Difficulty breathing is a good reason to call 911, whether it indicates a heart attack or not. But it's good to understand that shortness of breath isn't just associated with lung activity; it could also be a sign that your heart is calling for help.
Experiencing "shortness of breath" can mean several things in terms of how it feels -- it could mean you find yourself struggling to breathe. It could mean you suddenly feel the need to take deep, long breaths -- and maybe (but not always) you have trouble doing so. It could also mean your breathing speeds up and/or your breaths are shallow.
As with the other symptoms on this list, it's especially important that you call for help without wasting time if you experience breathing irregularities and also have another symptom of heart attack, like pain in your back or arms.
That other symptom might also be a racing heart, which is next up on the list of heart attack warning signs.