What Is a Nuclear Stress Test?

What to expect when undergoing this test

A nuclear stress test is a study aimed at measuring whether the blood flow to your heart muscle is normal or abnormal. The study utilizes a radioactive tracer to create an image of how well blood is reaching your heart muscle, both during exercise and while at rest. If you are unable to exercise, or your doctor thinks exercise is not a good idea for you, this study can still be performed by giving you a medication that produces the same kind of cardiovascular stress that is caused by exercise.

The nuclear stress test is most often used to estimate the severity of any blockages that might be present due to coronary artery disease. This study is generally thought to be the most accurate non-invasive method for diagnosing coronary artery blockages. In many cases, doing a nuclear stress test can make it unnecessary to perform an invasive cardiac catheterization.

what to expect during a nuclear stress test
 Illustration by Emily Roberts, Verywell

Purpose of Test

The nuclear stress test is most often performed to help diagnose whether coronary artery disease is the cause of unexplained symptoms, especially episodes of chest pain or dyspnea. If coronary artery blockages are present, this test can also help the doctor judge the severity of the blockages.

In people who are already known to have coronary artery disease, the nuclear stress test is often also quite helpful in developing an optimal treatment plan.

People who are being treated for coronary artery disease, whether with medication or a stent, will often have nuclear stress testing both to help judge the effectiveness of the therapy, and to get the information needed to provide objective advice about daily activities and exercise.

What the Test Assesses 

The idea of nuclear stress testing is to create two images of how blood is being distributed to the cardiac muscle—one during rest, and one during exercise. Normally, the blood should be evenly distributed to the heart muscle both at rest and while exercising, and the images will show this even distribution.

If both the rest and the exercise images show a fixed patch of poor blood flow, that indicates that a prior heart attack has occurred. If an area of poor blood flow is identified during stress testing that is not present during rest, that indicates that a blockage is likely in one of the coronary arteries that is producing a transient drop in blood flow.

So this test enables the doctor to detect the presence, location, and relative size of a prior heart attack; and the presence, location, and relative size of any blockages in the coronary arteries that are significant enough to impede blood flow during exercise.

The Radioactive Tracer

The choice of the optimal radioactive tracer for creating a useful image during a nuclear stress test has evolved over the years. In the past, thallium-201 (Tl-201) was most commonly used. Today, that has largely been replaced by technetium-99 (Tc-99), which can provide clearer images with less radiation exposure.

Rest and Exercise Images

In many laboratories, nuclear stress tests are two-day studies. In a two-day study, an exercise image is performed on one day, while the resting image is done the next day. Typically, the exercise test is performed first, since if the exercise image is entirely normal there is generally no need to do a rest image at all, and the second day of the test can be skipped.

Some labs are now routinely doing one-day nuclear stress testing. A resting image is performed first with a relatively small dose of Tc-99, followed a few hours later by stress imaging using a much larger dose. There are technical challenges to performing one-day testing, but these are often offset by avoiding the practical challenges (largely, to the patient) of two-day testing.

The resting image is performed by injecting the Tc-99 tracer intravenously, then making the image (by scanning the chest with a special camera called a gamma camera). The exercise image is performed by injecting the tracer during peak exercise. In both cases, the gamma scan should be done between 15 - 60 minutes after the tracer is injected.

In People Who Cannot Exercise

In people who are capable of performing adequate levels of exercise, exercise stress testing (usually on a treadmill, sometimes on a stationary bicycle) is the preferred method for performing a nuclear stress test. Because of physical limitations, however, some people who need nuclear stress testing cannot perform this exercise.

If you have such physical limitations, pharmacologic stress testing can be substituted for exercise stress testing. Drugs that can induce transient physiological cardiac stress sufficient for performing nuclear stress testing include vasodilators (such as adenosine or dipyridamole), and inotropic drugs (usually dobutamine). In general, vasodilators are preferred, but your doctor will choose the optimal pharmacologic agent for you if actual exercise is not an option.

Risks and Contraindications

When it is performed by experienced personnel, the nuclear stress test is quite safe. Still, there are known risks, which include:

  • Cardiac arrhythmias: Exercise-induced heart arrhythmias may occur. These are rarely dangerous, and almost always disappear when the exercise stops. Furthermore, while these arrhythmias are indeed considered a risk of stress testing, detecting them also has diagnostic value. Finally, if potentially dangerous arrhythmias are produced by modest exercise, it is better to find out about them in a controlled environment than out on the street.
  • Chest pain, dizziness, or other symptoms: In people with significant coronary artery disease, modest amounts of exercise can produce symptoms associated with cardiac ischemia (that is, insufficient blood flow to the heart muscle). Again, while symptoms like these are considered a risk of this test, it is often useful to reproduce such exercise-associated symptoms during the test, in order to determine whether they are indeed being caused by coronary artery disease. In fact, in people being specifically assessed for exercise-related symptoms, one of the goals of the test is to reproduce the symptoms.
  • Allergic reaction: While extremely rare, some people may have an allergic reaction to the radioactive tracer that is used in this test.
  • Heart attack: Also extremely rare, in a person with a critical blockage in a coronary artery, a bout of exercise might produce a heart attack. 

Life-threatening complications are estimated to occur only in 1 out of every 10,000 exercise tests that are done.

To minimize the risk of serious complications, some people should not have nuclear stress testing. Specific contraindications to the test include:

A typical nuclear stress test using the radioactive tracer Tc-99 will expose you to 11 millisieverts of radiation. This is about 3 times the dose of radiation you get from living a normal life for one year. It is considered a small amount, and by itself should not increase your lifetime risk of cancer by any measurable amount.

Before the Test

Knowing these details can help ease any anxiety you have about this test.

Timing and Location

Your doctor will discuss with you when and where the test will be performed. Most nuclear stress tests are performed in a hospital outpatient area. You will probably be asked to show up at least 30 minutes before the scheduled test and can expect to be there for at least two hours. You should discuss with your doctor how much time you should allow for this test.

Very importantly, you will need to know whether you are having a one-day test or a two-day test, so you can make appropriate arrangements. In general, a two-day test will require two or more hours each day; a one-day test will likely take at least four hours.

What to Wear

Since you will be probably walking on a treadmill or riding an exercise bike, you should bring comfortable, loose-fitting clothing and comfortable shoes. Many people will bring gym clothing with them, along with walking or running shoes. Most testing facilities offer a convenient place to change out of (and back into) your street clothing.

Food and Drink

You will be asked to avoid food, drink, and smoking for 4 to 6 hours before your test. It is also important to avoid caffeine for at least 24 hours before the test.

Medications

You should ask your doctor which of your prescription medications you should take on the day of testing, and which not to take. If you have diabetes, you should be given specific instructions on how to take your medications on the day of the test. You should also avoid taking any over-the-counter medications before the test unless you clear it first with your doctor.

Cost and Health Insurance

The nuclear stress test is relatively expensive, but its cost varies quite a bit from place to place, and also according to what type of insurance you have. Most medical insurance covers the nuclear stress test as long as a qualified cardiologist certifies that you need it—but you may want to check with your insurance company prior to testing just to be sure.

What to Bring

You are likely to spend at least a few hours of downtime during the testing process, so bring a book or a magazine to help you occupy your time.

Other Considerations

If at all possible you should bring a family member or a friend who can help you pass the time, and possibly, who can sit in with you and your doctor if you expect to get the test results on the day of testing.

During the Test

Pre-Test

After you arrive for testing, you will be interviewed by one of the healthcare team to see whether your symptoms or medical condition has changed since you last saw your doctor, and you will have a quick physical exam. You will also be instructed once again as to what to expect during the test and will be given an opportunity to ask any additional questions you may have.

Throughout the Test

All testing should be directly supervised by a physician, and the doctor conducting the test should clearly identify him/herself. 

To make the resting cardiac image, a technician will insert an intravenous line into one of your veins, and a small amount of the radioactive tracer will be injected. After about 20 minutes, you will be lying down under a gamma camera for 15 to 20 minutes while an image is made that shows how blood is being distributed to your heart muscle. While you are under the gamma camera, you will need to keep your arms up above your head, and you will have to lie still.

The exercise part of the study will be done later the same day if you are having a one-day test. If you are having a two-day test, generally the exercise portion of the study is done on the first day, and the resting portion is done the next.

For the exercise study, a technician will place electrodes on your chest so that an ECG can be recorded during exercise, and (unless you are having a one-day test and an IV has already been placed), an IV line will be started. 

You will begin walking on the treadmill (or pedaling a stationary bicycle) at a very slow pace at first. Every three minutes the level of exercise will be increased until you become fatigued, develop symptoms, or your target heart rate is reached. When you have achieved the maximum level of exercise you are being asked to do, the radioactive tracer will be injected into your IV, and you will stop exercising. Your ECG and vital signs will be monitored throughout the exercise and recovery periods. The exercise itself usually lasts for no more than 6 to 12 minutes.

Roughly 20 to 30 minutes after you have finished exercising, you will again lie under the gamma camera for 20 minutes or so, with your arms above your head, so the exercise cardiac image can be made.

After the Test

Most people are instructed to resume their normal meals, medications, and activities immediately after the study is ended. The amount of radiation you receive with nuclear stress testing is considered very small, and there are no special precautions you will need to take in this regard.

Testing centers vary on how the results of nuclear stress testing are communicated to the patient. Most laboratories will give you some indication of the results right away, at least to the extent of telling you whether the study has shown findings of immediate concern. If so, you will be given advice on what to do next.

Most often, however, the test is not immediately alarming, and formal results will not be available until the images are officially reviewed by a cardiologist. In this case, you will be instructed to contact your referring doctor for the results and to discuss what, if anything, should be the next steps.

Managing Side Effects

Except for a bit of fatigue from performing the exercise, you should expect no side effects at all. If you have any unusual symptoms after this test, you should bring them to the doctor’s attention.

Interpreting Results

The nuclear stress test is aimed at measuring whether blood flow to all areas of the heart muscle is sufficient, both during rest and during exercise. While the interpretation of the resting and exercise images has to be individualized, and while you will need to talk to your own doctor about the specific findings of the test in your own case, in general, the results of a nuclear stress test fall into three categories.

1. Both the resting and exercise cardiac images are normal.

This result suggests, first, that no permanent heart damage from a prior heart attack is detectable (because the resting scan is normal); and second, that no area of heart muscle is being deprived of blood flow during exercise (because the exercise scan is normal). This result would strongly suggest that no significant coronary artery blockages are present.

Based on these results, your doctor will suggest which next steps, if any, are recommended. If your nuclear stress test is normal (in which case, congratulations!) that may very well be the end of your cardiac testing. If strong evidence of coronary artery disease is present, or if a diagnosis can be made with confidence, then your doctor will likely recommend immediately instituting medical treatment to relieve symptoms and slow the progression of your coronary artery disease. If your symptoms are severe, or the risk of an early heart attack appears to be quite high, your doctor may also discuss the possibility of a cardiac catheterization, in order to consider a more invasive form of therapy, such as a stent.

2. The resting cardiac image is abnormal.

If the resting cardiac scan shows an area in which blood is not flowing normally to a part of the cardiac muscle, that is an indication that a prior heart attack has occurred, resulting in permanent heart muscle damage.

3. The exercise cardiac scan is abnormal, but the resting image is normal.

If a portion of the heart muscle is not getting sufficient blood flow during exercise but is getting normal flow while at rest, that is an indication that a significant blockage is likely present in the coronary artery that supplies that area of the heart muscle.

It is useful to remember that different portions of the heart muscle may display any of these three general kinds of findings. So, for instance, a person could have a “permanent” defect in one part of the heart muscle, indicating that a heart attack has occurred; and a “reversible” defect in another portion of heart muscle, indicating a blockage but no permanent damage.

A Word From Verywell

Nuclear stress testing is a generally safe and effective noninvasive method of assessing whether significant blockages in the coronary arteries are present​ and whether such blockages are responsible for symptoms such as chest pain, or have already produced permanent heart muscle damage. This kind of testing has proven to be quite valuable in diagnosing coronary artery disease, and in helping to guide its treatment.

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