Nuclear Cardiology: Stress Myocardial Perfusion Imaging and Radionuclide Angiography
Nuclear cardiac imaging plays a pivotal role in assessing total heart health. Nuclear cardiology uses noninvasive procedures to locate blockages in the coronary arteries and to determine if coronary artery bypass surgery and/or angioplasty are needed. It’s also useful in detecting blockages in coronary artery bypass grafts in patients who have undergone previous cardiac surgery. Cardiac function and heart size can also be precisely determined.
Stress myocardial perfusion imaging is one of the most popular and versatile of the nuclear cardiology tests used today to assess a patient’s cardiac risk and to guide appropriate therapy. Candidates for this test usually have symptoms suggestive of heart disease (such as chest discomfort/heaviness or shortness of breath). Patients with a history of diabetes mellitus, an abnormal resting electrocardiogram or a high CT calcium score result should also undergo testing.
During a stress perfusion imaging test, patients walk on a treadmill to assess how the heart performs under stress. In patients who cannot adequately exercise, several drugs are available which can be injected though a vein in the arm to simulate exercise. Two of the most commonly used drugs are Adenosine, and more recently Regadenoson. MDCA cardiologists were instrumental in investigating the use of these drugs in conjunction with perfusion imaging, and thus are very familiar with their actions and in deciding who might benefit most from this form of stress testing.
During the test, patients are injected with a small dose of a radioactive agent (radiotracer) which is tracked through the heart using a scanning device known as a gamma camera. An intravenous line is placed in an arm vein to inject the radiotracer and, if the patient can’t perform treadmill exercise, the stressor drug will also be injected. Healthy heart muscle has unrestricted blood flow and will avidly take up the radiotracer. However, heart tissue that is damaged or not receiving enough blood due to coronary artery blockages will not absorb as much of the tracer, and thus appear differently on the scanning results.
The initial scan takes approximately 15 minutes to complete, after which time another injection of the radiotracer is given under resting conditions. A second scan will then be performed. This allows physicians to see whether those areas of the heart muscle that could not absorb the radioactive tracer during stress can do so when the patient is resting. Patients with a normal stress myocardial perfusion study are very unlikely to have significant blockage in their coronary arteries and therefore are at very low risk for having a heart attack over the next year (less than 0.5%). However, patients with an abnormal test result are at a much higher risk, which increases with the amount of heart muscle affected.
On the morning of the test, patients should not have breakfast since they will be exercising on the treadmill. Morning medications can be taken with water. It is advisable to wear loose fitting clothing and comfortable shoes for the treadmill procedure. Patients who are scheduled for a pharmacologic stress test should not have any caffeine for at least 12 hours prior to the test since caffeine interferes with the drugs used. Caffeinated foods and drinks include coffee, tea, colas, and chocolate. Patients should prepare to spend several hours in the laboratory to complete both the stress and rest portions of the test.
Additional Nuclear Cardiology Procedures
Other tests performed in nuclear cardiology include radionuclide angiography and positron-emission tomography (PET), both noninvasive procedures. Radionuclide angiography is one of the most accurate methods to assess the heart’s pumping ability. Positron emission tomography is also a perfusion test which can accurately assess whether damaged muscle from a previous heart attack has the ability to rejuvenate with successful angioplasty or bypass surgery.
Nuclear cardiology tests can also determine to what extent damaged heart muscle might bring on arrhythmias, monitor heart status after transplantation, and determine whether heart tissue is damaged due to infection. Newer radiotracers under development can assess not only blood flow to the heart muscle, but also cardiac metabolism. This may help identify patients at high risk for heart attack and determine appropriate follow up treatment.