Why do people have a heart attack and why can't a doctor always tell who will have a heart attack?
One of the best descriptions that I ever heard for atherosclerosis likened it to acne. Although my first thought was "yuck", but when you think of it, atherosclerosis, like acne is a systemic problem of inflammation. Treating a single pimple, breakout or a single coronary artery plaque is not sufficient to treat the whole problem. This helps to explain the problem that doctors have. We may know that you have acne, but we can't predict where the first pimple will be. We also may know that you have plaque in your coronary arteries, but we can't tell when the first plaque will fissure or break, causing a heart attack.
How do doctors know that you have arteriosclerosis? Even at the young age 18, we begin to see changes related to cholesterol deposits in the arteries of people on a western diet. We cannot see these changes with normally-used diagnostic tests given to live people. As we age, the walls of the arteries thicken and so do these deposits. We are able to see these changes with some of our more advanced (and more expensive) technologies. We can also measure changes in the way that the artery walls relax to get an idea of how much the arteriosclerosis affects the blood pressure by doing ultrasound of the arm vessels. Ultrafast CT (computerized tomography, a form of x-ray) scans with contrast material can show us some blockages in the coronary arteries (but they expose the patient to more radiation that coronary angiography and also require a contrast agent to be injected intravenously that can bother the kidneys, mostly if they have previously been injured). MRI (magnetic resonance imaging) can also be used to look at the coronary arteries, but this technique is still in its infancy for this indication. Ultrasound is quite safe, and we can look at the carotid arteries (in the neck) to look at vessels to see how thick the walls are and how much plaque is present, but this technique doesn't really help in looking at the coronary arteries.
The heart receives blood oxygen and nutrients to survive through a small system of blood vessels called the coronary arteries. When plaque builds up (like sludge) in these vessels, they can gradually become narrow and limit flow to the heart muscle. While we used to believe that this plaque built up continuously and gradually, we have found out that this is not always the case. Indeed, even in patients who have heart attacks within the months following coronary angiograms, the vessel responsible for the damage is not always the one that we would have predicted.
While it would seem logical that the 60 or 70 per cent blockage would have progressed it often seems that it is a small plaque that was not significant on the last study that now has caused a problem. In fact, plaque (as opposed to the sludge in our home pipes) builds up within the wall of the vessel, and sometimes, it can "fissure" causing a tiny break on the inner line of the "pipe". When it does this, the body recognizes the area as if it were an infection or inflammation. White cells from the blood stream, and platelets collect to limit the problem but end up clogging the vessel and forming a clot. It is this process that limits the blood flow to the heart muscle and causes a heart attack.