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What is a nuclear stress test?
Nuclear stress test is a diagnostic tool that helps to examine the condition and functioning of a person's heart under conditions of stress (as during or shortly after physical exercise) as well as at rest using a radioactive imaging technique.
The following information can be derived from a nuclear stress test:
About the size of the heart
About the size of the heart's chambers
How efficiently the heart is pumping blood
How efficiently blood is flowing into the heart
Whether the heart has any damaged or dead muscle
Whether the arteries supplying blood to the heart are blocked or narrowed due to coronary heart disease
This cardiac nuclear stress test is quite similar to the routine exercise stress test except that in addition to electrocardiograms one also gets to see images of the heart and coronary arteries.
How does a nuclear stress test work?
During a nuclear test, traces of a radioactive substance (mostly thallium-201 isotope) are injected into the bloodstream. The radioactive substance then travels to the heart, where a gamma-ray camera picks up the radioactive signals and produces images of the heart and the arteries. These images are then sent to a monitor screen, where they are made visible to the eye. If a part of the heart is not receiving adequate flow of blood, it shows up as a light spot on the camera because the radioactive substance is not reaching there.
Images of the heart are observed shortly after strenuous exercise and also under rest conditions.
Preparation for nuclear stress test
There is no special preparation required for a nuclear stress test, compared to a non-nuclear test. General recommendations for any kind of a stress test can be followed as suggested by the physician and are covered by most individual health insurance plans and group health insurance plans.
Common variations of the test
The two common variations of nuclear stress test are:
Myocardial perfusion scan:
In this test the patient is made to exercise on a treadmill or on a stationary bike. After he reaches the target heart, the radioactive substance is injected into the bloodstream. Images are made of his heart shortly after exercise and a few hours later. This test reveals how well the blood is flowing into the heart and can detect narrowing of the coronary arteries (coronary artery disease).
Multigated acquisition (MUGA) scan:
In this test, the radioactive substance is injected before exercise unlike the previous test, where this is done after exercise. Images are made of the heart before and after exercise. A MUGA scan shows the motion of the heart and how well it pumps out blood (ejection fraction).
Interpreting the results of nuclear stress test
The results of the nuclear stress test can reveal the condition of the heart, whether or not it is functioning properly under physical stress and normal rest conditions. If the test shows that blood flow is normal during rest but not so while or shortly after exercising, this implies that blood flow to the heart is not adequate during times of stress. The heart normally pumps more blood during physical exertion. If the test results are not normal even during rest, this implies that a part of the heart is permanently deprived of blood or is scarred. If the radioactive substance cannot be detected in some part of the heart, it probably means that this particular section of heart muscle is dead, either due to a previous heart attack or because of blockage in the coronary arteries supplying blood to heart.
Nuclear stress test: safety concerns
Cardiac stress testing is an established method of investigating moderate-risk patients for coronary artery disease and obtaining prognostic information about the patient.High-risk patients would anyway not be advised about the stress test, whether nuclear or non-nuclear.
Regarding the risk associated with exposure to radioactive radiation, one has to weigh the benefits against the risks. Although the amount of radioactive substance injected into the body is miniscule, any kind of exposure to radioactive radiation brings long-term risk of radiation-induced cancer.
A nuclear test using thallium-201 exposes a patient to radiation equivalent of 250 chest x-rays, conferring a lifetime risk of about 0.1% for fatal cancer. This risk is perhaps not too high considering the benefits of the test. However when making frequent use of nuclear tests this factor should be kept in mind and the benefits should be weighed against the risk factor.
How accurate is a nuclear stress test?
If a patient is capable of achieving the target heart rate during exercise and if good-quality images are obtained, a nuclear stress test can accurately diagnose a serious disease in approximately 85% of patients with coronary artery.
Approximately 10% of patients may show a false positive test (i.e. when the test falsely diagnoses patient of not having coronary artery disease). Some technical problems can arise when a patient is obese. The results may not be true due to the enlarged diaphragm and breasts in men and woman respectively.
