Definition. A nuclear stress test measures blood flow to your heart at rest and while your heart is working harder as a result of exertion or medication.The test provides images that can show areas of low blood flow through the heart and damaged heart muscle.The test usually involves taking two sets of images of your heart — one while you're at rest and another after you heart is stressed, either by exercise or medication.he test provides images that can show areas of low blood flow through the heart and damaged heart muscle. The test usually involves taking two sets of images of your heart — one while you're at rest and another after you heart is stressed, either by exercise or medication.