There are many ways of measuring how the heart responds to exercise. One way is to exercise while wearing an electrocardiogram. This test is called a cardiac stress test. Another type of test uses exercise and an injected radioactive substance to measure heart response.

The thallium exercise scan uses radioactive thallium to test the blood flow to the heart muscle. It is different from the ECG stress test. The thallium exercise scan begins with a form of exercise, such as a treadmill or bicycle. The patient exercises until their heart rate reaches a certain level. The exercise is stopped and a very small amount of thallium is injected into a vein. Then they lie down under a special camera that measures the flow of thallium into the heart muscle. In a normal heart there is no blockage of blood flow and no damage to muscles. The thallium is spread evenly throughout the muscle and shows up on the picture taken by the camera.

If there is a problem with blood flow to a part of the heart, the thallium will not show up in that part. The first part of the test is to make a picture of the heart just after exercise. The second part is to make another picture several hours later. If there is a temporary blockage of blood, one that is worse with exercise, the first picture will show a blockage. The second picture will show that the thallium has finally spread throughout the heart. If the heart has permanent damage, usually a scar from an old heart attack, the thallium will not spread, even after rest.

The thallium exercise scan takes longer than an ECG stress test and costs more. It is used only in special situations. It is very helpful if a person is having chest pain and the cause is not clear. There is a special type of thallium stress test where a medication can be used to stimulate the heart. In this way, the test can be done in persons who are unable to exercise.

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