This is a type of nuclear scanning test or myocardial perfusion (mi"o-KAR'de-al per-FU'zhun) imaging test. It shows how well blood flows to the heart muscle. It's usually done along with an exercise stress test on a treadmill or bicycle.

The thallium stress test is useful to determine:

• Extent of a coronary artery blockage
• Prognosis of patients who've suffered a heart attack
• Effectiveness of cardiac procedures done to improve circulation in coronary arteries
• Cause(s) of chest pain
• Level of exercise that a patient can safely perform

When the patient reaches his or her maximum level of exercise, a small amount of a radioactive substance called thallium is injected into the bloodstream. Then the patient lies down on a special table under a camera ("gamma camera") that can see the thallium and make pictures. The thallium mixes with the blood in the bloodstream and heart's arteries and enters heart muscle cells. If a part of the heart muscle doesn't receive a normal blood supply, less than a normal amount of thallium will be in those heart muscle cells.

The first pictures are made shortly after the exercise test and show blood flow to the heart during exercise. The heart is "stressed" during the exercise test — thus the name "stress test." The patient then lies quietly for 2-3 hours and another series of pictures is made. These show blood flow to the heart muscle during rest.

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 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, there's 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.)

What if I can't perform an exercise test?

Sometimes you can't do an exercise test because you're too sick or have physical problems. In this case, a drug such as dipyridamole (di-pi-RID'ah-mol) or adenosine is given. This drug increases blood flow to the heart and thus "mimics" an exercise test. Then the thallium test is given.