A heart attack is caused by a complete obstruction of an artery supplying the heart muscle.  It is manifested by severe, crushing pain in the chest behind the breastbone that is not relieved with nitroglycerin.  It is accompanied by profuse sweating and nausea, as well as shortness of breath.  If you experience these symptoms, you should call 911 immediately.  Sometimes, precious time is wasted because patients think they are experiencing indigestion, and delay medical attention.