What is a thallium stress test?
This is a type of nuclear radio-isotope scanning test or myocardial perfusion imaging test. It shows how well blood flows to the heart muscle. It is usually done along with an exercise stress test on a treadmill or bicycle. It provides a view of the blood flow into the heart muscle and is used to evaluate how well the heart is perfused (supplied with blood) at rest as compared with activity.
Why is the test done?
The thallium stress tests are indicated when the doctor needs to evaluate:
- how well the heart responds to exercise
- the cause of chest pains
- the degree of blockage in the coronary arteries
- what to expect after one has had a heart attack
- how well a heart procedure done to improve blood flow in the coronary arteries
- level of exercise that a patient can safely perform
How is the test performed?
One is instructed to exercise as hard as one can on a treadmill or bicycle. If the doctor considers that exercise is not safe, or that one may be unable to exercise enough because of orthopaedic problems, then one will be given an intravenous medication that will challenge the heart as if one was exercising.
When one reaches the maximum level of exercise, a small amount of a radioactive substance (radiotracer), either thallium or sestamibi is injected in the vein. The radiotracer will travel in the bloodstream and, through the coronary arteries, will enter into the heart muscle as one completes the exercise session.
After one finishes exercising, the patient lies down on a special table under a bulky camera called a gamma camera. The gamma camera scans the heart and detects the radiotracer in it. The distribution of the radiotracer in the heart is processed by a computer to create pictures of the heart. The first pictures are made shortly after the exercise test, to show the circulation of blood to the heart during exercise. This is the part considered "a stress test" and is the most challenging for your heart.
The patient will need to lie quietly for 2-3 hours, and at that point the scanner will make another series of pictures of the heart. These images will show the circulation of blood through the heart muscle at rest.
If the doctor indicates that the test should be performed without exertion, then at the beginning of the test one will not exercise, but instead will receive an intravenous medication, a vasodilator (usually dipyridamole or adenosine). This medication will selectively dilate (widen) the coronary arteries if they are normal; arteries with blockages will receive less blood flow and will be less dilated, allowing less blood flow into the heart muscle. After this initial medication, one will receive the injection with the radiotracer. The test done using a vasodilator can potentially show a defect in the same way as the test with exercise does.
How to prepare for the test?
One is instructed to wear comfortable clothes and shoes. One will probably be asked not to eat or drink anything after midnight, except for a few sips of water if needed to take medicines.
For one entire day prior to the test, one will be required to abstain from caffeine and certain medications. Caffeine is present in food and beverages such as all regular and decaffeinated coffees or teas, chocolate products, many sodas, and certain pain relievers. Medications that one may need to stop taking before the test include some asthma medicines and medicines for chest pain (angina); it is necessary to check with the doctor. If one takes insulin to control the blood sugar, it is necessary to ask the doctor if or how much insulin should be taken on the day of the test.
The entire nuclear imaging test may last about 4 hours, so one will need to prepare accordingly. At some point during the stay for the test one will likely be given a long break and be allowed to have lunch or a snack. During the test one will not be sedated.
What does the thallium stress test show?
If the test is normal during both exercise and rest, then blood flow through the coronary arteries is normal. (The coronary arteries supply blood to the heart muscle). If the test shows that perfusion (blood flow) is normal during rest but not during exercise (a perfusion defect), then the heart is not 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, there is limited blood flow to that part of the heart at all times. If no thallium is seen in some part of the heart muscle, the cells in this part of the heart are dead from a prior heart attack. They have become scar tissue.
How does the test make one feel?
One may feel the same symptoms as with a regular stress test or with a session of strenuous exercise: fatigue, muscle cramps in the legs or feet, some shortness of breath, or chest pain. Some patients also develop a headache or nausea.
What are the risks?
Nuclear imaging stress tests are very safe. The estimated risk of complications are only about 1 in 1,000 to 1 in 2,000 cases. These complications involve rare skin rashes, large fluctuations in blood pressure, arrhythmias or irregular heart beats, and difficulty breathing or asthma-like reactions. These, and any additional risks that may apply in to will be explained in advance by the doctor who performs the test. Radiation exposure due to radiotracers can be a concern for the nuclear laboratory staff, but not for patients undergoing an occasional nuclear imaging test.