Is an enlarged heart and arrhythmia related related ischemic heart disease or exposure to agent orange?
Maybe. Maybe not. Cardiomegaly is the term used for an enlarged heart and that can be caused by many different things. Cardiomegaly isn't a disease in and of itself, it's a symptom of an underlying problem.
The underlying problem may be a virus, hypertension, valve problems, IHD and many more diseases that can lead to an enlarged heart. An arrhythmia (an erratic heart beat) may be caused by the enlarged heart and then may require a pacemaker or an AICD to stabilize the heart rhythm.
The only way to know the underlying cause of your enlarged heart is to ask your cardiologist. If the doctor says that Ischemic Heart Disease caused the heart to enlarge, and you're rated for IHD, then you may claim the enlarged heart and other conditions related to that as secondary to the rated IHD. If the doctor says that the enlarged heart is due to any other condition than IHD, VA isn't likely to accept that as service connected.