Greetings. Welcome to HCM and thank you for your question. I understand your concern. Usually, during flu and other infectious issues in the organism, that are accompanied with fever, there is a hyperdinamic state and the organism has to regulate itself to cope with this strain. One of the compensatory mechanisms in hyperdinamic states is the rapid heart rate. Furthermore, the centre of temperature regulation in the human body is settled in the hypothalamus, the highest regulator in the brain. Due to this engagement, also the heart rate goes up. As for the other concern, that heart beats fast when moving around, in case of dilated cardiomyopathy, it is accompanied with heart insufficiency. This insufficient heart cannot cope as easily as a normal heart with a workload put onto it. Therefore, also here, the heart rate compensates as much as it can, for the strain put onto the heart. To illustrate it more figuratively to understand it more clearly, imagine the heart is a donkey, and an enlarged heart is like an old donkey, and it is clear that the two donkeys cannot cope in the same way with the same loads put on their back. As about the atrial fibrillation, I would need further information to address your concern, because with the information provided, I am not clear if these is paroxysmal or chronic, under medication or not, rate controlled or rhythm controlled, arrhythmic heart rate. However, if this is an enlarged heart caused by ischemic reason, then an antiarrhythmic such as amiodarone, would be appropriate to treat or prevent this supraventricular arrhythmia. I hope I was helpful with my answer. Feel free to ask again. Take care. Regards, Dr. Meriton