Any cover for the USA for a senior citizen is going to be extremely expensive (with or without heart problems).
However this specialist broker is recommended on the websites of several leading charities (serving people with disabilities or medical condition) and has helped quite a few AB members, so it might be worth a phone call to see what they can offer:
http://www.able2travel.com/
Boots Travel Insurance also doesn't have an upper age limit and can cover people with pre-existing conditions:
http://www.bootstravelinsurance.com/
(A phone call, rather than trying to get an online quote, would probably be best)