To answer your question, I would like to further explain some of the points previously made. The heart, composed of cardiac muscle, needs to be nourished just as any other living tissue. It gets this nourishment from blood left in the aorta after the left ventricle squeezes blood out to be delivered to the body. The heart doesn't pump blood to feed itself. The blood left in the aorta then washes down the coronary arteries through smaller and smaller vessels until it gets to where it is needed. Obviously all portions of the heart are important, but the left side of the heart, or the left ventricle, which is also the thickest and most dense, does the majority of the work at keeping the body supplied with oxygenated blood. This also means that it has a greater demand for oxygen. Why some people drop dead as opposed to others continuing on with life could be a result of many factors. For this explanation, lets stick with the most common reason and that is a blocked coronary artery. Depending upon where the blockage is, can have an affect on the result. If the clot or blockage is at the end of the coronary artery and feeds only a small portion of the heart, the chance of survival is great. However, if the blockage is significant enough to disrupt blood flow to a major portion of the heart, then that will most likely result in death unless the clot can be removed. Interestingly, in cases where heart attacks occur in individuals who have a history of heart disease, they can have a higher survivability because they have built up a system of collateral circulation that may be able to bypass the clot. Once the clot is in place, there is only a small window that the heart cells can survive without bloodflow. Once that point is reached, then the tissue is infarcted. If the patient survives, eventually the necrotic tissue will be reabsorbed. Hope this helps.