Myocardial infarction, commonly known as a heart attack, is damage to the heart muscle caused by a lack of blood supply due to factors like high blood pressure, cholesterol, smoking, obesity or a sedentary lifestyle. Symptoms include chest pain or discomfort, shortness of breath, nausea and sweating. Diagnosis involves tests like ECG, blood tests, echocardiogram and stress tests. Complications can include heart failure, arrhythmias, cardiogenic shock or sudden cardiac arrest. In an emergency, call for help, take aspirin if recommended, and perform CPR if needed.