That’s because use of nuclear material, radioactive isotopes that act as a “dye,” can give physicians more accurate information about the size of a heart’s chambers, its efficiency in pumping blood and the presence of any damaged heart muscle. The test also has an estimated 85 percent accuracy rate in determining whether any of a patient’s major coronary vessels, namely the arteries supplying the heart with oxygen, blood and nutrients, is significantly blocked.
Sometimes called myocardial perfusion imaging, nuclear testing requires a small amount of a radiopharmaceutical like technetium-sestamibi (also called “mibi”), which is usually injected directly into the bloodstream. The radionuclide material sends out gamma ray energy that is tracked by a special camera and computer, resulting in detailed images of anatomic structure, function and distribution of blood flow to the heart while a patient is at rest and during exercise. Diseased or scarred areas of the heart and vessels will appear as cold spots, or dark areas, on the images.
The exercise portion of the test is usually performed while a patient walks a treadmill with a goal of achieving at least 85 percent of the maximum, age-appropriate predicted heart rate, a threshold at which the test becomes more diagnostically accurate. While exercise is a powerful prognostic tool and a great way for physicians to assess patient’s symptoms, for those who can’t exercise (due to orthopedic problems, and other co-morbidities), the dilation of blood vessels and increase in blood flow that is normally seen with exercise can also be induced chemically with medications called vasodilators (Lexiscan, Persantine).
Who Should Be Tested
A physician may prescribe a nuclear stress test if a patient has a history of heart problems, such as a prior heart attack or diagnosed coronary artery disease or experiences suspected heart-disease symptoms like shortness of breath, unexplained chest pain or chest pain induced by exercise and extreme fatigue. Cardiac nuclear testing is also preferred in patients who’ve had previous coronary stents placed and in patients who’ve had coronary artery bypass surgery as it is an accurate method of determining the effectiveness of specific treatments.
The nuclear testing process can be lengthy, taking between several hours to a couple of days depending on type of examination required and kind of radionuclide used to create images. Because the test involves a small amount of radiation exposure, women who are pregnant, may be pregnant or are breastfeeding should alert their physician. Certain medications, such as those to treat asthma or angina, may also interfere with test results.
Do Normal Results Mean You Are Safe?
Despite the accuracy of nuclear imaging, normal results obtained from any type of cardiac stress testing, including echo exams, which rely on ultrasound rather than on radioisotopes to create “pictures,” do not guarantee a patient is all-clear. In fact, experts agree that stress tests are not a screening tool to assess a person’s risk for coronary artery disease.
The first symptom of heart disease is likely to be a heart attack or sudden death, and this can happen when a blockage has narrowed a coronary artery by only 40 percent or less. Heart attacks occur when plaque in an artery ruptures, causing a blood clot to form. Heart stress tests oftentimes fail to detect accumulations of plaque blocking less than 65 percent of a heart artery.
Unless a patient has certain heart risk factors or symptoms of heart disease, stress tests in general may offer no benefits. The American College of Cardiology recommends any heart testing should be a joint decision of doctor and patient, with clear understanding of the test’s purpose and goal.
A. Garvey Rene, MD is a Johns Hopkins, Cornell, Columbia and Penn trained Clinical and Interventional Cardiologist with CareMount Medical. He is board-certified in internal medicine, echocardiography, cardiovascular disease and board-eligible in interventional and nuclear cardiology. He sees patients both in Mount Kisco and Fishkill and performs his cardiac catheterizations at Westchester Medical Center.