It may be worth looking into emergency release systems to put her at her ease. It may not feel 'like bondage' to you if she can escape at any time but you can see it as a physical equivilent to a safeword - she can only use it when it is an emergency.

Check self bondage sites for hints on how a sub who is tied in certain ways can get out quickly. Make sure there is a sharp knife where she can reach it to cut ropes in an emergency. Use velcro fastening bondage cuffs that she can pull out of easily. Get a pair of 'quick release hand cuffs'.

Another thing that may put her at her ease is if you go to a doctor and have regular checks on your heart (which you may be having already). Tell him that you are engaging in hard physical activity (which BDSM is, most of the time) and want a check to make sure you are ok for it. You can also look at health, fitness and diet in general - get him to suggest a regime which is suitable for you to maintain a healthy enough body to minimise the risks in the future.

The main concern is that, once the heart starts going wrong, it is more likely to keep going wrong. You can manage it with diet, exercise and drugs to minimise the risk but you cannot eliminate the risk altogether. Frankly, any of us, at any time, can have a heart attack no matter what your fitness or age but you are now in a high risk category and her fears are not unjustified.