With the development of surgical techniques and the experience of performers, the quality and number of open-heart operations performed has increased dramatically. However, despite the success achieved, openheart surgery may be associated with the development of cardiac arrhythmias. Studies have shown that pressure overload of the atria, the presence of an atriotomic scar and the resulting delayed conduction zones predispose to the development of complex rhythm disorders. To produce effective point effects, it is necessary to clearly represent the spread of excitation during tachycardia, the zone of critical conduction – the so-called breakthrough. The presented clinical case arouses interest in the possibility of determining the mechanism and localization of the critical zone of arrhythmia in a patient after surgical treatment of a congenital heart defect (ventricular septal defect, open ductus arteriosus) and the lack of access through the femoral veins.