What Causes Chest Pain, Rapid Heart Beats After Being Treated For Multiple Myeloma?

My husband has stage 111 multiple myleoma. He completed 10 radiation tx's a week ago. He experienced 2 nights of chest pain relieved somewhat by morphine. Yesterday he woke in severe pain in his chest radiating down both arms. At the ER he was in afib with HR as high as 150..he was treated with a cardizem drip which brought down the HR but he did not convert to normal rhythm. This morning his pain returned and did the HR. My question is ...could the radiation trigger the heart problems. He has never had any kind of heart condition. He has had COPD for many years, but it is managed well and he hasn't had any problems in 6months. None smoker. A stress test cannot be done yet..and a cardiac cath is not in the cards due to the myeloma.