Hi there, It s likely not due to his a-fib, as this will often cause the heart rate to be increased. Is he taking a beta blocker such as metoprolol? If so, he may be on too high of a dose, as these medications tend to bring down the heart rate. A heart below 50 in a patient with coronary artery disease may be dangerous, however I d be hesitant to make changes to his medications without further knowledge. I d recommend calling your husband s doctor and letting him know about the low heart rate. Since he knows your husband well, he can likely just make a change over the phone, or tell you to come in sooner. I hope I have answered your question. Please feel free to contact me if you have any further questions or if you feel I haven t adequately answered your question.