Key Tests for CAD

What else can be done to assess your risk for heart disease? Much of what has been discussed previously is risk factor identification and stratification based on traditional Framingham risk factors - the CAD risk factors that were identified in the landmark Framingham Heart Study. The purpose of the Framingham Heart Study was to learn why people develop cardiovascular disease, and how cardiovascular disease evolves and results in death in some patients. However, traditional Framingham risk factor scores fail to identify all patients who will develop a coronary event, and, conversely, many patients with Framingham risk factors will never have a coronary event. So, physicians and researchers are constantly seeking to understand CAD at a deeper level and to develop more robust testing methods for CAD. Some of the key tests include:

Laboratory Studies

Aside from Framingham risk factor data (lipid profiles), some other laboratory data may prove useful for risk stratification, such as C-reactive protein, homocysteine levels, and lipoprotein (a) values. These newer potential risk factors may have a role in sub-stratifying patients who are found to be at intermediate risk for coronary events.

Electrocardiograms

As discussed above, the ECG is a device designed to record the electrical activity of the heart. Occasionally an ECG will show changes in the heart's electrical activity that suggest prior MI. Alternatively, in the setting of a potential ACS (acute coronary syndrome), the ECG may show abnormal electrical activity that is caused by ongoing damage to heart muscle. The ECG is also used to assess for electrical changes that may occur when reduced blood flow to the heart occurs during exercise, due to the presence of coronary artery narrowing - this may occur during an exercise treadmill test, or stress test (see below).

Treadmill stress tests

Treadmill stress tests are the primary means by which the noninvasive evaluation of CAD has traditionally been performed. Unless plaque rupture occurs, CAD is usually silent until the cross sectional area of the coronary artery is narrowed by about 60-70%. With this degree of narrowing, during exercise, when the heart requires more oxygen and nutrients, the heart may be unable to deliver enough oxygen to keep up with the increased need caused by the extra work. Often this situation will produce chest pain (angina) or an anginal equivalent, such as shortness of breath. Even if this situation does not produce chest pain, the inability of the heart to deliver enough oxygen during exercise may create derangements during continuous monitoring of the ECG. These ECG changes will usually not be seen when the ECG is performed at rest. So, treadmill tests are designed to have patients exercise to their capacity to "unmask" any silent coronary disease. Blood pressure and heart rate are monitored while the patient exercises on a treadmill or stationary bicycle while the ECG is continuously monitored. The intensity of the exercise is slowly increased every 3 minutes according to a protocol - the most commonly used protocol for exercise treadmill testing is the Bruce protocol. The patient is monitored for the development of symptoms, changes in vital signs, and changes in the ECG. The test is stopped if the patient achieves 85% of the maximal predicted heart rate, if symptoms suggesting the presence of CAD develop, if vital signs deteriorate, or if the ECG shows potentially serious changes.

How reliable is the exercise treadmill test? On average, an exercise treadmill test is capable of diagnosing the presence of significant CAD in about 61-73% of patients with significant CAD. Accuracy is a bit higher if more than 1 artery is involved, and perhaps a bit less when only one artery is involved. Up to 20% of patients will have "false positive" results - the treadmill test will indicate a potential abnormality that will prompt further testing, and the additional tests will show that significant CAD is not present.

Other Non-Invasive Studies and Perfusion Imaging

A number of studies that provide measurements or data about the vascular system without entering the body to any significant extent, unlike percutaneous procedures or surgery (see invasive studies below) are termed "non-invasive" studies. Such techniques include measurements of carotid intimal-medial thickness, ankle-brachial indices, treadmill stress tests, echocardiography (including stress echocardiography), nuclear medicine perfusion imaging, computed tomography (CT) and magnetic resonance imaging (MRI).

Ankle-brachial indices (ABIs) are performed by measuring blood pressure at the ankle and in an arm while the patient is at rest, and then repeated after exercise. ABIs are often used to assess for peripheral vascular disease (PVD, or peripheral arterial disease). Normal values are approximately 1 to 1.1, whereas abnormal values are less than 1. Abnormal results indicate the blood flow in the measured leg is less than that of the arm, which implies that the vessels in the lower extremity are narrowed by atherosclerosis. What does detecting atherosclerosis in the leg have to do with CAD? It turns out that lower extremity atherosclerosis / PVD is associated with CAD. If a patient has atherosclerosis in one, easy to measure, arterial bed, they probably have it in another, harder to measure arterial bed. In fact, patients with proven PVD have a 4-6 fold increased rate of cardiovascular mortality compared to healthy patients

Carotid intimal-medial thickness (IMT) is a measurement of the wall thickness of the carotid artery, a major artery in the neck that supplies blood to the brain. Measuring the wall thickness of this vessel provides an overall assessment of the likelihood of atherosclerosis in other arterial circulations, such as the coronary circulation. This measurement is easily performed using ultrasound. However, much like ABI's, carotid IMT does not directly detect coronary artery atherosclerosis; rather, it is a marker, or surrogate, for the direct detection of coronary atherosclerosis. Much like other surrogate measures, abnormal IMT values may indicate a higher risk for CAD, but they do not directly assess for CAD, they cannot assess the total coronary atherosclerotic burden, and they cannot provide estimates of the severity or location of CAD when CAD is present. Currently, some physicians use measurements of the carotid IMT as a method to further categorize patients who would be considered at intermediate risk for CAD based on assessment of traditional Framingham risk factors (this process is described above).

Perfusion Imaging - The word perfusion means blood flow, and perfusion imaging implies that blood flow to a particular organ, in the case of CAD, the heart, is being measured by a technique that creates images of the human body.

Echocardiography, or echo, is an ultrasound study of the heart. Echo is a powerful tool that uses ultrasound (sound beams) and can directly visualize the heart in real time - this means that pictures of the heart are captured while the heart is beating, and the function of the heart, not just the anatomy, may be assessed. Echo is a superb tool for evaluating heart muscle thickness, for searching for fluid within the sac surrounding the heart, and for looking at the function of cardiac valves. The pattern and strength of cardiac muscle contraction may also be assessed. Using a technique called Doppler ultrasound, the direction and speed of blood flow may be visualized. Direct visualization of the coronary arteries with echo, however, is quite difficult owing to the small size of these vessels.

Echo can be used to assess for CAD by searching for changes in the pattern of heart muscle contraction while the patient undergoes stress - the test is referred to as stress echocardiography. The stress in this instance is not the usual connotation of the term, but rather refers to the use of exercise or certain medications to make the heart work harder. Recall from the discussion above that, as the heart works harder, much like any other organ, it requires more blood flow to supply the increased demand for oxygen and nutrients. If moderate-to-severe coronary artery blockages are present, during stress the area of the heart muscle supplied by the diseased coronary artery will not receive enough oxygen and will begin to function abnormally. Often the affected segment of heart muscle will not contract as strongly as it does during rest imaging and the heart chambers may dilate - this difference can be used to diagnose the presence of CAD. Overall, stress echocardiography is a fairly sensitive test for the detection of CAD. A recently published study reviewing stress echocardiography and other non-invasive methods for the assessment of CAD found that stress echocardiography correctly identifies the presence of CAD in nearly 80% of those patients undergoing the test, and when stress echocardiography is normal, the likelihood of CAD is very low.