Every year, 605,000 Americans have a first heart attack — and many come as a surprise. A study in the journal Circulation showed that two-thirds of people who have a heart attack have undiagnosed heart disease.

But is it really a total surprise?

A new study in The Lancet Digital Health shows that half of people who have what's called "sudden cardiac arrest," actually experience a symptom of its impending arrival 24 hours before it happens. Using data from two ongoing studies, Prediction of Sudden Death in Multi-Ethnic Communities (PRESTO) Study in Ventura County, Calif., and the Oregon Sudden Unexpected Death Study (SUDS), based in Portland, Ore., the researchers found that for women the warning sign was shortness of breath and for men it was chest pain and pressure.

Many women may experience shortness of breath off and on before a heart attack, but assume it is from allergies, fatigue, or not worth paying attention to.

And chest pains and pressure may come and go over time, too, making it easy for guys to dismiss them as "nothing."

That’s a big mistake. If you have such a symptom, even if you haven't been diagnosed with heart disease, it's smart to get a medical evaluation ASAP from your doctor, at an urgent care facility, or an emergency room. 

And to help prevent such an emergency, have regular screenings for blood pressure, LDL cholesterol, blood sugar, weight, and stress.