Nuclear Exercise Stress Test is a commonly performed cardiology test. During the test, a small amount of radioactive substance (isotope) is injected into the patient’s bloodstream. The isotope circulates to the heart muscle during exercise. The imaging done after the exercise portion of the test shows areas of the heart that take up none, or only a small portion, of the substance. In this way, the test can indicate whether there are areas of the heart muscle that have diminished blood flow.