Bradycardia (hea rate of 55) with atrial fibrillation suggests the probable diagnosis of a complete hea block. This patient is most likely to be on digitalis for hea failure due to mitral regurgitation. Digitalis intoxication is a common cause of complete hea block. Ref: Harrison's Internal Medicine 16th Edition, Page 1345, 1375, 1394; Essentials of Medical Pharmacology, KD Tripathi, 5th Edition, Chapter 35, Page 462 &463.