The patient was a 66-year-old widowed woman who developed cognitive decline, memory impairment, and mildly impaired ability of activity of daily living, associated with neuropsychiatric symptoms for 1–2 years before her first visit to our geropsychiatric clinic. She revealed exacerbated psychomotor agitation and aggression to the nursing home staff, with the delusional belief that somebody in the nursing home stole her laxative medications and money several days before the visit. She was therefore admitted to our inpatient ward. She had type 2 diabetes mellitus, with 10 units of Mixtard insulin injection for control, and no other significant medical, psychiatric, or family history. Her laboratory work-up, including complete blood count, liver, renal, and thyroid panels, vitamin B12 and folic acid levels, revealed unremarkable findings. The brain computerized tomogram showed cortical atrophy and compatible ventricle dilatation. The baseline electrocardiogram was normal sinus rhythm. Her Clinical Dementia Rating Scale was 1.0 and the Mini-Mental State Exam was 20. Based on the diagnosis of Alzheimer dementia with delusions, risperidone 2 mg per day was given on Day 1 of hospitalization, and rivastigmine, an acetylcholinesterase inhibitor, was started at 1.5 mg twice a day, as a cognitive enhancer from Day 3. However, on Day 11, she experienced irregular and rapid heartbeat with marked chest discomfort; the electrocardiogram showed AF with rapid ventricular response, 2-to-1 conduction, at 160–170 beats per minute. Emergency cardiology consultation was requested. Since the AF persisted after diltiazem 10 mg intravenous drip and codarone 300 mg drip at a rate of 2 mg/min treatment and digoxin 0.5 mg intravenous drip, emergent percutaneous catheter ablation was performed, and her cardiac rhythm returned to normal sinus rhythm 4 hours later. We discontinued rivastigmine because of the arrhythmia episode, and she has shown no more cardiac arrhythmia in the 4 years of follow-up, and relative control in her neuropsychiatric symptoms.