A 78-year-old male patient with a history of long-standing persistent atrial fibrillation, implanted with a single-chamber pacemaker for tachycardia–bradycardia syndrome (before atrial fibrillation became permanent), is admitted to the Cardiology Department due to an episode of palpitations with a sudden onset and regular rhythm, accompanied by dyspnea, lightheadedness, and chest pain, which had started 20 min before arriving at the Emergency Department. His ECG recorded at admittance to the Cardiology Department is presented in Fig. 1.