Coronary artery disease (CAD), also called heart disease or ischemic heart disease, results from a complex process known as atherosclerosis (commonly called "hardening of the arteries"). In atherosclerosis, fatty deposits (plaques) of cholesterol and other cellular waste products build up in the inner linings of the heart’s arteries. This causes blockage of arteries and prevents oxygen-rich blood from reaching the heart (ischemia). There are many steps in the process leading to atherosclerosis, some not fully understood.
Cholesterol and Lipoproteins. The atherosclerosis process begins with cholesterol and sphere-shaped bodies called lipoproteins that transport cholesterol.
- Cholesterol is a substance found in all animal cells and animal-based foods. It is critical for many functions, but under certain conditions cholesterol can be harmful.
- The lipoproteins that transport cholesterol are referred to by their size. The most commonly known are low-density lipoproteins (LDL) and high density lipoproteins (HDL). LDL is often referred to as "bad" cholesterol; HDL is often called "good" cholesterol.
Oxidation. The damaging process called oxidation is an important trigger of atherosclerosis:
- Oxidation is a chemical process in the body caused by the release of unstable particles known as oxygen-free radicals. It is one of the normal processes in the body, but under certain conditions (such as exposure to cigarette smoke or other environment stresses) these free radicals are overproduced.
- In excess amounts, they can be very dangerous, causing damaging inflammation and even affecting genetic material in cells.
- In heart disease, free radicals are released in artery linings and oxidize low-density lipoproteins (LDL). The oxidized LDL is the basis for cholesterol build-up on the artery walls and damage leading to heart disease.
Inflammatory Response. For the arteries to harden there must be a persistent reaction in the body that causes ongoing harm. Researchers now believe that this reaction is an immune process known as the inflammatory response.
There is growing evidence that the inflammatory response may be present not only in local plaques in single arteries but also throughout the arteries leading to the heart.
Blockage in the Arteries. Eventually the calcified (hardened) arteries become narrower (a condition known as stenosis).
- As this narrowing and hardening process continues, blood flow slows, preventing sufficient oxygen-rich blood from reaching the heart muscles.
- Such oxygen deprivation in vital cells is called ischemia. When it affects the coronary arteries, it causes injury to the tissues of the heart.
- These narrow and inelastic arteries not only slow down blood flow but also become vulnerable to injury and clot formation, which is what usually triggers a heart attack.
The End Result: Heart Attack. A heart attack can occur as a result of one or two effects of atherosclerosis:
- The plaque itself develops fissures or tears. Blood platelets stick to the site to seal off the plaque, and a blood clot (thrombus) forms. A heart attack can then occur if the blood clot completely blocks the passage of oxygen-rich blood to the heart.
- Less commonly, the artery becomes completely blocked by plaque itself, and ischemia becomes so extensive that oxygen-bearing tissues around the heart die.