NUCLEAR STRESS TESTING
One way to determine whether or not you have any significant blockages in your coronary arteries is by nuclear stress testing. A nuclear stress test is also used in patients who are known to have blockage in order to determine if enough blood is getting through the blockage to supply the heart muscle.
The heart arteries are the fuel lines that supply the blood to the heart muscle so that it can pump effectively. There are three coronary arteries that supply the main pumping chamber (left ventricle). The left anterior descending artery (LAD) supplies the front of the left ventricle, the left circumflex artery (LCA) supplies the side of the left ventricle and the right coronary artery (RCA) supplies the back of the left ventricle.
When not enough blood is getting through a coronary artery to supply the heart muscle, the muscle gets temporarily injured and the patient can feel pain in the chest (angina). If this goes on for too long, the heart muscle in that area dies (heart attack). Some people do not get any symptoms at all and may have critical blockages and feel fine. Usually a blockage starts to cause problems when it blocks that artery by 75%. A blockage is usually treated in one of three ways, all designed to get more blood to the area of the heart supplied by that blocked artery. Medicine can be used to get more blood through the blockage. This can be very effective. The blockage can sometimes be opened with a balloon and a small metal device called a "stent" inserted to hold it open. This is done at the time of the heart catheterization. Finally, coronary artery bypass surgery can be performed to physically put a bypass around the blockage.
This is the way the nuclear stress test works. An IV is started and then a nuclear isotope, which is a radioactive material that makes the heart radioactive for short period of time, is injected to obtain rest images of your heart. The patient is then placed under any "scanner," which is a large Geiger counter, which gets images of the heart while it is radioactive. There is no X-ray or dye used with this test. It is then necessary to increase the blood flow through the coronary arteries that supply the heart. This is done either by exercising the patient on a treadmill or with medication for those who can't walk on the treadmill. The medicine used is called Regadenoson. If Regadenoson is used, it is given through an IV over several minutes. After walking on the treadmill or getting the Regadenoson in the IV, the heart is scanned again.
In theory, even with a fairly significant blockage in a heart artery, enough blood is able to get through the blockage to adequately supply the heart muscle when the heart is at rest since the heart muscle does not need much blood flow at all at rest. When the heart is put under stress, such as with exercise on the treadmill or with the Regadenoson, that same blocked artery will not be able to supply enough blood to that area of the heart since the needs of the heart muscle are so much more with exercise. If there are no blockages, the whole heart should be radioactive for a short time and be able to be visualized well on both scans. If an artery has a significant blockage, that part of the left ventricle it supplies will get less radioactive isotope to it and not be visualized as well on the exercise scan. When an area of decreased radioactivity is seen on the scan, it is usually due to a significant blockage in the coronary artery supplying that area.
The reason for the two scans is to not only see if there are any areas of abnormality, but also to see if an area of abnormality or decreased uptake stays unchanged (fixed) or gets better (reversible). An area of temporary injury such as is caused by a blockage, will be abnormal on the exercise scan but be normal on the rest scan (a reversible defect). An area of scar, such as from a previous heart attack, will not improve with time and will look the same on both rest and exercise scans (a fixed defect).
If you are known to have a blockage already and are on medication to improve blood flow through that blockage, the nuclear stress test can be used to see if the medicine is doing the job. If you have a blockage and are on medications for it, and the nuclear images are normal, this usually means enough blood is getting through the blockage with exercise and the medication is working.
The nuclear stress test is very accurate but there can be instances where a blockage or blockages do not show an abnormality (false negative test) and instances where the test is abnormal and there are no significant blockages (false positive test). This is why your physician needs to use this test as a part of his clinical decision making.