It is when the rhythm of the heart changes from normal sinus rhythm to a different rhythm.

It's the same as a myocardinal infarction when the a part of the heart stops receiving enough oxygen and blood. That section will become damaged permanantly and die. Depending on the area affected, it could be the entire heart. The person will experience angina (chest pain) and shortness of breath and will need to go the hospital urgently. If left untreated then it becomes worse and worse and can lead on to other heart conditions. Usually it is the cornary artery that becomes blocked due to fat narrowing it and causing a heart attack. Medication is given at the hospital to dilate the artery and increase blood and oxygen to the heart.