Coronary Artery Disease Screening Tests
What is coronary artery disease?

Coronary artery disease (CAD) is arteriosclerosis of the inner lining of the blood vessels that supply blood to the heart. CAD is a common form of heart disease and is a major cause of illness and death. CAD begins when hard cholesterol substances (plaques) are deposited within a coronary artery. (The coronary arteries arise from the aorta, which is adjacent to the heart.) The plaques can cause a tiny clot to form which can obstruct the flow of blood to the heart muscle.
Symptoms of CAD include
1) chest pain (angina pectoris) from inadequate blood flow to the heart;
2) heart attack (acute myocardial infarction), from the sudden total blockage of a coronary artery; or
3) sudden death, due to a fatal rhythm disturbance

What is the purpose of screening tests for CAD?
In many patients, the first symptom of CAD is myocardial infarction or sudden death, with no preceding chest pain as a warning. For this reason, doctors perform screening tests to detect signs of CAD before serious medical events occur. Screening tests are of particular importance for patients with risk factors for CAD. These risk factors include a family history of CAD at relatively young ages, an abnormal serum cholesterol profile, cigarette smoking, elevated blood pressure (hypertension), and diabetes mellitus.

What are common initial screening tests for CAD?
Initial screening for CAD commonly involves stressing the heart under controlled conditions. These stress tests are able to detect the presence of flow-limiting blockages in the coronary arteries, generally in the range of at least a 50% reduction in the diameter of at least one of the three major coronary arteries. There are two basic types of stress tests; those that involve exercising the patient to stress the heart (exercise cardiac stress tests), and those that involve chemically stimulating the heart directly to mimic the stress of exercise (physiologic stress testing). Physiologic stress testing can be used for patients who are unable to exercise.

Exercise Cardiac Stress Test (Treadmill Stress Test)
Exercise cardiac stress testing (ECST) is the most widely used cardiac stress test. The patient exercises on a treadmill according to a standardized protocol, with progressive increases in the speed and elevation of the treadmill (typically changing at three minute intervals). During the ECST, the patient's electrocardiogram (EKG), heart rate, heart rhythm, and blood pressure are continuously monitored. If a coronary arterial blockage results in decreased blood flow to a part of the heart during exercise, certain changes may be observed in the EKG, as well as in the response of the heart rate and blood pressure.

The accuracy of the ECST in predicting significant CAD is variable, depending in part on the "pre-test likelihood" of CAD (also known as Bayes' theorem). In a patient at high risk for CAD (e.g.: advanced age, multiple coronary risk factors), an abnormal ECST is very predictive of the presence of CAD (over 90% accurate). However, a relatively normal ECST may not reflect the absence of significant disease in a patient with the same risk factors. Conversely, in a low- risk patient, a normal ECST is very predictive of the absence of significant CAD (over 90% accurate), but an abnormal test may not reflect the true presence of CAD (so-called "false-positive ECST"). The ECST may either miss the presence of significant CAD, or be a false-positive test, due to a variety of cardiac circumstances, which may include:

An abnormal EKG at rest, which may be due to abnormal serum electrolytes, abnormal cardiac electrical conduction, or certain medications, such as digitalis;
Heart conditions not related to CAD, such as mitral valve prolapse or hypertrophy (increased size) of the heart; or
An inadequate increase in the heart rate and/or blood pressure during exercise.

What if the initial ECST does not clarify the diagnosis?
When the doctor determines that the results of the ECST do not accurately reflect the presence or absence of significant CAD, additional tests are often used to clarify the condition. These additional options include radionucleide isotope injection and ultrasound of the heart (stress echocardiography) during the stress test.

Radionucleide Stress Test
Radionucleide stress testing involves injecting a radioactive isotope (typically thallium or cardiolyte) into the patient's vein after which an image of the patient's heart becomes visible with a special camera. The radioactive isotopes are absorbed by the normal heart muscle. Nuclear images are obtained in the resting condition, and again immediately following exercise. The two sets of images are then compared. During exercise, if a blockage in a coronary artery results in diminished blood flow to a part of the cardiac muscle, this region of the heart will appear as a relative "cold spot" on the nuclear scan. This cold spot is not visible on the images that are taken while the patient is at rest (when coronary flow is adequate). Radionucleide stress testing, while more time-consuming and expensive than a simple ECST, greatly enhances the accuracy in diagnosing CAD.

Stress Echocardiography
Another supplement to the routine ECST is stress echocardiography. During stress echocardiography, the sound waves of ultrasound are used to produce images of the heart at rest and at the peak of exercise. In a heart with normal blood supply, all segments of the left ventricle (the major pumping chamber of the heart) exhibit enhanced contractions of the heart muscle during peak exercise. Conversely, in the setting of CAD, if a segment of the left ventricle does not receive optimal blood flow during exercise, that segment will demonstrate reduced contractions of heart muscle relative to the rest of the heart on the exercise echocardiogram. Stress echocardiography is very useful in enhancing the interpretation of the ECST, and can be used to exclude the presence of significant CAD in patients suspected of having a "false-positive" ECST.