Abstract
A 47-year-old man was admitted to the Santa Casa de Misericórdia Hospital in Belo Horizonte, Brazil, with recurrent signs and symptoms of tachycardia, palpitation and fatigue. During medical examination, an electrocardiogram and 24-hour Holter monitoring were performed, which identified a predominant atrial flutter rhythm and, after transthoracic echocardiography, the patient was diagnosed with cor triatriatum sinister. The condition is a rare congenital heart disease characterised by the presence of a fibrous membrane that divides the left atrium into two separate chambers. The disease is especially evident during childhood; however, some cases show no signs until adulthood, which makes the presentation even more unique. In this case, clinical medication and electrical cardioversion were chosen as treatments, which caused progression to sinus rhythm and improvement of symptoms. Therefore, due to the stability of the condition, continuous follow-up with a cardiologist was implemented.