Coronary Angioplasty (cont.)
How does coronary artery disease develop?
Arteries that supply blood and oxygen to the heart muscles are called coronary arteries. Coronary artery disease (CAD) occurs when cholesterol plaque (a hard, thick substance comprised of varying amounts of cholesterol, calcium, muscle cells, and connective tissue, which accumulates locally in the artery walls) builds up in the walls of these arteries, a process called arteriosclerosis. Over time, arteriosclerosis causes significant narrowing of one or more coronary arteries. When coronary arteries narrow more than 50 to 70%, the blood supply beyond the plaque becomes inadequate to meet the increased oxygen demand during exercise. Lack of oxygen (ischemia) in the heart muscle causes chest pain (angina) in most patients. However, some 25% of patients experience no chest pain at all despite documented ischemia, or may only develop episodic shortness of breath instead of chest pain. These patients have "silent angina" and have the same risk of heart attack as those with angina. When arteries are narrowed in excess of 90-99%, patients often have angina at rest (unstable angina). When a blood clot (thrombus) forms on the plaque, the artery may become completely blocked, causing death of a part of the heart muscles (heart attack, or myocardial infarction).
The arteriosclerotic process can be accelerated by smoking, high blood pressure, elevated cholesterol levels, and diabetes. Patients are also at higher risk for arteriosclerosis if they are older (greater than 45 years for men and 55 years for women) or if they have a positive family history of coronary heart disease.
How is coronary artery disease diagnosed?
The resting electrocardiogram (EKG) is a recording of the electrical activity of the heart, and can show changes indicative of ischemia or heart attack. Often, the EKG in patients with coronary artery disease is normal at rest, and only becomes abnormal when heart muscle ischemia is brought on by exertion. Therefore, exercise treadmill or bicycle testing (stress tests) are useful screening tests for patients with significant coronary artery disease (CAD) and a normal resting EKG. These stress tests are 60 to 70% accurate in diagnosing significant CAD.
If the stress tests are not diagnostic, a nuclear agent (Cardiolite® or thallium) can be given intravenously during stress tests. Addition of one of these agents allows imaging of the blood flow to different regions of the heart, using an external camera. An area of the heart with reduced blood flow during exercise, but normal blood flow at rest, signifies substantial artery narrowing in that region.
Stress echocardiography combines echocardiography (ultrasound imaging of the heart muscle) with exercise stress testing. It is also an accurate technique for detecting CAD. When a significant narrowing exists, the heart muscle supplied by the narrowed artery does not contract as well as the rest of the heart muscle. Stress echocardiography and thallium stress tests are 80% to 85% accurate in detecting significant CAD.
When a patient cannot undergo an exercise stress test because of neurological or arthritic difficulties, medications can be injected intravenously to simulate the stress on the heart normally brought on by exercise. Heart imaging can be performed with either a nuclear camera or echocardiography.
Cardiac catheterization with angiography (coronary arteriography) is a technique that allows x-ray pictures to be taken of the coronary arteries. It is the most accurate test to detect coronary artery narrowing. Small hollow plastic tubes (catheters) are advanced under x-ray guidance to the openings of coronary arteries. Iodine contrast "dye" is then injected into the arteries while an x-ray video is recorded. Coronary arteriography gives the doctor a picture of the location and severity of narrowed artery segments. This information is important in helping the doctor select medications, PCI, or coronary artery bypass graft surgery (CABG) as the preferred treatment option.
A newer, less invasive technique is the availability of high speed CT coronary angiography. While it still involves radiation and dye exposure, no catheters are needed in the arterial system, which does decrease the risk of the procedure somewhat. This is a very new modality, and its role in the evaluation and management of CAD is still evolving. It is important to remember that risk of serious complications from conventional coronary angiography is very low (well under 1%). For more, please read the CT Coronary Angiography article.
Next: How is CAD treated? »
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