FCA1-0551

An 80-year old female was taken to the emergency room for chest pain. She has a medical history of coronary artery disease and previous episodes of atrial fibrillation. She was immediately attached to the cardiac monitor, which showed tachycardia at 148 beats per minute. The 12-lead ECG revealed atrial fibrillation. Digoxin was given as an anti-arrhythmic at 500 micrograms, which is higher than the maintenance dose routinely given. Why is this so?