I don't know about any research that has proven a link between anxiety and dementia, but I can offer anecdotal evidence about my mother. My mother was diagnosed with dementia in January, but has clearly been suffering from it for at least the past several years, and probably longer.
Sometime in the past two years, my mother began reporting she was having a difficult time with anxiety. Everything LS reports, is absolutely true about my mother. I now understand that the dementia was a direct cause of the anxiety, because my mother was struggling so much to cope and not appear that there was a problem, that it made her anxious, all the time. Everything made her anxious: shopping, getting the mail, trying (and failing) to pay bills, remembering directions, chores, going to the doctor, going to the chemist, literally everything made her anxious, ALL THE TIME. Her GP did give her medication, which might have helped, had she been able to remember to take it.
Since her hospitalization and subsequent move into a care home, the anxiety has disappeared. I think this is a combination of better medications for her depression and anxiety, but also because the stress of daily living has been reduced. No more mail, laundry, cleaning, shopping, cooking, paying bills, et cetera.
Your aunt may well feel anxious because she can't remember things, or is struggling to remember. She may also be confusing physical symptoms and anxiety with each other, for example, if she feels short of breath or unwell from her cardiac condition. She could also be experiencing something physical, related to the heart attack last year, or something else entirely. I would definitely talk to her doctor. It couldn't hurt to get the GP to assess her both physically and also to try medication for the anxiety.
I hope you're able to help sort something out for your aunt. Good luck.