Is it possible for a person having a heart attack and not know it?

Yes: Everyone's pain perception and threshold is different. Some, like someone with diabetes, may have their pain thresholds affected so they do not feel the normal warning signs or any pain. The symproms may also present as other than chest pain; shoulder or jaw pain, nausea/vomiting, shortness of breath and other symptoms that may make diagnosis difficult.

Absolutely: One of the best known research studies to examine this issue was the framingham heart study. In the study, several thousand people were followed for 50 years. One quarter of all patients with heart attacks did not have symptoms that were recognized by the patient and were not admitted to the hospital.