A stress test is a general screening tool to test the effect of exercise on the heart and the body's increased demand for oxygen. It can be performed to help determine causes of chest pain, the exercise capacity of the heart or appropriate exercise levels in those starting an exercise program. It can also identify rhythm disturbances during exercise.
During an exercise stress test, the patient engages in controlled exercise (such as walking on a treadmill or pedaling a stationary bicycle) and an electrocardiogram (EKG) is used to record the heart's response to the increased workload. Blood pressure readings are also taken. For patients who are unable to perform the physical exertion - due to illness or physical limitations - an intravenous medication such as dipyridamole or dobutamine may be used to produce a similar cardiovascular response as during exercise. This is referred to as a pharmacologic stress test.
In stress echocardiography, ultrasound is used in addition to the electrocardiogram, to evaluate the effects of increased activity. Following exercise or pharmacologic stress to the heart, ultrasound is performed to identify areas of the patient's heart with decreased function that may be a result of not receiving enough blood and therefore not enough oxygen. The ultrasound portion of this test is performed in that same manner as an echocardiogram.
Stress tests may also be enhanced through the use of nuclear imaging. These studies involve the use of small amounts of a radioactive substance (radionuclide) that targets the heart and a radionuclide detector that traces the absorption of the radioactive substance. The images produced may reveal problems in heart muscle and blood vessels, especially when the images of the heart at work and at rest are compared.
Myocardial Perfusion Imaging
Myocardial perfusion (blood flow) imaging is the most widely used type of nuclear cardiology test and provides more information than routine exercise stress tests. It can be used to identify areas of the heart muscle that have an inadequate blood supply, quantify the extent of the heart muscle with a limited blood flow, provide information about the heart's pumping function, assess the amount of scarring from a heart attack, and evaluate the results of coronary bypass surgery or angioplasty.
Thallium stress testing is done along with an exercise or pharmacologic stress test to provide additional information by using a radioactive substance to show how well blood flows to the heart muscle. It is useful in assessing the extent of a coronary artery blockage, damage from a heart attack, the effectiveness of procedures done to improve circulation in coronary arteries, the cause(s) of chest pain, or the level of exercise that a patient can safely perform.
The patient is asked to exercise on a treadmill or bicycle as in a standard stress test. When the patient reaches his/her maximum level of exertion, a small amount of thallium, a radioactive substance, is injected into the bloodstream. It travels with the blood to the heart. A thallium-sensitive "gamma camera" produces images showing where the blood flow has transported the substance. If a part of the heart muscle doesn't receive a normal blood supply, then less than a normal amount of thallium will be in those heart muscle cells.
The first pictures are made shortly after the stress test and show blood flow to the heart during exercise. The patient then lies quietly for a certain period and another series of pictures is made to show the heart muscle during rest.
If the test is normal during both exercise and rest, then blood flow through the coronary arteries is normal. If the test shows that blood flow is normal during rest but not during exercise, then the heart isn't getting enough blood when it must work harder than normal. This may be due to a blockage in one or more coronary arteries.
If the test is abnormal during both exercise and rest, blood flow to that part of the heart is always limited. When no thallium is seen in some part of the heart muscle, the cells in that part of the heart are dead from a prior heart attack and have become scar tissue.
Ventricular Function Testing
The multiple-gated acquisition (MUGA) scan or ventricular function test, is primarily used to evaluate how well the heart is pumping. Like an echocardiogram, this test shows how much blood the heart can pump with each beat. It can also help assess the health of the cardiac chambers and valves, as well as monitor the effect of different drugs on the heart muscle.
The left ventricle, the strongest chamber in the heart, is the main pump of blood through the body. The left ventricle is assessed by measuring the amount (fraction) of all blood in the ventricle that is pumped (ejected) with each heartbeat (the ejection fraction), ventricle filling, and the blood flow into the pumping chamber. A normal ejection fraction is 50 percent or more. The heart's ejection fraction is one of the most important measures of its performance. The right ventricle's ability to pump blood to the lungs is also assessed, and any abnormalities in the heart wall are identified.
During the MUGA scan, electrodes are placed on the patient's body so that an electrocardiogram (ECG) can be conducted. The imaging equipment and computer are synchronized with the ECG so that images of the heart can be recorded without motion or blur. Then a small amount of a mildly radioactive substance called technetium is injected, usually into an arm vein.
The technetium, also called a "tagging agent," attaches to the red blood cells and passes through the heart in the circulating blood. While the patient lies motionless on the test table, a "gamma scintillation camera" follows the movement of the technetium. The camera, which looks like an x-ray machine and is suspended above the table, moves back and forth over the patient. It displays multiple images of the heart in motion and records them on a computer for later analysis.
The MUGA scan can be done with the patient at rest or exercising (called a stress MUGA). The stress MUGA is often performed in patients who have or are suspected of having coronary artery disease. The resting MUGA is compared with the stress MUGA and changes in the heart's pumping performance are analyzed.
If the patient's heart is normal, the technetium will appear to be evenly distributed in the scans. (In a stress MUGA, patients with normal hearts will exhibit an increase in ejection fraction or no change.)
An uneven distribution of technetium in the heart indicates that the patient has coronary artery disease, a cardiomyopathy, or blood shunting within the heart.