The normal human heart is a strong, muscular pump a little larger than a fist. It pumps blood continuously through the circulatory system. Each day the average heart “beats” (or expands and contracts) 100,000 times and pumps about 2,000 gallons of blood. In a 70-year lifetime, an average human heart beats more than 2.5 billion times.
The heart pumps blood through a circulatory system, which is a network of elastic tubes through which blood flows as it carries oxygen and nutrients to all parts of the body. The circulatory system includes the heart, lungs, arteries, arterioles (small arteries), and capillaries (minute blood vessels). It also includes venules (small veins) and veins, the blood vessels through which blood flows as it returns to the heart.
The circulating blood brings oxygen and nutrients to all the organs and tissues of the body, including the heart itself. The blood also picks up waste products from the body's cells. These waste products are removed as they're filtered through the kidneys, liver and lungs.
Over time, the coronary arteries which supply the heart muscle with blood can become clogged. One cause of clogged arteries is due to a condition called atherosclerosis, or hardening of the arteries. Atherosclerosis causes a constriction of the inner lumen of the affected artery when the lumen of the arteries become more narrow due to a pathological accumulation of cells, fats and cholesterol called plaque. The descriptive term given to this narrowing of the coronary arteries is “stenosis.” Stenosis means constriction or narrowing. A coronary artery that is constricted or narrowed is referred to as stenosed. When stenosis of the coronary artery is sufficient to deprive the heart muscle of the oxygen levels necessary for cell viability, the result is typically myocardial infarction, typically referred to as a heart attack.
A heart attack occurs when the blood supply to part of the heart muscle itself, the myocardium, ceases or is severely reduced. This occurs when one or more of the arteries supplying blood to the heart muscle (coronary arteries) becomes partially or completely obstructed by plaque stenoses. If cessation of the blood supply occurs for a long time, heart muscle cells suffer irreversible injury and die. Severe disability or death can result, depending on how much heart muscle is damaged.
Coronary artery bypass surgery is a heart operation used to treat coronary artery disease. In coronary artery bypass surgery a blood vessel is used to go around or “bypass” clogged coronary (heart) arteries. During the “bypass” procedure, a blood vessel from the patient's chest or leg is used as the “bypass” conduit. For venous “bypass” grafts, one end of the vessel is attached to the aorta (the large artery coming out of the heart) and the other end is attached to the coronary artery below the point where it's clogged. Once the clog has been bypassed, blood can once again flow through the bypass graft to the heart, in a manner that prevents ischemia and infarction. Almost half a million coronary bypass operations are performed each year in the USA.
Another procedure for opening clogged coronary arteries is to perform percutaneous transluminal coronary angioplasty, or balloon angioplasty. Balloon angioplasty is an established and effective therapy for some patients with coronary artery disease. Balloon angioplasty is used to dilate (widen) arteries narrowed by plaque. During the procedure, a catheter with a deflated balloon on its tip is passed into the narrowed part of the artery. The balloon in then inflated, and the narrowed area is widened. Balloon angioplasty is a less traumatic and less expensive alternative to bypass surgery for some patients with coronary artery disease. However, in 25 to 30 percent of patients the dilated segment of the artery renarrows (restenosis) within six months after the procedure. The patient may then require either to repeat the balloon angioplasty or to undergo coronary bypass surgery.
One approach to preventing restenosis has been to insert a “stent” across the stenosed area of coronary artery. A stent is a metallic wire mesh tube that is used to prop open an artery that has been recently dilated using balloon angioplasty. The stent is collapsed to a small diameter, placed over an angioplasty balloon catheter and moved into the area of the blockage. When the balloon is inflated, the stent expands, locking in place to form a rigid support (structural scaffolding) which holds the artery lumen open. The stent remains in the artery permanently to help improve blood flow to the heart muscle. However, reclosure (restenosis) remains an important issue with the stent procedure.
Several approaches have been taken to reduce the occurrence of restenosis associated with the stent procedure. Stents have been impregnated with drugs and chemicals that emit radiation (gamma-rays) in an attempt to reduce the frequency of restenosis. Also, drug eluting stents have been used in an attempt to reduce the occurrence of restenosis. However, a need still exists for additional safe and effective treatments to prevent restenosis after the placement of an intravascular stent.