A 78 year old male with a history of diabetes, hypertension, dyslipidemia, and atrial fibrillation presents to the emergency room with complaints of nausea, vomiting, generalized weakness, and an episode of syncope. He states that he has had no appetite for 3 days now. He was watching TV and passed out for about 2 minutes with some preceding dizziness. His temperature is 37.0, blood pressure 100/60, heart rate 50, respirations 20, and oxygen saturation 95% on room air. Physical examination reveals clear lung sounds, an irregularly irregular, bradycardic rhythm with no murmurs, and no lower extremity edema. His laboratory studies reveal a creatinine of 2.5 and a potassium of 6.1. Blood glucose is normal. His ECG is below. Which of the following would best aid in his diagnosis?