Inappropriate sinus tachycardia or…something else?

We report the case of a 45-year old female patient with a past medical history of rheumatoid arthritis who presented to our cardiology department with a suspicion of inappropriate sinus tachycardia. Echocardiography showed a nondilated left ventricle with a preserved ejection fraction. A careful reinterpretation of her 12-lead ECG reoriented the diagnosis toward an incessant atrial tachycardia.