A hospitalized patient in their 80s became tachycardic. The patient initially presented with changes in mental status and generalized weakness, which were attributed to urinary tract infection. An electrocardiogram (ECG) showed a narrow-complex tachycardia with nonspecific ST-T wave changes (Figure, A). The patient was asymptomatic and denied having palpitations, chest pain, and shortness of breath; blood pressure was normal. The patient had a remote history of coronary artery disease. A left carotid sinus massage (CSM) was performed at the bedside (Figure, B).