A patient in their 70s presented to the emergency department with a history of syncope that occurred 7 days before admission without preceding signs or symptoms. Since then, the patient had been experiencing fatigue, mainly from exertion. The patient had a history of hypertension, heart failure, and aortic valve replacement surgery in 2020, complicated on follow-up by prosthetic valve thrombosis. The patient had been taking warfarin, furosemide, metoprolol succinate, and spironolactone. Initial evaluation revealed a heart rate of 61 beats/min, blood pressure of 114/74 mm Hg, oxygen saturation of 96%, and temperature of 36.6 °C. The physical examination was unremarkable, and a 12-lead electrocardiogram (ECG) was obtained (Figure).