A middle-aged man with a history of ischemic heart disease presented at the emergency department having had a forty-minute long precordial pain at home and with an electrocardiogram showing a wide complex tachycardia with left bundle branch block shaped QRS. While preparing for urgent electrical cardioversion, the physician practiced a carotid sinus massage which helped to i) unveil the supraventricular origin of the arrhythmia (by slowing down heart frequency and thus displaying p waves previously hidden) and ii) to exclude that the aberrancy was the expression of a transmural ischemia, thanks to the narrowing of the QRS complex. The final diagnosis was atrial tachycardia. The patient consequently received amiodarone IV and was discharged in normal sinus rhythm. We emphasize the importance of correctly diagnosing any rhythm disorder before administering the treatment, if the patient’s clinical condition permits it, in order to ensure the best treatment in urgency and the most appropriate prophylaxis.