A 76-year-old woman presents to the office for evaluation of symptoms of weight loss, anxiety, and palpitations. The symptoms started 1 month ago, and are involuntary. She has no prior history of anxiety or palpitations and her only medical history is hypertension for which she is taking losartan.On examination, the blood pressure is 120/70 mm Hg, heart rate is 100/min and regular. On auscultation the heart sounds are normal and the lung clear. There is a thyroid goiter, warm skin, and a fine tremor in her hands. Which of the following is the most likely cardiac finding?