M-TEER was the strongest predictor for spontaneous closure (Odds ratio 3.78; 95% confidence interval 1.26–11.33, p = 0.01) and an eccentricity index of < 1.9 provided a sensitivity of 77% at a specificity of 83% for iASD persistence (Area under the curve 0.83, p < 0.001) within 6-months post M-TEER.At follow-up, a numerical difference in the endpoint of HF hospitalization between the spontaneous closure and the residual shunt group (0% vs. 20%, p = 0.25) was observed. The eccentricity of the iASD was the strongest predictor for spontaneous closure at 1-months and an eccentricity index of < 1.9 is associated with a high persistence rate for 6 month after M-TEER. Clinical Trial Registration ClinicalTrials.gov https://clinicaltrials.gov/ct2/show/NCT03024268 Identifier: NCT03024268. Graphical Abstract a (red) is reflecting the mayor lengthwise dimension and b (blue) the mayor oblique dimension. The eccentricity index is calculated by dividing a through b. (Open circle) is depicting an example for a round iASD and (Open rhombus) an example for an eccentric iASD 1 month after M-TEER.