A 33-year-old male patient with a history of dextrocardia, L-transposition of the great arteries, ventricular septal defect (VSD), atrial septal defect (ASD), treated with repeated surgical interventions (arterial switch procedure, closure of the VSD and of the ASD, and surgical relief of the subpulmonary obstruction), and paroxysmal atrial fibrillation is admitted to the Cardiology Department for an episode of palpitations accompanied by dyspnea, anxiety, and lightheadedness that had occurred 20 min prior to his presentation to the hospital. His ECG recorded at admittance is presented in Fig. 1.