Recurrent syncope: a slow heart rate?

A 70-year-old female patient was admitted to the geriatrics department with an overall deterioration, including lethargy, and with recurrent syncope. She had no palpitations or chest pain or other symptoms preceding the syncopal episodes. She is known with a history of an anterior myocardial infarction more than 20 years ago and psychotic depression. Her current medication consisted of lithium 400 mg, olanzapine 10 mg and nortriptyline 50 mg, all once a day, along with cardiac medication. Physical examination, initial laboratory tests and echocardiography did not demonstrate any abnormalities. A 12-lead surface electrocardiogram on admission is shown in Fig. 1. What is your diagnosis?