The specific language you are looking for here is "indications for use". The way the FDA works, you cannot advertise (even directly) a medical device for a usage you have not indicated in your filing and backed up with the required evidence. The level and type of evidence will depend on the type of filing and claims. A medical professional can advise for a different use (called "off label") but you, as the device developer, cannot even hint at it.

Aside: as others note, though, you can't really detect heart attacks well from something this simple.