BACKGROUND: Transseptal puncture (TSP) is the first step in pulmonary vein isolation and catheter ablation, as well as in left atrial appendage closure in atrial fibrillation. Although TSP has been reported to be successful in patients with device closure of interatrial septal communications, questions pertinent to its feasibility in patients with large devices still remain. We sought to determine whether a "safe zone" for TSP could be visualised by computer tomography (CT), especially if larger device sizes for interatrial septal communication closure (IASC-C) had been used. METHODS: Retrospective observational study of 20 patients who underwent CT for de novo chest pain occurring after IASC-C or as a diagnostic test for suspected or proven coronary artery disease (CAD). Clinical follow-up was for 20.5 ± 17.6 (6-84) months. CT was done18 ± 10 (2-28) weeks after IASC-C. Device size and dimensions of both atria in the long and short axes were measured, as was the minimal distance of the device edge to the inferior and inferoposterior atrial floor. RESULTS: The calculated minimal distance from the device edge to the inferior aspect (at 6 o'clock) of the (right or left) atrial floor was 7.2 ± 6.5 (0-27) mm while that to the inferoposterior aspect (at 07:30 o'clock) was 5.3 ± 4.2 (0-15) mm. In both locations, a distance of >6 mm was documented in ten patients (50%) while in nine patients (45%) a space of <6 mm was shown in both locations. There was no correlation between atrial dimensions or device size and minimal device distance to either wall. CONCLUSION: With the exception of cases with the smallest devices (18 and 20 mm), neither device size nor atrial dimensions allow us to predict the feasibility of TSP in patients with a clamshell-type interatrial septal device in place, so that CT may be of help in determining whether a safe puncture space does exist in these patients.