A nuclear cardiac stress test is a specialized imaging procedure that helps medical professionals evaluate how well blood flows to the heart muscle. It involves injecting a small amount of a radioactive substance, called a tracer or radiopharmaceutical, into the bloodstream. This tracer is then absorbed by the heart muscle, making it visible to a special camera that takes pictures of blood flow within and around the heart. This diagnostic tool is also known as myocardial perfusion imaging (MPI), cardiac PET (positron emission tomography), or cardiac SPECT (single-photon emission computed tomography).
Why a Nuclear Stress Test is Performed
Doctors order a nuclear stress test to diagnose and monitor heart conditions, particularly coronary artery disease (CAD). This condition occurs when the arteries supplying blood to the heart become damaged or narrowed, limiting the flow of oxygen and nutrients. The test helps to pinpoint areas of reduced blood flow or damage within the heart muscle.
The test also assesses heart damage after a heart attack and determines if the heart muscle is pumping effectively. Patients experiencing symptoms such as chest pain, shortness of breath, or unexplained fatigue may undergo this test to identify underlying heart issues. It can also guide treatment decisions for those already diagnosed with heart disease, evaluating current therapies or determining the need for further interventions.
The Nuclear Stress Test Procedure
Preparation for a nuclear stress test involves specific instructions for accurate results. Patients are asked to fast for several hours before the test and to avoid caffeine for at least 24 hours prior, including coffee, tea, sodas, chocolate, and certain pain relievers. Wear comfortable, loose-fitting clothing and walking shoes, and avoid applying lotions or powders to the chest area. Patients should also discuss all medications with their doctor, as some may need to be adjusted or temporarily stopped.
The procedure begins with the insertion of an intravenous (IV) line, in an arm or hand. A small, safe dose of a radioactive tracer, such as technetium-99m sestamibi or thallium-201, is then injected through the IV. This tracer circulates through the bloodstream and is absorbed by the heart muscle. After a waiting period, 15 to 45 minutes, a special camera, a gamma camera, takes the first set of images of the heart while the patient is at rest. This camera rotates around the chest to create three-dimensional pictures.
Following the resting images, the stress phase begins. For those able to exercise, this involves walking on a treadmill or pedaling a stationary bike. The intensity gradually increases until a target heart rate is achieved. During peak exercise, a second dose of the radioactive tracer is injected.
If a patient cannot exercise, a pharmacological stress test is performed, where medication is administered through the IV to induce a stress response in the heart. After the second injection and a short waiting period, a second set of images is taken, showing blood flow to the heart under stress. The entire test takes about three to four hours to complete.
Understanding Your Results
After the nuclear stress test, a cardiologist or radiologist analyzes the images obtained from both the resting and stress phases. The distribution of the radioactive tracer within the heart muscle reveals how well blood is flowing to different areas. A normal result indicates that the heart receives sufficient and unrestricted blood flow during both rest and activity, suggesting no significant blockages in the coronary arteries.
If the stress images show areas with less tracer uptake compared to the resting images, it suggests reduced blood flow during exertion, known as ischemia. This points to a narrowing or blockage in one or more coronary arteries. If an area shows reduced tracer uptake in both resting and stress images, it can indicate scar tissue or damage from a previous heart attack, where blood flow is permanently impaired. These interpretations help doctors diagnose heart conditions like coronary artery disease, assess the severity of blockages, and determine the need for further medical or surgical interventions.
Risks and Important Considerations
Nuclear stress tests are considered safe, with minimal risks. The amount of radiation exposure from the radioactive tracer is low, 3 to 22 millisieverts, comparable to about three times the average annual background radiation exposure. While the body remains slightly radioactive for 1 to 2 days, the risk of radiation exposure to others is low. Drinking plenty of fluids after the test helps to flush the tracer from the body.
During the stress phase, some individuals may experience side effects like headache, nausea, dizziness, or shortness of breath. Chest pain or irregular heart rhythms can also occur, but medical staff closely monitor patients to manage these symptoms. Serious complications, such as a heart attack or severe allergic reactions to the tracer or pharmacological agents, are very rare. Patients should contact their doctor if they experience any worsening or unusual symptoms after the test.